Radio frequency filters realized by a cascade of hybridly coupled resonators

Abstract

An RF filter includes a filter housing, a plurality of hybridly coupled resonators configured in a cascade arrangement in the filter housing, a three-dimensional structure for providing inductive coupling in the RF filter. Each of the hybridly coupled resonators includes a conductive support structure extending vertically from a base of the filter housing and a folded resonator head supported on and electrically connected to the conductive support structure. The three-dimensional structure includes at least one conductive ridge extending vertically from the base of the filter housing between and electrically connected to the conductive support structures of adjacent resonators among the hybridly coupled resonators. The folded resonator head of each of the hybridly coupled resonators is configured to provide capacitive coupling in the RF filter.

Description

RADIO FREQUENCY FILTERS REALIZED BY A CASCADE OF HYBRIDLY COUPLED RESONATORSCROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] This application claims the benefit of priority to Italian Patent Application No. 102024000018088 filed on August 1, 2024 with the IPTO (Italian Patent and Trademark Office), the entire contents of which are incorporated herein by reference.BACKGROUND

[0002] The present disclosure relates generally to communication systems, and more particularly, to frequency-dependent couplings in radio frequency (RF) filters.

[0003] In the context of telecommunications, the advent of the fifth-generation (5G) technology standard for cellular networks has created an increasing demand for miniaturization. RF filters commonly used in 5G systems often present an impediment to miniaturization, since the layout and dimensions of the RF filters are typically configured into a grid shape. In such a grid configuration, the realization of cross-couplings between nonadj acent resonators for generating transmission zeros in the filter response is challenging or not possible.

[0004] To overcome some of these challenges, transmission zeros may be realized using an inline structure by means of frequency-dependent couplings (FDC). Standard realizations of FDC in coaxial filters typically employ some bulky form of lumped coupling capacitance between adjacent resonators to provide the required coupling slope. This capacitance is often realized by using some floating metal part assembled between open ends of resonating rods or through direct proximity of the open ends themselves, which are typically enlarged to facilitate capacitive coupling. However, this results in small air gaps that are subjected to wide variations in assembly and manufacturing tolerances, and whose dimensional variation can only be compensated with post-manufacturing tuning to a limited extent. Power handling of the fdters using this approach is reduced due to localized high electromagnetic fields that may be present in these air gaps. Additionally, standard realizations often employ expensive parts that cannot be widely reused inside the filter; that is, each cavity has its own dedicated resonator.SUMMARY

[0005] The present invention, as manifested in one or more embodiments thereof, is directed to RF filters that realize frequency-dependent couplings (FDC) using a cascade of hybridly coupled resonators (e.g., employing a coaxial cavity section and printed circuit board (PCB) metallization). In some embodiments, transmission zeros in the RF filter are realized using a combination of a three-dimensional (3D) structure (e.g., ridges and / or windows into a housing of the RF filter) for providing magnetic coupling (i.e., inductive) and a planar structure (e.g., PCB metallization) providing electrical coupling (i.e., capacitive). This hybrid approach may be oriented to higher frequency solutions (e.g., 1500 MHz -10000 MHz). In other embodiments, the RF filter may include resonators employing a “folded mushroom” (i.e., deep drawn) design for the resonator heads. This folded mushroom approach, which may be used to provide capacitive coupling between adjacent resonators either in place or in addition to the PCB metallization, may be oriented to lower frequency solutions (e.g., about 500 MHz - 1500 MHz).

[0006] According to an embodiment of the present disclosure, an RF filter includes a filter housing, hybridly coupled resonators configured in a cascade arrangement in the filter housing, a planar structure for providing negative coupling in the RF filter and configured to provide capacitive coupling between adjacent resonators, and a three-dimensional structure for providing positive coupling in the RF filter. Each of the hybridly coupled resonators includes a conductive pedestal extending upwardly from a bottom of the filter housing and a resonator head supported on and electrically connected to the conductive pedestal. The planar structure includes a printed circuit board (PCB). The resonator heads of the respective hybridly coupled resonators are realized using a first metallization layer on a surface of the PCB. The three- dimensional structure includes at least one conductive ridge extending upwardly from the bottom of the filter housing between and electrically connected to the conductive pedestals of adjacent resonators of the hybridly coupled resonators. In some embodiments, the planar structure may include at least one conductive probe between adjacent resonator heads of the hybridly coupled resonators, the conductive probe being realized using a second metallization layer on at least one surface of the PCB and being configured to provide capacitive coupling between the adjacent resonator heads.

[0007] According to another embodiment of the present disclosure, an RF fdter includes a filter housing, a plurality of hybridly coupled resonators, a plurality of conductive probes configured to provide capacitive coupling between adjacent resonators of the plurality of hybridly coupled resonators, and a printed circuit board having a first surface and a second surface opposite the first surface. Each of the plurality of hybridly coupled resonators includes a conductive pedestal extending upwardly from a bottom of the filter housing and a resonator head supported on and electrically connected to the conductive pedestal. Each of a plurality of resonator heads of the respective plurality of hybridly coupled resonators is formed in a first metallization layer on the first surface of the printed circuit board. A first one of the plurality of conductive probes is formed in a second metallization layer on the second surface of the printed circuit board, and a second one of the plurality of conductive probes is formed in a third metallization layer on the first surface of the printed circuit board, the first and second conductive probes being electrically connected to one another using one or more conductive vias extending through the printed circuit board from the first surface to the second surface. In one or more embodiments, the RF filter may further include a first conductive ridge extending upwardly from the bottom of the filter housing between and electrically connected to the conductive pedestals of adjacent resonators of the plurality of hybridly coupled resonators. In some embodiments, the RF filter may further include a second conductive ridge extending outwardly from a sidewall of the filter housing and configured to provide positive coupling as a function of a proximity of the second conductive ridge to an adjacent pair of resonators to reduce capacitive coupling between the adjacent pair of resonators.

[0008] According to yet another embodiment of the present disclosure, an RF filter incorporating FDC includes a filter housing and a plurality of resonators configured in a cascade arrangement in the filter housing. The plurality of resonators are coupled using a combination of capacitive coupling and inductive coupling to realize the FDC. The inductive coupling is provided by one or more conductive ridges extending vertically from a bottom of the filter housing, each of the conductive ridges electrically connecting a pair of adjacent resonators of the plurality of resonators. The capacitive coupling is provided by one or more conductive probes configured between the pair of adjacent resonators.

[0009] In accordance with an embodiment of the present disclosure, an RF filter includes a filter housing, a plurality of hybridly coupled resonators configured in a cascade arrangement in thefilter housing, a three-dimensional structure for providing inductive coupling in the RF filter. Each of the hybridly coupled resonators includes a conductive support structure extending vertically from a base of the filter housing and a folded resonator head supported on and electrically connected to the conductive support structure. The three-dimensional structure includes at least one conductive ridge extending vertically from the base of the filter housing between and electrically connected to the conductive support structures of adjacent resonators among the hybridly coupled resonators. The folded resonator head of each of the hybridly coupled resonators is configured to provide capacitive coupling in the RF filter.

[0010] In accordance with an embodiment of the present disclosure, an RF filter includes a filter housing and a plurality of hybridly coupled resonators, each of the hybridly coupled resonators including a conductive support structure extending vertically from a base of the filter housing and a folded resonator head supported on and electrically connected to the conductive support structure. At least one cross-coupling between non-adjacent resonators among the hybridly coupled resonators comprises an FDC, the FDC being configured as a parallel inductor-capacitor (LC) circuit. A capacitance of the FDC is configured as a function of a proximity of a pair of adjacent folded resonator heads among the respective folded resonator heads of the plurality of hybridly coupled resonators.

[0011] In accordance with an embodiment of the present disclosure, an RF filter incorporating frequency-dependent coupling includes a plurality of resonators, each of the plurality of resonators including a folded resonator head and a conductive support structure supporting the folded resonator head. The RF filter further includes at least one conductive ridge extending vertically between and electrically connected to the conductive support structures of adjacent resonators of the plurality of resonators. An inductance in the FDC is provided by the at least one conductive ridge, and a capacitance in the FDC is provided by capacitive coupling between adjacent folded resonator heads among the plurality of resonators.

[0012] Aspects of the present inventive concept can provide substantial beneficial technical effects. By way of example only and without limitation, techniques according to embodiments of the present disclosure may provide one or more of the following advantages, among other benefits:reduces the size of RF filters through its FDC realization using a cascade of hybridly coupled resonators;• reduces variations in assembly and manufacturing tolerances;• increases the extent to which post-manufacturing tuning can be performed to compensate for variations in tolerances due to assembly and / or manufacturing processes; increases the number of transmission zeros that can be accommodated for a given number of resonators, thereby resulting in a lighter and more compact filter by reducing the total number of resonators required; reduces the overall cost of manufacturing the RF filter; allows resonator reuse for all or nearly all cavities of the RF filter.

[0013] These and other features and advantages of the present inventive concept will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are included to provide a further understanding of the inventive concept and are incorporated in and constitute a part of this application, are presented by way of example only and without limitation, wherein like reference numerals (when used) indicate corresponding elements throughout the several views, and wherein:

[0015] FIGS. 1A - 1C are schematic plan views depicting at least a portion of an example inline filter realized using a cascade of hybridly coupled resonators, according to one or more embodiments;

[0016] FIG. 2 is a schematic top plan view depicting the illustrative in-line filter shown in FIGS. 1A - 1C with its tuning cover in place, according to one or more embodiments;

[0017] FIGS. 3A - 3C are schematic top plan and perspective views depicting an illustrative inline filter with its tuning cover removed to expose the printed circuit board and interior filter housing, according to one or more embodiments;

[0018] FIG. 4 is a graph depicting illustrative frequency response characteristics for the example in-line filter shown in FIGS. 3A - 3C, according to one or more embodiments;

[0019] FIG. 5 is a schematic diagram depicting an example cross-coupled filter including frequency dependent couplings configured into a single triplet topology, according to one or more embodiments;

[0020] FIGS. 6A and 6B are schematic top plan and perspective views, respectively, depicting a physical realization of the example single triplet filter of FIG. 5 configured to generate two transmission zeros above the filter passband, according to one or more embodiments;

[0021] FIG. 7 is a graph depicting frequency response characteristics of the illustrative filter shown in FIGS. 6A and 6B, according to one or more embodiments;

[0022] FIGS. 8A and 8B are schematic top plan and perspective views, respectively, depicting a physical realization of an example single triplet filter configured to generate two transmission zeros below the filter passband, according to one or more embodiments;

[0023] FIG. 9 is a graph depicting frequency response characteristics of the illustrative filter shown in FIGS. 8A and 8B, according to one or more embodiments;

[0024] FIGS. 10A and 10B are schematic top plan and perspective views, respectively, depicting a physical realization of an example single triplet filter configured to generate a transmission zero below the filter passband and a transmission zero above the filter passband, according to one or more embodiments;

[0025] FIG. 11 is a graph depicting frequency response characteristics of the illustrative filter shown in FIGS. 10A and 10B, according to one or more embodiments;

[0026] FIGS. 12A - 12C are schematic top plan and perspective views depicting a physical realization of an example cross-coupled filter utilizing a cascaded triplet topology including five resonators and configured to generate four transmission zeros, according to one or more embodiments;

[0027] FIGS. 13A and 13B are graphs depicting simulated frequency response and insertion loss (IL) characteristics, respectively, of the illustrative filter shown in FIGS. 12A - 12C, according to embodiments of the inventive concept;

[0028] FIGS. 14A - 14E conceptually depict some example fundamental cross-coupled fdter topologies in which aspects of the inventive concept may be used, according to one or more embodiments of the inventive concept;

[0029] FIG. 15A is a schematic top perspective view depicting an illustrative folded resonator head, according to one or more embodiments of the inventive concept;

[0030] FIG. 15B is a schematic cross-sectional view depicting the illustrative folded resonator head shown in FIG. 15A taken along line A -A';

[0031] FIG. 16 is a schematic top perspective view depicting a pair of adjacent resonators employing folded resonator heads for implementing a frequency variable coupling, according to one or more embodiments of the inventive concept;

[0032] FIGS. 17A and 17B are top plan and perspective views, respectively, depicting a physical realization of an illustrative filter configured in a single triplet topology, according to one or more embodiments of the inventive concept;

[0033] FIG. 18 is a graph depicting frequency response characteristics of the illustrative triplet filter shown in FIGS. 17A and 17B, according to one or more embodiments of the inventive concept;

[0034] FIGS. 19A and 19B are top plan and perspective views, respectively, depicting a physical realization of an illustrative filter configured in a single triplet topology, according to another embodiment of the inventive concept;

[0035] FIG. 20 is a graph depicting frequency response characteristics of the illustrative triplet filter shown in FIGS. 19A and 19B, according to one or more embodiments of the inventive concept;

[0036] FIG. 21 is a schematic diagram depicting an illustrative cross-coupled filter including frequency dependent couplings configured into a single quadruplet topology, according to one or more embodiments of the inventive concept;

[0037] FIGS. 22A and 22B are top plan and perspective views, respectively, depicting a physical realization of the illustrative filter shown in FIG. 21, according to one or more embodiments of the inventive concept;

[0038] FIG. 23 is a graph depicting frequency response characteristics of the illustrative single quadruplet filter shown in FIGS. 22A and 22B, according to one or more embodiments of the inventive concept;

[0039] FIG. 24 is a schematic diagram depicting an illustrative filter including two mixed couplings, according to one or more embodiments of the inventive concept;

[0040] FIG. 25 is a top plan view depicting at least a portion of a physical realization of the illustrative filter shown in FIG. 24, according to one or more embodiments of the inventive concept;

[0041] FIG. 26 is a graph illustrating simulated frequency response characteristics of the illustrative filter shown in FIG. 25 compared to simulated frequency response characteristics of a classical quadruplet filter topology, according to one or more embodiments of the inventive concept;

[0042] FIG. 27 is a top plan view depicting at least a portion of an illustrative filter, according to another embodiment of the inventive concept;

[0043] FIG. 28 is a graph illustrating simulated frequency response characteristics of the illustrative filter shown in FIG. 27 compared to simulated frequency response characteristics of a classical quadruplet filter topology, according to one or more embodiments of the inventive concept;

[0044] FIG. 29A is a top plan view depicting at least a portion of an illustrative filter, according to embodiments of the inventive concept;

[0045] FIG. 29B is an enlarged view of a region A of the filter shown in FIG. 29A, with a resonator head of the common resonator removed and including isolation ridges, according to one or more embodiments of the inventive concept;

[0046] FIG. 30 is a graph depicting frequency response of the filter of FIG. 29B before introducing corrections for spurious coupling, according to one or more embodiments of the inventive concept; and

[0047] FIGS. 31A and 3 IB are graphs depicting insertion loss of a filter incorporating frequency-dependent coupling, according to one or more embodiments of the inventive concept.

[0048] It is to be appreciated that elements in the figures may be illustrated for simplicity and clarity. Common but well-understood elements that may be useful or necessary in a commercially feasible embodiment are not necessarily shown in order to facilitate a less hindered view of the illustrated embodiments.DETAILED DESCRIPTION

[0049] Principles of the present inventive concept, as manifested in one or more embodiments thereof, may be described herein in the context of frequency-dependent couplings (FDCs) for RF fdters and printed circuit board (PCB) filters which may be used in antenna filter units (AFUs), base station antennas and the like. Additionally, a “folded mushroom” resonator head design according to embodiments of the inventive concept, which may provide capacitive coupling between adjacent (i.e., proximate) resonators in place of or in addition to PCB metallization, may be well-suited for implementation in tower-mounted amplifiers (TMAs). It is to be appreciated, however, that embodiments of the present invention are not limited to the specific devices, systems and / or methods illustratively shown and described herein. Rather, it will become apparent to those skilled in the art given the teachings herein that numerous modifications to the embodiments shown are contemplated and are within the scope of the present inventive concept. That is, no limitations with respect to the embodiments shown and described herein are intended or should be inferred.

[0050] It should be understood that the terms used herein are only used to describe specific embodiments, and are not intended to limit the scope of the present disclosure. All terms used herein (including technical terms and scientific terms) have meanings normally understood by those skilled in the art unless otherwise defined. For brevity and / or clarity, well-known functions or structures may not be described in detail herein.

[0051] As used herein, when an element is said to be “on” another element, “attached” to another element, “connected” to another element, “coupled” to another element, or “in contact with” another element, etc., the element may be directly on another element, attached to another element, connected to another element, coupled to another element, or in contact with another element, or an intermediate element may be present. In contrast, if an element is described as “directly on” another element, “directly attached” to another element, “directly connected” toanother element, “directly coupled” to another element or “directly in contact with” another element, there will be no intermediate elements. As used herein, when one feature is arranged “adjacent” to another feature, it may mean that one feature has a part overlapping with the adjacent feature or a part located above or below the adjacent feature (i.e., vertically adjacent), or it may mean that the one feature is arranged to the side of the adjacent feature (i.e., laterally adjacent).

[0052] In this specification, elements, nodes or features that are “coupled” or “connected” together may be described. Unless explicitly stated otherwise, “coupled” means that one element / node / feature can be mechanically, electrically, logically or otherwise connected with another element / node / feature in a direct or indirect manner to allow interaction, even though the two features may not be directly connected. That is, the term “coupled” or “connected” is intended to comprise direct and indirect physical and / or electrical connection of components or other features, including connection using one or a plurality of intermediate components.

[0053] As used herein, spatial relationship terms such as, for example, “upper,” “lower,” “left,” “right,” “front,” “back,” “high,” “low,” and the like, may be used to describe the relationship between one feature and another in the drawings. It should be appreciated, however, that in addition to the orientations shown in the attached drawings, the terms expressing spatial relations also contemplate different orientations of a device in use or operation. For example, when a device shown in the attached drawings is rotated reversely (i.e., 180 degrees), the features originally described as being “below” other features may now be described as being “above” the other features.” The device may also be oriented by other means (e.g., rotated by 90 degrees, 45 degrees, or at other orientations), and at this time, a relative spatial relation will be explained accordingly.

[0054] As may be used herein, the term “exemplary” means “serving as an example, instance or explanation,” not as a “model” to be accurately copied. Therefore, any realization approach described as being “exemplary” herein may not necessarily be interpreted as being preferable or advantageous over other realization approaches. Furthermore, the present disclosure is not limited by any expressed or implied theory given in the above technical field, background art, summary of the invention or embodiments.

[0055] As may be used herein, the term “basically” (or like terms such as “substantially”) is intended to encompass any minor changes caused by design or manufacturing defects, device or component tolerances, environmental influences, and / or other factors. The term “basically” also allows for the divergence from the perfect or ideal situation due to parasitic effects, noise, and other practical considerations that may be present in the actual realization.

[0056] In addition, for reference purposes only, ordinal terms such as, for example, “first,” “second,” and similar terms, as may be used herein, are not intended to be limiting. Unless the context clearly indicates otherwise, the terms “first,” “second,” and other such words involving structures or elements are generally intended to distinguish one structure or element from another structure or element and are not intended to imply a particular sequence or order.

[0057] It should also be understood that when the term “comprise” or “include” is used herein, it is intended to indicate the presence of the specified feature, entirety, step, operation, unit and / or component, but does not exclude the presence or addition of one or a plurality of other features, steps, operations, units and / or components, or combinations thereof.

[0058] With an increasing demand for miniaturization in communications systems (e.g., fifthgeneration (5G) communications), RF filters that may be used in AFUs are often fixed into a grid pattern. In a grid configuration, the realization of cross-couplings between nonadjacent resonators, to generate transmission zeros in the filter response is difficult, if not impossible. T o overcome these challenges, an in-line realization of transmission zeros is preferred.Transmission zeros can be realized in an in-line structure by means of FDCs. Factors affecting the realization of transmission zeros in an in-line structure using FDCs may include, but are not limited to, (i) the coupling coefficient at a center frequency of the filter and (ii) a slope of the coupling coefficient.

[0059] To control both the coupling coefficient at the center frequency of the filter and the slope of the coupling coefficient, a mix of both electric and magnetic coupling can be employed. In accordance with one or more embodiments of the inventive concept, transmission zeros in an in-line structure may be realized using a combination of a three-dimensional (3D) structure for providing magnetic coupling (i.e., inductive or positive coupling) and a planar structure for providing electric coupling (i.e., capacitive or negative coupling). In some embodiments, the 3D structure may include one or more ridges and / or windows in a housing (i.e., enclosure) of thefilter to provide inductive coupling for controlling magnetic coupling in the filter. Capacitive coupling to provide electric coupling in the filter may be achieved using a planar structure including probes or other connections which may be realized as a metallization layer in a printed circuit board (PCB). The capacitive coupling on the PCB can be realized, for example, using conductive (e.g., metal) probes (i.e., conductive traces) between resonators or by using a prescribed configuration of resonator heads (e.g., resonator heads with protrusions) that allow stronger electric coupling between adjacent resonators. Because filter elements, such as resonator heads and conductive probes, may be implemented with a single-piece PCB, variation during manufacturing can be reduced and assembly tolerances can be highly-controlled.

[0060] Metallization on the PCB may be provided on one side or on both sides (e.g., top and bottom surfaces) of the PCB; additional metallization layers may also be provided within the PCB, such as stacked in a vertical dimension with insulating layers between adjacent metallization layers. When multiple metallization layers are provided within and / or on the PCB, conductive vias may be used to electrically connect the metallization layers, which may reduce losses in the filter structure. By using a single PCB having one or more metallization layers to realize FDCs, the filter is able to achieve improved tolerance robustness as well as increased peak power handling capability. Compared to classical filter arrangements that can only generate M2 transmission zeros using N resonators, where N is an integer greater than one representing the number of resonators in the filter, the number of transmission zeros that can be generated using the filter arrangements according to embodiments of the inventive concept is N- 1, which represents a significant improvement over conventional approaches. This means that the total number of resonators in the filter can be reduced, compared to conventional filter designs, to achieve the same or similar frequency response characteristics, resulting in a lighter, less expensive and more compact filter design.

[0061] FIGS. 1A - 1C are schematic diagrams depicting at least a portion of an example in-line filter 100 realized using a cascade of hybridly-coupled resonators, according to one or more embodiments. FIG. 1A is a plan view depicting a top surface 102 of a single PCB 101 of the filter 100, FIG. IB is a pian view depicting a bottom surface 104 of the PCB 101, opposite the top surface 102, and FIG. 1C is a top plan view depicting a housing 106 (i.e., enclosure) of the filter 100 with its cover removed to illustrate an interior of the housing 106. The filter housing 106 is typically connected to ground and may be considered a ground plane of the filter 100.

[0062] By way of example only and without limitation or loss of generality, the illustrative filter 100 may be designed having a passband in a frequency range of about 3.4 - 3.8 gigahertz (GHz) and having two transmission zeros, one above and one below the passband, for example at about 2.930 GHz and 4.365 GHz, respectively. It is to be appreciated, however, that embodiments are not limited to this or any particular frequency of operation, or number or frequency of the transmission zeros.

[0063] Referring to FIGS. 1A and IB, the filter 100 includes a plurality of resonators, planar portions of which are realized as first through fourth conductive structures (i.e., resonator heads) 108, 110, 112 and 114, respectively, of a first patterned metallization layer provided on the top surface 102 of the PCB 101, and may include corresponding fifth through eighth conductive structures 116, 118, 120 and 122, respectively, of a second patterned metallization layer provided on the bottom surface 104 of the PCB 101. The first through fourth conductive structures 108, 110, 112 and 114 on the top surface 102 of the PCB 101 are arranged in the x-direction, parallel to the top surface 102 of the PCB 101. Similarly, the fifth through eighth conductive structures 116, 118, 120 and 122 on the bottom surface 104 of the PCB 101 are arranged along the x- direction, with each of the conductive structures 116, 118, 120 and 122 at least partially overlapping a corresponding one of the conductive structures 108, 110, 112 and 114 in the z- direction (i.e., vertical direction) perpendicular to the top surface 102 of the PCB 101.

[0064] In some embodiments, only one surface of the PCB 101 is provided with a patterned metallization layer. In other embodiments, a multi-layer PCB, including a plurality of stacked metallization layers (e.g., copper or copper foil) with electrically insulating material disposed between adjacent metallization layers, may be used to form the resonators heads. When the PCB 101 includes multiple metallization layers, conductive vias (e.g., through-holes, etc.) 124 may be provided, each conductive via 124 extending in a vertical direction (i.e., z-direction) through the PCB 101 to electrically connect one of the conductive structures 108, 110, 112, 114 disposed on the top surface 102 of the PCB 101 with a corresponding one of the conductive structures 116, 118, 120, 122 disposed on the bottom surface 104 of the PCB 101. Thus, the first conductive structure 108 on the top surface 102 of the PCB 101 may be electrically connected to the fifth conductive structure 116 on the bottom surface 104 of the PCB 101 by way of a corresponding one of the conductive vias 124. Likewise, respective conductive vias 124 may be used to electrically connect the second conductive structure 110 to the sixth conductivestructure 118, the third conductive structure 112 to the seventh conductive structure 120, and the fourth conductive structure 114 to the eighth conductive structure 122.

[0065] Each of the conductive structures 108, 110, 112 and 114 provided on the top surface 102 of the PCB 101 and each of the conductive structures 116, 118, 120 and 122 provided on the bottom surface 104 of the PCB 101 forming the plurality of resonator heads may be configured to be generally circular in shape, although embodiments are not limited thereto. The dimensions of the conductive structures 108, 110, 112, 114, 116, 118, 120 and 122 need not be the same. For example, in the illustrative embodiment shown in FIG. 1A, the first conductive structure 108 may be larger in diameter relative to the second conductive structure 110, and the second conductive structure 110 may be larger in diameter than the third conductive structure 112. Furthermore, the dimensions of a given conductive structure on the top surface 102 of the PCB 101 need not be equal to the dimensions of a corresponding conductive structure on the bottom surface 104 of the PCB 101. The resonant frequency of each of the resonators in the filter 100 can be controlled as a function of the dimensions (e.g., diameter) of the respective conductive structures 108, 110, 112, 114, 116, 118, 120, 122.

[0066] In one or more embodiments, the RF filter 100 may generate transmission zeros by means of one or more probes realized as electrically conductive traces on one or both surfaces of the PCB 101. Negative (i.e., capacitive) coupling in the filter 100 is provided using the probes. Referring to FIG. 1A, the top surface 102 of the PCB 101 may include a first probe 126 between the first and second conductive structures 108 and 110 and a second probe 128 between the third and fourth conductive structures 112 and 114, with each of the probes 126, 128 being electrically separated from their corresponding conductive structures 108, 110, 112, 114. Each of the probes 126, 128 may be configured as rectangular metal traces between adjacent conductive structures, although embodiments are not limited to any specific shape of the probes. Moreover, the respective probes 126, 128 may not necessarily have the same dimensions relative to one another.

[0067] Referring to FIG. IB, each of the conductive structures 116 through 122 forming at least a portion of the respective resonators may be configured having a protrusion. Specifically, the fifth conductive structure 116 is configured having a first protrusion portion 130 extending in the x-direction, and the adjacent sixth conductive structure 118 is configured having a secondprotrusion portion 132 extending in the x-dircction to face the first protrusion portion 130. The first and second protrusion portions 130, 132 are spaced apart in the x-direction so as to be electrically separated from one another, with a distance between the first and second protrusion portions 130, 132 being configured to provide a prescribed capacitive coupling associated therewith. Likewise, the seventh conductive structure 120 is configured having a third protrusion portion 134 extending in the x-dircction, and the eighth conductive structure 122 is configured having a fourth protrusion portion 136 extending in the x-direction to face the third protrusion portion 134. The third and fourth protrusion portions 134, 136 are spaced apart in the x-direction so as to be electrically separated from one another, with a distance between the third and fourth protrusion portions 134, 136 being configured to have a prescribed capacitive coupling associated therewith.

[0068] In some embodiments, the first probe 126 on the top surface 102 of the PCB 101 may be configured to at least partially vertically overlap the first and second protrusion portions 130, 132 on the bottom surface 102 of the PCB 101. Similarly, the second probe 128 on the top surface 102 of the PCB 101 may be configured to at least partially vertically overlap the third and fourth protrusion portions 134, 136 on the bottom surface 102 of the PCB 101. In this manner, a capacitive plate coupling may be provided between the first probe 126 and the respective first and second protrusion portions 130, 132, and between the second probe 128 and the respective third and fourth protrusion portions 134, 136. This capacitive plate coupling is in addition to capacitive edge coupling provided by the respective protrusion portions 130, 132, 134, 136.

[0069] As previously stated, transmission zeros can be realized in-line as a combination of electric and magnetic couplings. The electric couplings may be generated using the probes on one or both surfaces of the PCB 101, as shown in FIGS. 1A and IB. Magnetic couplings may be generated using ridges and / or windows in the filter housing. For example, referring to FIG. 1C, the filter housing 106 may include a plurality of conductive pedestals 138, 140, 142 and 144 and one or more conductive ridges 146, 148 and 150 integrated into the filer housing 106.

[0070] Each of the conductive pedestals 138, 140, 142, 144 may be formed as cylindrical metal structures (i.e., conductive rods) extending upwardly (i.e., in the z-direction) from a bottom (i.e., floor) of the housing 106, although embodiments are not limited to any particular shape, sizeand / or configuration of the conductive pedestals. For example, in some embodiments, one or more of the conductive pedestals 138, 140, 142, 144 may be cuboid in shape (i.e., having a square or rectangular horizontal cross section). The conductive pedestals 138, 140, 142, 144 may be formed integrally with the filter housing 106, such as, for example, using a die casting or computer numerical control (CNC) machining process, although embodiments are not limited thereto. In one or more embodiments, each of at least a subset of the conductive pedestals 138, 140, 142, 144 may be formed having a constant diameter (or horizontal width). In other embodiments, each of at least a subset of the conductive pedestals 138, 140, 142, 144 may be configured having a diameter that varies with a vertical distance from the bottom of the filter housing 106. In one or more embodiments, each of at least a subset of the conductive pedestals 138, 140, 142, 144 may comprise aluminum, although other materials for forming the conductive pedestals 138, 140, 142, 144 are similarly contemplated (e.g., copper).

[0071] Each of the resonators in the filter 100 comprises a three-dimensional portion, which may be implemented by a corresponding one of the conductive pedestals 138, 140, 142, 144, providing magnetic (i.e., inductive) coupling, and a planar portion (i.e., resonator head), which may be implemented by a corresponding one of the first through fourth conductive structures 108, 110, 112, 114 and, in some embodiments, a corresponding one of the fifth through eighth conductive structures 116, 118, 120, 122, providing electric (i.e., capacitive) coupling. Each of the conductive pedestals 138, 140, 142, 144 is electrically connected to a corresponding one of the resonator heads, implemented using conductive structures 108 through 122. A top portion of each of the conductive pedestals 138, 140, 142, 144 may be configured in the shape of a pin which is adapted to be received into a corresponding hole in the PCB 101, such as through a center of one or more conductive structures 108 through 122. A given one of the conductive pedestals may be electrically connected to a corresponding one of the conductive structures by press-fitting the PCB 101 onto the pin on the top portion of the conductive pedestal and then soldering the pin to the conductive structure on the PCB 101. The conductive pedestals 138, 140, 142, 144 may be plated with silver or copper, for example, to facilitate solderability of the conductive pedestals to the respective conductive structures 108 through 122.

[0072] Each of the ridges 146, 148, 150 electrically connects a pair of adjacent conductive pedestals and may be configured having a longitudinal axis along the x-direction and extending upwardly from the bottom of the housing 106. More particularly, a first ridge 146 mayelectrically connect first and second conductive pedestals 138, 140, a second ridge 148 may electrically connect second and third conductive pedestals 140, 142, and a third ridge 150 may electrically connect third and fourth conductive pedestals 142, 144. A vertical height of the ridges 146, 148, 150 above the bottom of the filter housing 106 may be adjusted to provide a prescribed amount of magnetic coupling; the higher the ridges, the more the inductive coupling. In some embodiments, the ridges may be configured to be in a non-contact relationship with respect to the resonators, rather than being electrically connected to the resonators, although the effect is minor with respect to the ridges directly connected to the conductive pedestals. When the ridges are in proximity to the resonators, a similar effect may be produced, with the amount of magnetic coupling being a function of the proximity between the ridges and the resonators.

[0073] The respective heights of the ridges 146, 148, 140 need not be the same relative to one another. In one or more embodiments, each of the ridges 146, 148, 150 may have a height above the bottom of the filter housing 106 that is less than a height of a corresponding one of the conductive pedestals 138, 140, 142, 144, so that the ridges do not contact the conductive structures 108 through 122 attached to the top of the conductive pedestals. The ridges 146, 148, 150, like the conductive pedestals 138, 140, 142, 144, may be formed, for example, using a die casting or CNC machining process, although embodiments are not limited thereto. As previously stated, transmission zeros of the filter 100 may be realized in-line, in one or more embodiments, as a hybrid mix of electric couplings, generated by means of the probes 126, 128 and protrusion portions 130, 132, 134, 136 on one or both sides of the PCB 101, and magnetic couplings, generated by means of the conductive ridges 146, 148, 150 integrated into the filter housing 106 (see FIGS. 1A - 1C).

[0074] When the filter design requires positive coupling at center frequency and a negative slope, this can be realized by means of a dominance of magnetic coupling (by appropriately configuring the ridges 146, 148, 150 on the filter housing 106) and using the electric coupling (by appropriately configuring the probes 126, 128 and / or protrusion portions 130, 132, 134, 136 on one or both sides of the PCB 101) to control the slope. When the filter design requires negative coupling at center frequency and positive slope, this can be realized by means of a dominance of electric coupling (by appropriately configuring the probes 126, 128 and / or protrusion portions 130, 132, 134, 136 on one or both sides of the PCB 101) and using the magnetic coupling (by appropriately configuring the ridges 146, 148, 150 on the filter housing106) to control the slope. Although it may be difficult for the illustrative filter arrangement shown in FIGS. 1A - 1C to realize positive slope, the filter 100 may be configured in such a way as to have a negative slope by changing the sign of the couplings, as will be understood by those skilled in the art.

[0075] FIG. 2 is a top plan view depicting the illustrative in-line filter 100 shown in FIGS. 1A - 1C with its tuning cover in place, according to one or more embodiments. Referring to FIG.2, the in-line filter 100 includes a tuning cover 202 over the filter housing 106. The tuning cover 202, which may be formed of metal (e.g., thin sheet metal), may include a plurality of spirals 204, 206, 208 and 210, and / or a plurality of tabs 212, 214 and 216, each of the spirals and tabs formed as patterned cutouts in the tuning cover 202. Each of the spirals 204, 206, 208, 210 may be configured to at least partially overlap, in the vertical direction, a respective one of the conductive structures 108, 110, 112 and 114 forming a corresponding one of the resonator heads on the PCB 101 (see FIGS. 1A and IB). Likewise, each of the tabs 212, 214, 216 may be configured to at least partially overlap, in the vertical direction, a respective one of the probes 128, 128 and / or protrusion portions 130, 132, 134, 136 on the PCB 101 (see FIGS. 1A and IB). A distance between the tabs 212, 214, 216 and / or spirals 204, 206, 208, 210 and the underlying PCB (e.g., 101 in FIGS. 1A and IB) can be adjusted such as by bending the tabs and / or spirals inward (i.e., towards the PCB) or outward (i.e., away from the PCB) to tune the resonant frequency of the in-line filter 100 as needed, post- assembly; the tabs may tune the coupling between resonators in a similar manner as the spirals.

[0076] The in-line filter 100 further includes a first connector 218 disposed at a first end of the housing 106 and a second connector 220 disposed at a second end of the housing 106, longitudinally opposite to the first end, each of the first and second connectors 218, 220 serving as input / output ports of the in-line filter 100. Each of the first and second connectors 218, 220 may be coaxial connectors, although embodiments are not limited thereto.

[0077] By way of example only and without limitation, FIGS. 3 A - 3C are top plan and perspective views depicting an illustrative in-line filter 300 with its tuning cover removed to expose the PCB and interior filter housing, according to one or more embodiments. FIG. 3 A is a top plan view conceptually depicting the different couplings between resonators provided by the in-line filter 300; FIG. 3B is a top plan view conceptually depicting the PCB as partiallytransparent to show the ridges integrated with the filter housing; and FIG. 3C is a perspective view of the overall in-line filter 300.

[0078] Referring to FIGS. 3A - 3C, the in-line filter 300 includes a filter housing 302 having a bottom and sidewalls extending upwardly from the bottom to form an interior space for containing elements of the filter 300. The filter housing 302 may be cuboid in shape, although embodiments are not limited thereto. The filter 300 further includes a PCB 304 on which planar portions of a plurality of resonators are disposed along a longitudinal axis thereof. Specifically, the PCB 304, in one or more embodiments, includes a first conductive structure 306 forming a planar portion of a first resonator 308, a second conductive structure 310 forming a planar portion of a second resonator 312, a third conductive structure 314 forming a planar portion of a third resonator 316, and a fourth conductive structure 318 forming a planar portion of a fourth resonator 320. The PCB 304 further includes a first probe 322 between the first and second conductive structures 306, 310, and a second probe 324 between the third and fourth conductive structures 314, 318. As previously explained in conjunction with FIG. 1A, the probes 322, 324 may be used to provide electric (i.e., capacitive) couplings for generating transmission zeros of the filter 300.

[0079] The in-line filter 300 further includes a plurality of conductive pedestals 326, 328, 330 and 332. A first conductive pedestal 326 forms a 3D portion of the first resonator 308, a second conductive pedestal 328 forms a 3D portion of the second resonator 312, a third conductive pedestal 330 forms a 3D portion of the third resonator 316, and a fourth conductive pedestal 332 forms a 3D portion of the fourth resonator 320. Each of the conductive pedestals 326, 328, 330, 332 may be formed as cylindrical metal structures extending upwardly from the bottom (i.e., floor) of the filter housing 302, although embodiments are not limited thereto. The filter housing 302 may also include a plurality of ridges 334, 336 and 338 providing magnetic (i.e., inductive) couplings for generating transmission zeros of the filter 300. Each of the ridges 334, 336, 338 electrically connects a pair of adjacent conductive pedestals and may be configured having a longitudinal axis parallel to a direction in which the resonators are arranged and extending upwardly from the bottom of the filter housing 302. More particularly, a first ridge 334 may electrically connect the first and second conductive pedestals 326, 328 of the first and second resonators 308, 312, respectively, a second ridge 336 may electrically connect the second and third conductive pedestals 328, 330 of the second and third resonators, respectively, and athird ridge 338 may electrically connect the third and fourth conductive pedestals 330, 332 of the third and fourth resonators 316, 320, respectively.

[0080] The in-line filter 300 further includes a first connector 340 disposed at a first end of the filter housing 302 and a second connector 342 disposed at a second end of the filter housing 302, longitudinally opposite to the first end, each of the first and second connectors 340, 342 serving as input / output ports of the in-line filter 300. Each of the first and second connectors 340, 342 may be coaxial connectors, although embodiments are not limited thereto. A first port connection 344 may electrically connect the first resonator 308 to the first connector 340, and a second port connection 346 may electrically connect the fourth resonator 320 to the second connector 342.

[0081] In the illustrative in-line filter 300, a first coupling between the first and second resonators 308, 312 is configured to be positive (i.e., inductive) with a negative slope. To obtain a positive coupling between adjacent first and second resonators 308, 312 having both inductive and capacitive couplings, the first ridge 334 may be configured to provide greater inductive coupling relative to the amount of capacitive coupling provided by the first probe 322. A second coupling between the third and fourth resonators 316, 320 is configured to be negative (i.e., capacitive) with a negative slope. To obtain a negative coupling between adjacent third and fourth resonators 316, 320 having both inductive and capacitive couplings, the second probe 324 may be configured to provide greater capacitive coupling relative to the amount of coupling provided by the third ridge 338. A third coupling between adjacent second and third resonators 312, 316 will be positive (i.e., inductive) provided by the second ridge 336.

[0082] FIG. 4 is a graph depicting illustrative frequency response characteristics for the example in-line filter 300 shown in FIGS. 3A - 3C. Referring to FIG. 4, the filer may be configured having a passband 402 of about 3.4 - 3.8 GHz. The second coupling between adjacent third and fourth resonators (316, 320 in FIG. 3 A) may be configured to realize a transmission zero 404 at about 2.930 GHz, which is below the passband, and the first coupling between the first and second resonators (308, 312 in FIG. 3A) may be configured to realize a transmission zero 406 at about 4.365 GHz, which is above the passband, although embodiments are not limited thereto.

[0083] The filter arrangements described in conjunction with FIGS. 1A through 4 are in-line filters, wherein the plurality of resonators are disposed along a common center axis that is parallel to a longitudinal axis of the filter housing. Aspects of the inventive concept may be similarly employed to realize frequency dependent couplings incorporating a cascade of hybridly coupled resonators into other topologies, such as single and cascaded triplet filter topologies.

[0084] FIG. 5 is a circuit diagram depicting an example filter 500 including frequency dependent couplings configured into a cross-coupled arrangement, according to one or more embodiments. FIGS. 6A and 6B are top plan and perspective views, respectively, depicting a physical realization of the example filter 500 of FIG. 5, according to one or more embodiments. Referring to FIGS. 5, 6A and 6B, the filter 500 includes a first resonator 502, a second resonator 504 and a third resonator 506 configured into a single triplet topology. In a cross-coupled filter topology, the resonators are not arranged in a straight line (as in an in-line filter topology), but can have coupling between resonators that are not adjacent along an electrical path of the filter. For example, in the single triplet topology of the filter 500, the resonators 502, 504, 506 may be disposed at respective vertices of a triangle. The filter 500 further includes a first input / output (source) port 508 coupled to the first resonator 502 and a second input / output (load) port 510 coupled to the third resonator 506.

[0085] As shown in FIG. 5, the filter 500 is configured having a first main coupling between the first resonator 502 and the second resonator 504, and a second main coupling between the second resonator 504 and the third resonator 506. The first and second main couplings may be considered direct couplings (i.e., either only inductive or only capacitive). The resonant frequency of the first, second and third resonators 502, 504 and 506, respectively, may be configured to control a center frequency of the filter 500, while a coupling strength of the first and second main couplings may be configured to control a width of the passband of the filter 500.

[0086] The filter 500 further comprises an FDC 512 included in a cross-coupling between the first resonator 502 and the third resonator 506. The FDC 512 may be a mixed coupling, conceptually depicted as a capacitor, C, and an inductor, L, connected as a parallel inductorcapacitor (LC) circuit between the first and third resonators 502, 506. By including the FDC512 into the cross-coupling, two transmission zeros can be generated using only three resonators, rather than generating only one transmission zero using a classical triplet filter topology.

[0087] As shown in FIGS. 6A and 6B, the first resonator 502 comprises a planar portion providing electric (i.e., capacitive) coupling, realized as a first conductive structure (i.e., first resonator head) 514, and a 3D portion providing magnetic (i.e., inductive) coupling, realized as a first conductive pedestal 516 electrically connected to the first conductive structure 514. Similarly, the second resonator 504 comprises a planar portion providing electric coupling, realized as a second conductive structure (i.e., second resonator head) 518, and a 3D portion providing magnetic coupling, realized as a second conductive pedestal 520 electrically connected to the second conductive structure 518. The third resonator 506 comprises a planar portion providing electric coupling, realized as a third conductive structure (i.e., third resonator head) 522, and a 3D portion providing magnetic coupling, realized as a third conductive pedestal 524 electrically connected to the third conductive structure 522. The first, second and third conductive structures 514, 518 and 522, respectively, may be formed as one or more metallization layers on a PCB 526, although embodiments are not limited thereto.

[0088] The PCB 526 may include a capacitive coupling 528 between the first and third conductive structures 514 and 522, respectively. The capacitive coupling 528 may be implemented using at least one conductive probe 530 formed as part of the patterned metallization layer on at least a bottom surface of the PCB 526, in a manner consistent with the probes (e.g., 126 and 128) shown in FIG. 1A. The conductive probe 530 may be configured to at least partially overlap the conductive structures 514 and 522 (provided on the top surface of the PCB 526) of the first and third resonators 502 and 506, respectively, in the vertical direction to thereby generate the capacitive coupling. In order to reduces losses introduced by the dielectric in the PCB 526, a conductive trace may be provided on the top surface of the PCB 526 that is electrically connected to the conductive probe 530 on the bottom surface of the PCB 526 using one or more conductive vias or through holes. In one or more embodiments, the capacitor C in the FDC 512 shown in FIG. 5 may be realized using the capacitive coupling 528.

[0089] The filter 500 further comprises a filter housing 532. The filter housing 532 includes a bottom (i.e., floor) and a plurality of sidewalls extending upwardly (i.e., in a vertical direction) from the bottom of the filter housing 532. In one or more embodiments, the sidewalls of thefilter housing 532 may be formed integrally with the bottom of the filter housing (e.g., using a die casting or CNC machining process). The sidewalls and bottom of the filter housing 532 are configured to define an interior space of the filter 500. The shape and dimensions of the filter housing 532 are not limited to the arrangement shown in FIGS. 6A and 6B. The filter housing 532 is typically connected to ground and may be considered a ground plane of the filter 500.The first through third resonators 502, 504, 506 and the PCB 526 are disposed within the interior space of the filter housing 532.

[0090] Each of the first through third conductive pedestals 516, 520, 524 may be formed as cylindrical metallic structures extending upwardly (i.e., in the vertical direction) from the bottom of the filter housing 532, although embodiments are not limited thereto. For example, in some embodiments, one or more of the conductive pedestals 516, 520, 524 may be formed as a cuboid structure or other structure having a polygonal cross section. In one or more embodiments, the first through third conductive pedestals 516, 520, 524 may be formed integrally with the bottom of the filter housing 532 (e.g., using a die casting process).

[0091] The filter 500 further includes one or more conductive ridges 534, 536 and 538 integrated into the filer housing 532. A first conductive ridge 534 is configured to extend upwardly from the bottom of the filter housing 523 and electrically connects conductive pedestals 516 and 524 of the adjacent first and third resonators 502 and 506, respectively. The first conductive ridge 534 may be used to implement inductive (i.e., magnetic) coupling. In one or more embodiments, the inductor L in the FDC 512 shown in FIG. 5 may be realized using the first conductive ridge 534.

[0092] Coupling between the first and second resonators 502, 504 and between the second and third resonators 504, 506 are based on proximity of the respective resonators to one another, which is primarily inductive (positive) coupling. Each of a second conductive ridge 544 and a third conductive ridge 546 may be configured to extend outwardly from a corresponding one of the sidewalls of the filter housing 532 and along the bottom of the filter housing 532. The second conductive ridge 544 may be disposed between the adjacent first and second resonators 502, 504 and the third conductive ridge 546 may be disposed between the adjacent second and third resonators 504, 506. Each of the second and third conductive ridges 544, 546 is configured to reduce (or block) magnetic (i.e., inductive) coupling between proximateresonators, and may therefore be referred to as an isolation structure or isolation wall, to reduce inductive coupling when the inductive coupling between a corresponding pair of adjacent resonators is too strong.

[0093] Specifically, the second conductive ridge 544 is configured to reduce magnetic coupling between the first and second resonators 502, 504 and the third conductive ridge 546 is configured to reduce magnetic coupling between the second and third resonators 504, 506. Unlike the conductive ridge 534 electrically connecting two adjacent resonator heads and providing inductive coupling, the conductive ridges (i.e., isolation structures) 544, 546 are connected to a ground plane of the filter 500 and are not directly connected to any of the resonators. The degree by which magnetic coupling is reduced between a corresponding adjacent pair of resonators may be controlled as a function of a distance by which the conductive ridge extends outwardly from the sidewall of the filter housing 532; that is, a proximity between the conductive ridge and the corresponding adjacent pair of resonators. The greater the distance by which a given one or the conductive ridges 544, 546 extends from the sidewall of the filter housing 532 (and thus the closer in proximity the conductive ridge is to the corresponding pair of resonators), the greater the reduction in magnetic coupling achieved.

[0094] The filter 500 may include a first transmission line 536 configured to electrically connect the first resonator 502 with a first input / output terminal 538, and a second transmission line 540 configured to electrically connect the third resonator 506 with a second input / output terminal 542. Each of the first and second transmission lines 536, 540 may be implemented as coaxial conductors extending between a given one of the resonators 502, 506 and a corresponding one of the input / output terminals 538, 542.

[0095] By way of example only and without limitation, the filter 500 may be configured having a passband from about 3.3 GHz - 3.5 GHz and two transmission zeros above the passband. The center frequency of the passband may be controlled as a function of the dimensions (e.g., diameter of resonator heads, etc.) of the resonators 502, 504, 506 and / or coupling between adjacent resonators. The frequency of each of the transmission zeros may be controlled as a function of the capacitance C and inductance L values forming the FDC 512 (FIG. 5) between the first and third resonators 502, 506. For example, the capacitance C may be increased or decreased by increasing or decreasing, respectively, the size of the conductivetrace on the bottom surface of the PCB 526 between the first and third conductive structures 514, 522. The inductance L may be increased or decreased by increasing or decreasing, respectively, the height of the first conductive ridge 534 above the bottom of the filter housing 532.

[0096] FIG. 7 is a graph depicting the frequency response characteristics of the illustrative filter 500 shown in FIGS. 6A and 6B, according to one or more embodiments. Referring to FIG. 7, the filter 500 has a passband 702 from about 3.3 GHz - 3.5 GHz, a first transmission zero 704 at about 3.7 GHz and a second transmission zero 706 at about 3.8 GHz.

[0097] In one or more embodiments, the filter 500 may be modified to generate two transmission zeros below the passband instead of above the passband. By way of example only and without limitation, FIGS. 8A and 8B are top plan and perspective views, respectively, depicting a physical realization of an example filter 800 configured to generate two transmission zeros below the filter passband, according to one or more embodiments. Referring to FIGS. 8A and 8B, the filter 800 may be formed in a manner consistent with the illustrative filter 500 shown in FIGS. 6A and 6B, except with different capacitance C and inductance L values in the FDC between the first and third resonators 502, 506. Specifically, in the filter 800, the size of the conductive probe 530, implemented as a conductive trace on the bottom surface of the PCB 526, has been increased, compared to the conductive probe 530 in the filter 500 (FIGS. 6A and 6B), to thereby increase the capacitance C, and a height of the first conductive ridge 534 above the bottom of the filter housing 532 has been increased, relative to the conductive ridge 534 in the filter 500, to thereby increase the inductance L. By increasing the capacitance and inductance of the FDC between the first and third resonators 502, 506, the two transmission zeros can be shifted down in frequency.

[0098] As previously stated in conjunction with FIGS. 6A and 6B, the proximity of a conductive ridge (i.e., isolation structure) extending outwardly from the sidewall of the filter housing 532 proximate to a pair of adjacent resonators can reduce magnetic (i.e., positive) coupling. In the filter 800, negative coupling at the center frequency and negative slope can be realized by having a dominance of electric coupling, which may be provided by conductive probes 530 on the PCB 526, and by using magnetic coupling to regulate the slope, which may be provided by changing the height of the first conductive ridge 534 that connects the conductivepedestals 516 and 524 of the adjacent first and third resonators 502 and 506, respectively. Compared to the second and third conductive ridges 544, 546 shown in FIGS. 6A and 6B, the second and third ridges 802, 803 in the filter 800 of FIGS. 8A and 8B may extend further from the sidewall of the filter housing 532 due primarily to the fact that the couplings between the first and second resonators 502, 504 and between the second and third resonators 504, 506 are lower, with respect to the illustrative filter 500 of FIGS. 6A and 6B, in order to shift the transmissions zeros to a lower frequency, below the passband of the filter 800.

[0099] FIG. 9 is a graph depicting the frequency response characteristics of the illustrative filter 800 shown in FIGS. 8A and 8B, according to one or more embodiments. Referring to FIG. 9, the filter 800 has a passband 902 from about 3.3 GHz - 3.5 GHz, a first transmission zero 904 at about 3.15 GHz and a second transmission zero 906 at about 3.2 GHz. The difference in frequency between the transmission zeros may be controlled as a function of the slope of the coupling coefficient between the first and third resonators 502, 506.

[0100] By way of example only and without limitation, FIGS. 10A and 10B are top plan and perspective views, respectively, depicting a physical realization of an example single triplet filter 1000 configured to generate a transmission zero below the filter passband and a transmission zero above the passband, according to one or more embodiments. Referring to FIGS. 10A and 10B, the filter 1000 includes a first resonator 502, a second resonator 504 and a third resonator 506 configured into a single triplet topology, in a manner consistent with the filter 500 shown in FIGS. 6A and 6B. The first resonator 502 comprises a planar portion (i.e., first resonator head) realized as the first conductive structure 514 and a 3D portion realized as the first conductive pedestal 516 supporting and electrically connected to the first conductive structure 514. The second resonator 504 comprises a planar portion (i.e., second resonator head) realized as the second conductive structure 518 and a 3D portion realized as the second conductive pedestal 520 supporting and electrically connected to the second conductive structure 518. The third resonator 506 comprises a planar portion (i.e., third resonator head) realized as the third conductive structure 522 and a 3D portion realized as the third conductive pedestal 524 supporting and electrically connected to the third conductive structure 522. The first, second and third conductive structures 514, 518 and 522, respectively, may be formed as patterned metallization layers on at least the top surface of the PCB 526, although embodiments are not limited thereto.

[0101] The filter further comprises a filter housing 532, which may be consistent with the filter housing 532 shown in FIGS. 6A and 6B. Sidewalls and a bottom of the filter housing 532 are configured to define an interior space of the filter 1000. The shape and dimensions of the filter housing 532 are not limited to the arrangement shown in FIGS. 10A and 10B. The filter housing 532 is typically connected to ground and may be considered a ground plane of the filter 1000. The first through third resonators 502, 504, 506 and the PCB 526 are disposed within the interior space of the filter housing 532.

[0102] In one or more embodiments, in order to generate transmission zeros above and below the passband, the filter 1000 comprises an FDC between the first and third resonators 502, 506, negative coupling between the first and second resonators 502, 504, and positive coupling between the second and third resonators 504, 506. The FDC is a mixed (i.e., hybrid) coupling including a first capacitive coupling 1002 and an inductive coupling. The first capacitive coupling 1002 may be realized as at least one conductive probe 1003, which may be provided as a conductive trace on the bottom surface of the PCB 526 and configured to at least partially overlap the first and third conductive structures 514 and 522, respectively, of the first and third resonators 502, 506 in the vertical direction. In one or more embodiments, the conductive probe 1003 may be a patterned metallization layer on the bottom surface of the PCB 526. In order to reduce dielectric loss introduced by the PCB 526, a conductive trace 1004 may be optionally provided on the top surface of the PCB 526 which is electrically connected to the conductive probe 1003 on the bottom surface of the PCB 526 using one or more conductive vias or through holes.

[0103] The inductive coupling in the FDC may be realized as a first conductive ridge 1006 extending upwardly (i.e., vertically) from the bottom of the filter housing 532 and electrically connecting the conductive pedestals 516 and 524 of the first and third resonators 502, 506. The first conductive ridge 1006 may be curved rather than straight (e.g., as in the illustrative filter 500 shown in FIGS. 6A and 6B). The curved configuration of the first conductive ridge 1006 serves to move the first ridge 1006 away from a conductive support post 1008 in the center of the triplex arrangement of resonators 502, 504, 506 to facilitate manufacturing of the filter 1000. Specifically, the curved configuration of the first conductive ridge 1006 may facilitate postmanufacturing tuning of the filter 1000 by bending a tab (e.g., 212, 214, 216 in a cover 202 shown in FIG. 2) placed over the first conductive ridge 1006 without interference from the PCB526, which would otherwise be interposed between the first conductive ridge 1006 and the cover of the filter housing. The amount of inductance provided by the first conductive ridge 1006 may be controlled as a function of a height of the first conductive ridge 1006 above the bottom of the filter housing 532.

[0104] The negative coupling provided between the first and second resonators 502, 504 may be implemented using a second capacitive coupling 1009. The second capacitive coupling1009, like the first capacitive coupling 1002, may be realized as at least one conductive probe1010, which may be provided as a conductive trace on the bottom surface of the PCB 526 and configured to at least partially overlap the first and second conductive structures 514 and 518, respectively, of the first and second resonators 502, 504 in the vertical direction. In one or more embodiments, the conductive probe 1010 may be a patterned metallization layer on the bottom surface of the PCB 526. In order to reduce dielectric loss introduced by the PCB 526, a conductive trace 1012 may be optionally provided on the top surface of the PCB 526 which is electrically connected to the conductive probe 1010 on the bottom surface of the PCB 526 using one or more conductive vias or through holes.

[0105] Due to physical constraints of the filter 1000, the amount of capacitance provided by the second capacitive coupling 1009 between the first and second resonators 502, 504 may be very small, particularly if there is positive coupling which would otherwise reduce the negative coupling provided by the second capacitive coupling 1009. Thus, in order to reduce positive coupling and thereby increase the capacitance provided by the second capacitive coupling 1009, a second conductive ridge 1014 may be provided that is configured to extend outwardly (i.e., horizontally) from a sidewall of the filter housing 532, along the bottom of the filter housing 532, into a center of the triplex arrangement of resonators 502, 504, 506. The conductive support post 1008 may be used to secure a first end of the second conductive ridge 1014 opposite a second end of the second conductive ridge 1014 contacting the sidewall of the filter housing 532. The second conductive ridge 1014 may include a window (i.e., opening) adapted to allow the PCB 526 to pass therethrough and avoid electrical connection between the second conductive ridge 1014 and the conductive probe 1010 on the bottom surface of the PCB 526 forming the second capacitive coupling 1009. In this manner, the second conductive ridge 1014 serves as an isolation structure between the first and second resonators 502, 504 that is configured to block all or most of the positive coupling between the first and second resonators 502, 504. Becauseonly negative (i.e., capacitive) coupling is needed between the first and second resonators 502, 504, a conductive ridge electrically connecting the first and second conductive pedestals 516 and 520 of the adjacent first and second resonators 502 and 504, respectively, is not required.

[0106] The positive coupling provided between the second and third resonators 504, 506 may be implemented using a third conductive ridge 1020. The third conductive ridge 1020 is configured to extend upwardly from the bottom of the filter housing 523 and electrically connects the second and third conductive pedestals 520 and 524 of the adjacent second and third resonators 504 and 506, respectively. The amount of inductance provided by the third conductive ridge 1020 may be controlled as a function of a height of the third conductive ridge 1020 above the bottom of the filter housing 532. The positive coupling provided by the third conductive ridge 1020 may be sufficient enough such that an isolation structure between the second and third resonators 504, 506 is not needed.

[0107] FIG. 11 is a graph depicting frequency response characteristics of the illustrative filter shown in FIGS. 10A and 10B, according to one or more embodiments. Referring to FIG. 11, the filter 1000 has a passband 1102 from about 3.6 GHz - 3.8 GHz, a first transmission zero 1104 below the passband at about 3.35 GHz and a second transmission zero 1106 above the passband at about 4.05 GHz.

[0108] The single triplet filter topology described herein can be extended to realize more complex cross-coupled filter arrangements, according to embodiments of the present disclosure. For example, a standard filter requiring six resonators and generating two transmission zeros can be realized, according to one or more embodiments, using a filter having five resonators and generating four transmission zeros by incorporating FDCs. By way of illustration only and without limitation, FIGS. 12A - 12C are schematic top plan and perspective views depicting a physical realization of an example cross-coupled filter 1200 utilizing a cascaded triplet topology including five resonators and configured to generate four transmission zeros, according to one or more embodiments. In one or more embodiments, the filter 1200 may be configured, using two FDCs and capacitive and inductive couplings, to have the example frequency response characteristics shown in Table 1 below.Table 1

[0109] Referring to FIGS. 12A - 12C, the filter 1200 includes a first triplet comprising a first resonator 1202, a second resonator 1204, and a third resonator 1206, and a second triplet comprising the third resonator 1206, a fourth resonator 1208, and a fifth resonator 1210. The first and second triplets are configured in a cascaded arrangement in that the third resonator 1206 is shared by both the first and second triplets. Each of the first through fifth resonators 1202 through 1210 forms a vertex of two adjacent triangles, with the third resonator 1206 forming a shared vertex of both triangles.

[0110] Each of the resonators comprises a planar portion and a 3D portion, consistent with the resonators 502, 504, 506 previously described in conjunction with FIGS. 6A and 6B. More particularly, the first resonator 1202 includes a planar portion (i.e., first resonator head) realized as a first conductive structure 1212 and a 3D portion realized as a first conductive pedestal 1214 electrically connected to the first conductive structure 1212. The second resonator 1204 includes a planar portion (i.e., second resonator head) realized as a second conductive structure 1216 and a 3D portion realized as a second conductive pedestal 1218 electrically connected to the second conductive structure 1216. The third resonator 1206 includes a planar portion (i.e., third resonator head) realized as a third conductive structure 1220 and a 3D portion realized as a third conductive pedestal 1222 electrically connected to the third conductive structure 1220. The fourth resonator 1208 includes a planar portion (i.e., fourth resonator head) realized as a fourth conductive structure 1224 and a 3D portion realized as a fourth conductive pedestal 1226 electrically connected to the fourth conductive structure 1224. The fifth resonator 1210 includes a planar portion (i.e., fifth resonator head) realized as a fifth conductive structure 1228and a 3D portion realized as a fifth conductive pedestal 1230 electrically connected to the fifth conductive structure 1228.

[0111] The first through fifth conductive structures 1212, 1216, 1220, 1224, 1228 of the first through fifth resonators 1202 through 1210, respectively, may be implemented as one or more patterned metallization layers on a PCB 1232. Forming the resonator heads as a metallization layer on the PCB 1232 provides superior control over manufacturing tolerances in forming the capacitive coupling between resonators compared to standard approaches, among other benefits. The first through fifth resonators 1202 through 1210 and the PCB 1232 are enclosed within an interior space of a filter housing 1234 including a bottom and sidewalls extending upwardly (i.e., vertically) from the bottom (i.e., floor) of the filter housing 1234. In the embodiment shown in FIGS. 12A - 12C, the filter housing 1234 may be rectangular in shape, although embodiments are limited to any specific shape or dimensions of the filter housing 1234.

[0112] Each of the first through fifth conductive pedestals 1214, 1218, 1222, 1226 and 1230, respectively, may be formed as cylindrical metal structures extending upwardly from a bottom of the filter housing 1234, although embodiments are not limited thereto. For example, one or more of the conductive pedestals 1214, 1218, 1222, 1226, 1230 may be shaped as a rectangular cuboid, or other structure having a polygonal cross section. The conductive pedestals 1214, 1218, 1222, 1226, 1230 may be formed integrally with the filter housing 1234, such as, for example, using a die casting or CNC machining process, although embodiments are not limited thereto. In one or more embodiments, each of at least a subset of the conductive pedestals 1214, 1218, 1222, 1226, 1230 may be formed having a constant diameter. In other embodiments, one or more of the conductive pedestals 1214, 1218, 1222, 1226, 1230 may be configured having a diameter that varies with a height of the conductive pedestal above the bottom of the filter housing 1234. In one or more embodiments, each of the conductive pedestals 1214, 1218, 1222, 1226, 1230 may be formed as a solid metal structure, such as, for example, aluminum, although other materials for forming the conductive pedestals 1214, 1218, 1222, 1226, 1230 are similarly contemplated (e.g., copper).

[0113] In one or more embodiments, each of the first and second triplets includes at least one FDC. Specifically, the first triplet includes a first FDC coupled between the first and third resonators 1202, 1206, and the second triplet includes a second FDC coupled between the thirdand fifth resonators 1206, 1210. The first FDC is a hybrid or mixed coupling comprising a first capacitive coupling 1236 and a first inductive coupling. The first inductive coupling may be realized using a first conductive ridge 1238 extending upwardly from the bottom of the filter housing 1234 and electrically connecting the first and third conductive pedestals 1214, 1222. The first conductive ridge 1238 may be formed integrally with the filter housing 1234 (e.g., using die casting or CNC processing), although embodiments are not limited thereto.

[0114] The first capacitive coupling 1236 may be realized using a first conductive probe 1240 provided as a conductive trace on the bottom surface of the PCB 1232. The first conductive probe 1240 may be formed as at least a portion of a patterned metallization layer on the bottom surface of the PCB 1232 that is configured to at least partially overlap the first and third conductive structures 1212, 1220 in the vertical direction. In order to reduce dielectric loss introduced by the PCB 1232, a conductive trace 1242 may be optionally provided on the top surface of the PCB 1232 that is electrically connected to the first conductive probe 1240 on the bottom surface of the PCB 1232 using one or more conductive vias or through holes.

[0115] In place of or in addition to the conductive probes on the PCB 1232, capacitive coupling, in one or more embodiments, may be provided using the resonator heads themselves. For example, the resonator heads of adjacent resonators may be configured (e.g., using resonator heads having protrusions, as shown in FIG. IB) to provide capacitive coupling based on a proximity between the resonator heads, with or without a conductive probe.

[0116] The second FDC is a hybrid or mixed coupling comprising a second capacitive coupling 1244 and a second inductive coupling. The second inductive coupling may be realized using a second conductive ridge 1246 extending upwardly from the bottom of the filter housing 1234 and electrically connecting the third and fifth conductive pedestals 1222 and 1230, respectively. The second capacitive coupling 1244 may be realized using a second conductive probe 1248 provided as a conductive trace on the bottom surface of the PCB 1232. The second conductive probe 1248 may be formed as at least a portion of a patterned metallization layer on the bottom surface of the PCB 1232 that is configured to at least partially overlap the third and fifth conductive structures 1220, 1228 in the vertical direction. In order to reduce dielectric loss introduced by the PCB 1232, a conductive trace 1250 may be optionally provided on the topsurface of the PCB 1232 that is electrically connected to the second conductive probe 1248 on the bottom surface of the PCB 1232 using one or more conductive vias or through holes.

[0117] In order to increase the amount of capacitance provided by the first capacitive coupling 1236, a first isolation structure 1252 may be provided configured to reduce magnetic coupling between the first and third resonators 1202, 1206. The first isolation structure 1252 is configured to extend outwardly from a first conductive post 1254 disposed at or near a center of the first triplet (comprising the first, second and third resonators 1202, 1204, 1206) between the first and third resonators 1202, 1206. The first isolation structure 1252 extends vertically from the bottom of the filter housing 1234; a height of the first isolation structure 1252 above the bottom of the filter housing 1234 may be greater than a height of the capacitive coupling 1236. A first end of the first isolation structure 1252 is attached to the first conductive post 1254 and a second end of the first isolation structure 1252, opposite the first end, may have a notch or opening therein configured to allow the first capacitive coupling 1236 to pass over the second end of the first isolation structure 1252 without making direct contact thereto.

[0118] A frequency of the respective transmission zeros may be controlled as a function of the amount of capacitance provided by the first and second capacitive couplings 1236 and 1244, respectively, and an amount of inductance provided by the first and second conductive ridges 1238 and 1246, respectively. The inductance of each of the first and second conductive ridges 1238, 1246 can be varied as a function of a height of the conductive ridge above the bottom of the filter housing 1234. The capacitance of each of the first and second capacitive couplings 1236, 1244 can be varied as a function of one or more factors, including, for example, a proximity of the conductive probes 1240, 1248 on the bottom surface of the PCB 1232 to corresponding conductive structures 1212, 1220, 1228 on the top surface of the PCB 1232 and / or a dielectric constant of the PCB 1232.

[0119] The filter 1200 further comprises capacitive (i.e., negative) couplings or inductive (i.e., positive) couplings between one or more pairs of adjacent resonators. The capacitive and inductive couplings can be used to further control the frequency response characteristics of the filter 1200. Specifically, a third capacitive coupling may be provided between the first and second resonators 1202, 1204 based on a proximity of the first conductive structure 1212 to the second conductive structure 1216. In order to reduce magnetic coupling between the first andsecond resonators 1202, 1204, a second isolation structure 1256 may be provided. The second isolation structure 1256 extends outwardly from the first conductive support post 1254 toward an area between the first and second resonators 1202, 1204. A height of the second isolation structure 1256 above the bottom of the filter housing 1234 is configured to allow the PCB 1232 between the first and second resonators 1202, 1204 to pass without making direct contact with the second isolation structure 1256. The degree to which the second isolation structure 1256 extends along the bottom of the filter housing 1234 from the first conductive support post 1254 toward the first and second resonators 1202, 1204 can be configured as a function of the amount of desired reduction in magnetic coupling.

[0120] A third inductive coupling may be provided between the second and third resonators 1204, 1206. The third inductive coupling may be realized using a third conductive ridge 1258 extending upwardly from the bottom of the filter housing 1234 and electrically connecting the second and third conductive pedestals 1218, 1222 of the second and third resonators 1204, 1206.

[0121] A fourth inductive coupling may be provided between the third and fourth resonators 1206, 1208. The fourth inductive coupling may be realized using a fourth conductive ridge 1260 extending upwardly from the bottom of the filter housing 1234 and electrically connecting the third and fourth conductive pedestals 1222, 1226 of the third and fourth resonators 1206, 1208.

[0122] In order to reduce magnetic coupling between the second and fourth resonators 1204, 1208, a third isolation structure 1262 may be provided. The third isolation structure 1262 is configured to extend from a sidewall of the filer housing 1234 towards the third resonator 1206, passing between the second and fourth resonators 1204, 1208, and extends upwardly from the bottom of the filter housing 1234. A height of the third isolation structure 1262 may be configured to provide a desired reduction in magnetic coupling between the second and fourth resonators 1204, 1208.

[0123] A fourth capacitive coupling may be provided between the fourth and fifth resonators 1208, 1210 based on a proximity of the fourth conductive structure 1224 to the fifth conductive structure 1228. In order to reduce magnetic coupling between the fourth and fifth resonators 1208, 1210, a fourth isolation structure 1264 may be provided. The fourth isolation structure 1264 extends outwardly (i.e., horizontally), along the bottom of the filter housing 1234, from asecond conductive support post 1266 toward an area between the fourth and fifth resonators 1208, 1210. The second conductive support post 1266 may be disposed at or near the center of the second triplet (comprising the third, fourth and fifth resonators 1206, 1208, 1210). A height of the fourth isolation structure 1264 above the bottom of the filter housing 1234 is configured to allow the PCB 1232 between the fourth and fifth resonators 1208, 1210 to pass without making direct contact with the fourth isolation structure 1264. The degree to which the fourth isolation structure 1264 extends along the bottom of the filter housing 1234 from the second conductive support post 1266 toward the fourth and fifth resonators 1208, 1210 can be configured as a function of the amount of desired reduction in magnetic coupling.

[0124] A fifth isolation structure 1268 may be provided between the third and fifth resonators 1206, 1210 to reduce magnetic coupling between the third and fifth resonators. The fifth isolation structure 1268 may extend outwardly (i.e., horizontally), along the bottom of the filter housing 1234, from the second conductive support post 1266 toward an area between the third and fifth resonators 1206, 1210. The amount of reduction in magnetic coupling can be controlled as a function of distance that the fifth isolation structure 1268 extends from the second conductive support post 1266 toward the third and fifth resonators 1206, 1210.

[0125] The filter 1200 further comprises a first input / output terminal 1270 electrically coupled to the first conductive pedestal 1214 of the first resonator 1202 and a second input / output terminal 1272 electrically coupled to the fifth conductive pedestal 1230 of the fifth resonator 1210.Input and output signals associated with the filter 1200 may be accessible via the first and second input / output terminals 1270 and 1272.

[0126] FIGS. 13A and 13B are graphs depicting simulated frequency response and insertion loss (IL) characteristics, respectively, of the illustrative filter 1200 shown in FIGS. 12A - 12C, according to embodiments of the inventive concept. As shown in FIG. 13 A, the filter 1200 is configured to generate two transmission zeros below the passband and two transmission zeros above the passband. In this example, the filter 1200 is configured having a passband 1302 of about 3.6 - 3.7 GHz, a first transmission zero 1304 at about 3.25 GHz, a second transmission zero 1306 at about 3.55 GHz, a third transmission zero 1308 at about 3.75 GHz, and a fourth transmission zero 1310 at about 4.1 GHz. Referring to FIG. 13B, the filter 1200 is configured having an insertion loss of about 1 dB or less from about 3.6 - 3.7 GHz representing thepassband. The filter 1200 exhibits a sharp attenuation above and below the passband (i.e., below about 3.6 GHz and above about 3.7 GHz). It is to be appreciated that the frequency response characteristics shown in FIGS. 13A and 13B are merely illustrative, and that the filter 1200 may be configured to achieve various other frequency response design parameters as will become apparent to those skilled in the art given the teachings herein.

[0127] The illustrative filter arrangements shown in FIGS. 6A, 6B, 8A, 8B, 10A, 10B, 12A and 12B included an FDC integrated into the cross-coupling of a single triplet or a cascaded triplet topology. However, it is to be understood that embodiments of the inventive concept are not limited to a triplet topology or to using an FDC in the cross-coupling. By way of example only and without limitation, FIGS. 14A - 14E conceptually depict some other example fundamental cross-coupled filter topologies in which aspects of the inventive concept may be used, according to one or more embodiments. In FIGS. 14A - 14E, each resonator is numbered and represented as a vertex (solid dot) of a corresponding shape, straight lines between vertices without arrows represent constant couplings between adjacent resonators, straight lines with arrows represent FDCs, and open circles represent input / output terminals of the filters. As shown in FIGS. 12A - 12C, each of these fundamental filter blocks can be expanded to form more complex filters (e.g., cascaded triplet filter, cascaded quadruplet filter, etc.), as will become apparent to those skilled in the art given the teachings herein.

[0128] Referring to FIG. 14A, a single triplet filter topology 1402 including an FDC as a mainline coupling is shown. In the filter topology 1402, the FDC is included between the second resonator (2) and the third resonator (3). A first input / output terminal is connected to the first resonator and a second input / output terminal is connected to the third resonator.

[0129] FIG. 14B depicts a single quadruplet filter topology 1404 including one FDC as the cross-coupling. In the filter topology 1404, the FDC is included between the first resonator (1) and the fourth resonator (4). A first input / output terminal is connected to the first resonator and a second input / output terminal is connected to the fourth resonator.

[0130] In FIG. 14C, a single quadruplet filter topology 1406 including two FDCs is shown. In the filter topology 1406, a first FDC is included as the cross-coupling between the first resonator (1) and the fourth resonator (4) and a second FDC is included as the mainline coupling betweenthe second resonator (2) and the third resonator (3). A first input / output terminal is connected to the first resonator and a second input / output terminal is connected to the fourth resonator.

[0131] Referring to FIG. 14D, a single quadruplet filter topology 1408 including one FDC as the mainline coupling is shown. In the filter topology 1408, the FDC is included between the second resonator (2) and the third resonator (3). A first input / output terminal is connected to the first resonator and a second input / output terminal is connected to the fourth resonator.

[0132] In FIG. 14E, a box section filter topology 1410 including one FDC is shown. Specifically, in the filter topology 1410, the FDC is included between the second resonator (2) and the fourth resonator (4). A first input / output terminal is connected to the first resonator and a second input / output terminal is connected to the fourth resonator.

[0133] According to one or more embodiments, the resonator head may be configured as a folded (i.e., deep drawn) resonator head, for example in the shape of a folded mushroom, to realize frequency variable couplings (FVCs). The unique folded resonator head configuration according to embodiments of the inventive concept may provide capacitive coupling between adjacent (i.e., proximate) folded resonator heads in place of or in addition to PCB metallization and / or resonator head designs having protrusions or fingers (e.g., protrusion portions 130, 132, 134, 136 shown in FIG. IB). As will be described in further detail herein, the folded resonator head design may also provide improved selectivity in comparison to non-FVC counterpart resonator heads for the same number of resonators in a given RF filter. This folded resonator head design may be well-suited, for example, in filter applications in which selectivity (i.e., quality factor (Q)) is not as critical, since this design uses a resonator head having a larger radius than a radius of the conductive support pedestal (i.e., post or stalk) supporting the resonator head, which reduces selectivity compared to other FVC resonator head designs. The folded resonator head design may be particularly suitable for implementation in tower-mounted amplifiers (TMAs), among other applications.

[0134] FIG. 15A is a schematic top perspective view depicting an illustrative folded resonator head 1500, according to one or more embodiments. FIG. 15B is a schematic cross-sectional view depicting the illustrative folded resonator head 1500 taken along line A-A' in FIG. 15A. Referring to FIGS. 15A and 15B together, the folded resonator head 1500 may be configured having a flat disk-shaped top cover 1502 forming an upper surface of the folded resonator head1500 and an exterior sidewall 1504 surrounding (i.e., extending around) and extending downward from a perimeter of the top cover 1502. The top cover 1502 includes an opening 1506 in a center portion thereof. The top cover 1502 and sidewalls 1504 of the folded resonator head 1500 comprise a conductive material, such as a metal, and may be formed using a deep drawn metal manufacturing process, although embodiments are not limited thereto.

[0135] The folded resonator head 1500 includes a center support structure 1508 which may be shaped as a hollow cylinder in one or more embodiments. An upper edge of the center support structure 1508 may contact an inner rim of the opening 1506. The center support structure 1508 may comprise a conductive material, such as a metal. The center support structure 1508 may include a base 1510 having an opening 1512 therein which may be adapted to receive a conductive pedestal (e.g., any of conductive pedestals 326 through 332 shown in FIG. 3C) for supporting the folded resonator head 1500 in a filter housing.

[0136] The folded resonator head 1500 may be configured such that inner surfaces of the sidewalls 1504 (facing the center support structure 1508), an underside of the top cover 1502, and exterior sidewalls of the center support structure 1508 (facing the sidewalls 1504) define an inverted U-shaped interior space 1514 surrounding the center support structure 1508.

[0137] In some embodiments, the center support structure 1508, the top cover 1502 and the sidewalls 1504 may be formed from a continuous piece of material. For example, in some embodiments the folded resonator head 1500 may be manufactured using a metal stamping process whereby a flat sheet of metal is bent into a folded resonator head configuration as shown in FIGS. 15A and 15B.

[0138] Frequency variable couplings are generated by the coexistence of capacitive and inductive couplings between pairs of resonators under analysis. Aspects of the inventive concept overcome one or more limitations of standard resonator realizations in that resonators formed in accordance with embodiments of the present disclosure are easier to fabricate (e.g., using a deep drawn manufacturing process), are less expensive, are reusable in all cavities of the filter product due to its rounded shape, and require less precision in terms of orientation to achieve a particular filter response since the desired capacitive coupling can be obtained by changing a gap between resonators, among other factors.

[0139] FIG. 16 is a schematic top perspective view depicting a pair of resonators employing folded resonator heads for implementing a frequency variable coupling, according to one or more embodiments. Referring to FIG. 16, a first resonator includes a first folded resonator head 1602 and a first conductive pedestal (i.e., resonator stalk or post) 1604 on which the first folded resonator head 1602 is supported. Likewise, a second resonator includes a second folded resonator head 1606 and a second conductive pedestal 1608 on which the second folded resonator head 1606 is supported. In one or more embodiments, each of the first and second folded resonator heads 1602, 1606 are conductive and may be formed in a manner consistent with the folded resonator head 1500 previously described in conjunction with FIGS. 15A and 15B.

[0140] Each of the first and second conductive pedestals 1604, 1608 may be formed as pillarshaped support structures extending upwardly (i.e., in the vertical direction) from a bottom of the filter housing, although embodiments are not limited thereto. For example, in some embodiments, one or both of the conductive pedestals 1604, 1608 may be formed as a cylindrical structure (i.e., having a circular horizontal cross section), or as a cuboid structure (i.e., having a rectangular horizontal cross section) or other structure having a polygonal cross section. In one or more embodiments, the first and second conductive pedestals 1604, 1608 may be formed integrally with the bottom of the filter housing, such as by using a die casting or metal stamping process.

[0141] Capacitive (i.e., electric) coupling 1610 for realizing the frequency variable coupling may be provided by the first and second resonator heads 1602, 1606 based on a proximity of the first and second resonator heads 1602, 1606 to one another; that is, a capacitance of the capacitive coupling 1610 may be controlled as a function of a distance (i.e., gap size) between the first and second resonator heads 1602, 1606. A conductive ridge 1612 may be used to provide inductive (i.e., magnetic) coupling between the first and second resonators for realizing the frequency variable coupling. The conductive ridge 1612 may be integrated with the filter housing and is configured to extend upwardly from the bottom of the filter housing and electrically connects the first and second conductive pedestals 1604, 1608 of the first and second resonators, respectively.

[0142] In a cross-coupled configuration, filters may be realized by a cascade of one or more triplets and / or quadruplets formed by FVC topologies according to embodiments of the inventive concept. A circuit diagram depicting an example fdter 500 including frequency dependent couplings configured into a cross-coupled triplet arrangement was previously described in connection with FIG. 5. With reference again to FIG. 5, the filter 500 is configured having first and second main couplings between the first resonator 502 and the second resonator 504 and between the second resonator 504 and the third resonator 506, and having the FDC 512 included in the cross-coupling between the first and third resonators 502, 506. The first and second main couplings may be considered direct couplings (i.e., either only inductive or only capacitive) and the FDC may be considered a mixed coupling. A center frequency of the filter 500 may be controlled as a function of the resonant frequency of the first, second and third resonators 502, 504 and 506, respectively, and a coupling strength of the first and second main couplings may be configured to control a width of the passband of the filter 500, as previously stated.

[0143] By way of example only and without limitation, embodiments of cross-coupled filter arrangements using folded resonator heads will now be discussed. FIGS. 17A and 17B are top plan and perspective views, respectively, depicting a physical realization of an illustrative filter 1700 configured in a single triplet topology, according to one or more embodiments. The filter 1700 may be implemented in a manner consistent with the example filter 500 shown in FIGS. 6A and 6B, except that each of the resonators in the filter 1700 of FIGS. 17A and 17B employs a folded resonator head rather than the PCB-implemented resonator head used in the filter 500. The filter 1700 is configured to provide two transmission zeros: one above the passband and one below the passband.

[0144] Capacitive coupling, which may be implemented as cross-coupling between non-adjacent resonators, creates a signal path for the RF signal that interferes destructively with the main signal path at certain frequencies, resulting in a transmission zero. Similarly, inductive coupling can be used to generate transmission zeros. For example, inductive-window bandpass filters can use higher-order mode excitation due to inductive coupling to create transmission zeros. Also, incorporating cross-coupling grounding inductors can help to generate and manipulate the position of the transmission zero(s).

[0145] Referring to 17A and 17B, the filter 1700 includes a first resonator 1702, a second resonator 1704 and a third resonator 1706 configured in a single triplet topology. The first resonator 1702 includes a first resonator head 1707 and a first support structure 1708 supporting and electrically connected to the first resonator head 1707. The second resonator 1704 includes a second resonator head 1709 and a second support structure 1710 supporting and electrically connected to the second resonator head 1709. The third resonator 1706 includes a third resonator head 1711 and a third support structure 1712 supporting and electrically connected to the third resonator head 1711. Each of the first through third resonator heads 1707, 1709, 1711 is implemented as a folded resonator head consistent with the folded resonator head depicted in FIGS. 15A and 15B.

[0146] The filter 1700 may further include a filter housing 1713. In one or more embodiments, the filter housing 1713 includes a base (i.e., floor or bottom) and a plurality of sidewalls extending upwardly (i.e., in a vertical direction) from the base of the filter housing 1713. The sidewalls of the filter housing 1713 may be formed integrally with the base of the filter housing (e.g., using a die casting or CNC machining process). The sidewalls and base of the filter housing 1713 are configured to define an interior space of the filter 1700. It is to be appreciated that the particular shape and dimensions of the filter housing 1713 are not limited to the arrangement shown in FIGS. 17A and 17B. The filter housing 1713 may be connected to ground and may therefor serve as a ground plane of the filter 1700. The first through third resonators 1702, 1704, 1706 may be disposed within the interior space of the filter housing 1713.

[0147] In one or more embodiments, each of the first through third support structures 1708, 1710, 1712 may be formed as cylindrical metallic structures extending upwardly (i.e., in the vertical direction) from the base of the filter housing 1713, although embodiments are not limited thereto. For example, one or more of the first through third support structures 1708, 1710, 1712 may be formed as cuboid structures or other structures having a polygonal cross section. In some embodiments, the first through third support structures 1708, 1710, 1712 may be formed integrally with the base of the filter housing 1713, for example using a die casting or other manufacturing process.

[0148] The filter 1700 further includes a first input / output (source) port 1714 coupled to the first resonator 1702, such as via a first transmission line, and a second input / output (load) port1716 coupled to the third resonator 1706, such as via a second transmission line. Each of the first and second transmission lines may be implemented as coaxial conductors extending between a given one of the resonators 1702, 1706 and a corresponding one of the input / output terminals 1714, 1716.

[0149] Coupling between the resonators 1702, 1704, 1706 is based on proximity of the respective resonators to one another, which is primarily inductive (positive) coupling. A first conductive ridge 1718 may be disposed between the adjacent first and second resonators 1702, 1704 to provide inductive coupling between the first and second resonators 1702, 1704, a second conductive ridge 1720 may be disposed between the adjacent second and third resonators 1704, 1706 to provide inductive coupling between the second and third resonators 1704, 1706, and a third conductive ridge 1722 may be disposed between the adjacent third and first resonators 1706, 1702 to provide inductive coupling between the third and first resonators 1706, 1702. Each of the first, second and third conductive ridges 1718, 1720, 1722 may be configured to extend upwardly from the base of the filter housing 1713 between the support structures 1708, 1710, 1712 of respective pairs of adjacent resonators 1702, 1704, 1706.

[0150] Although not explicitly shown in FIGS. 17A and 17B, the filter 1700 may include one or more isolation structures, consistent with the conductive ridges 544 and 546 shown in FIGS. 6A and 6B, that extend outwardly from corresponding sidewalls and along the base of the filter housing 1713. The isolation structures may be configured to reduce (or block) magnetic (i.e., inductive) coupling between proximate resonators to thereby reduce inductive coupling when the inductive coupling between a corresponding pair of adjacent resonators is too strong.

[0151] Capacitive coupling in the filter 1700 may be provided based on a proximity of the conductive resonator heads to one another. The amount of capacitive coupling is directly proportional to the area of the conductive surfaces of the resonator heads and is inversely proportional to the distance between the resonator heads; that is, C = so • (A / d), where C is amount of capacitive coupling, A is the area of the conductive surfaces of the resonator heads, d is the distance between resonator heads, and so is the permittivity of free space. The first through third resonator heads 1707, 1709, 1711 may provide increased capacitive coupling compared to other resonator head designs such as, for example, the PCB-based resonator headarrangement shown in FIGS. 6A and 6B, due at least in part to an increased surface area provided by their folded resonator head configuration.

[0152] In the filter 1700, a distance between the first and third resonators 1702, 1706, which forms the primary capacitance component of the cross-coupling, may be less than a distance between the first and second resonators 1702, 1704 or between the second and third resonators 1704, 1706. This capacitance, in conjunction with an inductance attributable to the third conductive ridge 1722, controls a resonant frequency of the cross-coupling. The resonant frequency of the first, second and third resonators 1702, 1704 and 506, respectively, may be configured to control a center frequency of the filter 1700, while a coupling strength of the first and second main couplings may be configured to control a width of the passband of the filter 1700.

[0153] The filter 1700 may further include a first conductive post 1724, a second conductive post 1726, and a third conductive post 1728. In one or more embodiments, each of the conductive posts 1724, 1726, 1728 may be implemented as turning screws (i.e., conductive post of variable electrical and mechanical length) connected to ground; specifically, to a cover of the filter 1700 (not explicitly shown). The first conductive post 1724 may be disposed between the first and second resonator heads 1707, 1709, the second conductive post 1726 may be disposed between the second and third resonator heads 1709, 1711, and the third conductive post 1728 may be disposed between the second and third resonator heads 1709, 1711. Each of the first through third conductive posts 1724, 1726, 1728 may be supported on a base plate extending horizontally from a corresponding one of the resonator support structures, such as the second support structure 1710, although embodiments are not limited thereto. Each of the first through third conductive posts 1724, 1726, 1728 may be configured to control an amount of capacitive coupling between a corresponding pair of resonator heads.

[0154] A fourth conductive post 1730 and a fifth conductive post 1732 may be disposed between the first and third resonators 1702, 1706. Each of the fourth and fifth conductive posts 1730, 1732 may extend vertically from a base of the filter housing 1713. The fourth and fifth conductive posts 1730, 1732 may be used to control a capacitance of the cross-coupling between the first and third resonators 1702, 1706. An upper surface of the fourth and fifth conductive posts 1730, 1732 may be coplanar with an upper surface of the first and third resonator heads1707, 1711, although embodiments are not limited thereto. In some embodiments, the respective upper surfaces of the fourth and fifth conductive posts 1730, 1732 may be at different heights relative to the base of the filter housing 1713. In contrast to the first through third conductive posts 1724, 1726, 1728, the fourth and fifth conductive posts 1730, 1732 may be fixed in length and connected (i.e., shorted) to both ground planes — the base and cover of the filter 1700. In one or more embodiments, the fourth and fifth conductive posts 1730, 1732 provide boundaries to the coupling windows between resonators. Since the magnetic coupling by proximity through the coupling windows may be insufficient, depending on the specific frequency response characteristics required, coupling ridges may be additionally used in the filter 1700.

[0155] FIG. 18 is a graph depicting frequency response characteristics of the illustrative triplet filter 1700 shown in FIGS. 17A and 17B, according to one or more embodiments. Referring to FIG. 18, the filter 1700 has a passband 1802 from about 700 MHz - 740 MHz, a first transmission zero 1804 below the passband at about 630 MHz, and a second transmission zero 1806 above the passband at about 810 MHz.

[0156] FIGS. 19A and 19B are top plan and perspective views, respectively, depicting a physical realization of an illustrative filter 1900 configured in a single triplet topology, according to another embodiment of the inventive concept. The filter 1900 may be implemented in a manner consistent with the illustrative filter 1700 shown in FIGS. 17A and 17B, but configured to provide two transmission zeros above the passband. In order to achieve this, a distance between the first and third resonator heads 1707, 1711 may be reduced compared to the arrangement of the first and third resonator heads 1707, 1711 of the filter 1700 shown in FIGS. 17A and 17B, thereby increasing the capacitance component of the cross-coupling (e.g., 512 in FIG. 5). The third conductive post 1728 (see FIG. 17A), which was disposed between the first and third resonator heads 1707, 1711, on an imaginary line drawn between the first and third support structures 1708, 1712, may be removed and replaced by a sixth conductive post 1902 disposed between the first and third resonator heads 1707, 1711 in a first horizontal direction (an x-axis direction) and offset from the imaginary line in a second horizontal direction (a r-axis direction). The sixth conductive post 1902 may be supported by the fifth conductive post 1732, although embodiments of the inventive concept are not limited thereto. The sixth conductive post 1902 may be implemented as a tuning screw configured to control the coupling between thefirst and third resonator heads 1707, 1711, which may affect the frequency response of the filter 1900.

[0157] Additionally, the third conductive ridge 1722 (see FIG. 17B) providing inductive coupling between the first and third resonators 702, 706 may be removed and replaced by a fourth conductive ridge 1904. Rather than being disposed parallel to the imaginary line drawn between the first and third support structures 1708, 1712, as in the case of the third conductive ridge 1722 (FIG. 17B), the fourth conductive ridge 1904 is disposed perpendicular to the line drawn between the first and third support structures 1708, 1712. More particularly, the fourth conductive ridge 1904 may be disposed along the base of the filter housing 1713, extending in the vertical (z-axis) direction and electrically connecting the first and third support structures 1708, 1712. Configured in this manner, the fourth conductive ridge 1904 may provide weak inductive coupling.

[0158] FIG. 20 is a graph depicting frequency response characteristics of the illustrative triplet filter 1900 shown in FIGS. 19A and 19B, according to one or more embodiments. Referring to FIG. 20, the filter 1900 has a passband 1902 from about 700 MHz - 740 MHz, which is consistent with the passband of the filter 1700 shown in FIGS. 17A and 17B, a first transmission zero 2004 above the passband at about 765 MHz, and a second transmission zero 2006 above the passband at about 800 MHz.

[0159] FIG. 21 is a schematic diagram depicting an illustrative cross-coupled filter 2100 including frequency dependent couplings configured into a single quadruplet topology, according to one or more embodiments. Referring to FIG. 21, the filter 2100 includes a first resonator 2102, a second resonator 2104, a third resonator 2106, and a fourth resonator 2108, configured into a single quadruplet topology. In a cross-coupled filter topology, the resonators are not arranged in a straight line (as in an in-line filter topology), but can have coupling between resonators that are not adjacent along an electrical path of the filter. For example, in the single quadruplet topology of the filter 2100, the resonators 2102, 2104, 2106, 2108 may be disposed at respective vertices of a four-sided polygon. The filter 2100 further includes a first input / output (source (S)) port 2110 coupled to the first resonator 2102 and a second input / output (load (L)) port 2112 coupled to the fourth resonator 2108.

[0160] As shown in FIG. 21, the filter 2100 is configured having a first main coupling between the first resonator 2102 and the second resonator 2104, a second main coupling between the second resonator 2104 and the third resonator 2106, and a third main coupling between the third resonator 2106 and the fourth resonator 2108. The first, second and third main couplings may be considered direct couplings (i.e., either only inductive or only capacitive). The resonant frequency of the first through fourth resonators 2102, 2104, 2106 and 2108, respectively, may be configured to control a center frequency of the filter 2100, while a coupling strength of the first through third main couplings may be configured to control a width of the passband of the filter 2100.

[0161] The filter 2100 further comprises a first cross-coupling between the first and third resonators 2102, 2106, and a second cross-coupling between the first and fourth resonators 2102, 2108. The second cross-coupling may comprise an FDC 2114. The FDC 2114 may be a mixed coupling, conceptually depicted as a capacitor, C, and an inductor, L, connected in a parallel LC circuit configuration between the first and fourth resonators 2102, 2108. By including the FDC 2114 into the cross-coupling, three transmission zeros can be generated using only four resonators, rather than generating only two transmission zeros using a classical quadruplet filter topology.

[0162] By way of example only and without limitation, FIGS. 22A and 22B are top plan and perspective views, respectively, depicting a physical realization of the illustrative filter 2100 of FIG. 21, according to one or more embodiments. Referring to FIGS. 22A and 22B, the first resonator 2102 includes a first resonator head 2202 and a first support structure 2204 supporting and electrically connected to the first resonator head 2202. The second resonator 2104 includes a second resonator head 2206 and a second support structure 2208 supporting and electrically connected to the second resonator head 2206. The third resonator 2106 includes a third resonator head 2210 and a third support structure 2212 supporting and electrically connected to the third resonator head 2210. The fourth resonator 2108 includes a fourth resonator head 2214 and a fourth support structure 2216 supporting and electrically connected to the fourth resonator head 2214. Each of the first through fourth resonator heads 2202, 2206, 2210, 2214 may be implemented as a folded resonator head consistent with the folded resonator head depicted in FIGS. 15A and 15B. As previously stated, the folder resonator head design provides increased capacitive coupling between adjacent resonators in the filter 2100.

[0163] The filter 2100 may further include a filter housing 2217. In one or more embodiments, the filter housing 2217 includes a base (i.e., floor or bottom) and a plurality of sidewalls extending upwardly (i.e., in a vertical (z-axis) direction) from the base of the filter housing 2217. The sidewalls of the filter housing 2217 may be formed integrally with the base of the filter housing (e.g., using a die casting or CNC machining process), although embodiments are not limited thereto. Like the filter housing 1713 shown in FIGS. 17A and 17B, the sidewalls and base of the filter housing 2217 are configured to define an interior space of the filter 2100 in which the first through fourth resonators 2102, 2104, 2106, 2108 are disposed. It is to be appreciated that the particular shape and dimensions of the filter housing 2217 are not limited to the arrangement shown in FIGS. 22A and 22B. The filter housing 2217 may be connected to ground and may therefor serve as a ground plane of the filter 2100.

[0164] In one or more embodiments, each of the first through fourth support structures 2204, 2208, 2212, 2216 may be formed as cylindrical metallic structures extending upwardly (i.e., in the vertical direction) from the base of the filter housing 2217, although embodiments are not limited thereto. For example, in some embodiments, one or more of the first through fourth support structures 2204, 2208, 2212, 2216 may be formed as cuboid structures or other structures having a polygonal cross section. In some embodiments, the first through fourth support structures 2204, 2208, 2212, 2216 may be formed integrally with the base of the filter housing 2217, for example using a die casting or other manufacturing process, although embodiments are not limited thereto.

[0165] The filter 2100 further includes a first input / output (source) port 2110 coupled to the first resonator 2102, such as via a first transmission line, and a second input / output (load) port 2112 coupled to the fourth resonator 2108, such as via a second transmission line. Each of the first and second transmission lines may be implemented as coaxial conductors extending between a given one of the resonators 2102, 2108 and a corresponding one of the input / output terminals 2110, 2112.

[0166] Coupling between the resonators 2102, 2104, 2106, 2108 is based on proximity of the respective resonators to one another, which is primarily inductive (positive) coupling. A first conductive ridge 2218 may be disposed between the adjacent first and second resonators 2102, 2104 to implement the first main coupling, by providing inductive coupling between the first andsecond resonators 2102, 2104. A second conductive ridge 2220 may be disposed between the adjacent second and third resonators 2104, 2106 to implement the second main coupling, by providing inductive coupling between the second and third resonators 2104, 2106, and a third conductive ridge 2222 may be disposed between the adjacent third and fourth resonators 2106, 2108 to implement the third main coupling, by providing inductive coupling between the third and fourth resonators 2106, 2108. Each of the first, second and third conductive ridges 2218, 2220, 2222 may be configured to extend upwardly from the base of the filter housing 2217 between the support structures 2204, 2208, 2212, 2216 of respective pairs of adjacent resonators 2102, 2104, 2106, 2108.

[0167] In the filter 2100, the first cross-coupling may be implemented as a fourth conductive ridge 2224 to provide inductive coupling between the first and third resonators 2102, 2106. The FDC 2114 (see FIG. 21) in the second cross-coupling may be implemented using a combination of capacitance (C in FIG. 21) provided by the capacitive coupling between the first and fourth resonator heads 2202, 2214, and inductance (L in FIG. 21) provided by inductive coupling which may be implemented using a fifth conductive ridge 2226 between the first and fourth resonators 2102, 2108. The fifth conductive ridge 2226 may be configured perpendicular to an imaginary line drawn between a center of the first and fourth support structures 2204, 2216. More particularly, the fifth conductive ridge 2226 may be arranged in the second horizontal direction ( r-axis direction) along the base of the filter housing 2217 between fifth and sixth support structures, 2228 and 2230, respectively, and extend vertically (z- axis direction), electrically connecting the fifth and sixth support structures 2228, 2230. Configured in this manner, the fifth conductive ridge 2226 may provide weak inductive coupling.

[0168] One or more conductive posts may be provided in the filter housing 2217 to control an amount of capacitive coupling between adjacent resonators and thereby tune the frequency response characteristics of the filter 2100. For example, a first conductive post 2232 may be disposed between the first and second resonator heads 2202, 2206, a second conductive post 2234 may be disposed between the second and third resonator heads 2206, 2210, a third conductive post 2236 may be disposed between the third and fourth resonator heads 2210, 2214, a fourth conductive post 2238 may be disposed between the fourth and first resonator heads 2214, 2202, and a fifth conductive post 2240 may be disposed between the first and thirdresonator heads 2202, 2212. An amount of reduction in capacitive coupling between adjacent resonator heads will be a function of the arrangement of the conductive posts relative to corresponding proximate resonators; that is, each of the conductive posts 2232, 2234, 2236, 2238, 2240, which may be implemented as turning screws having variable electrical and mechanical length, may be configured to reduce or otherwise offset the amount of coupling (mostly capacitive) between a corresponding pair of adjacent resonator heads by adding a selected amount of inductive coupling; the respective lengths of the conductive posts 2232, 2234, 2236, 2238, 2240 may control the amount of coupling.

[0169] In some embodiments, each of the conductive posts 2232, 2234, 2236, 2238, 2240 may be supported by a corresponding support structure in the filter 2100 that is connected to the top ground plane (filter cover). Furthermore, a given support structure may support more than one conductive post. For example, the first and second conductive posts 2232, 2234 may be supported by a seventh support structure 2242. The third and fifth conductive posts 2236, 2240 may be supported by the sixth support structure 2230, and the fourth conductive post 2238 may be supported by the fifth support structure 2228. In one or more embodiments, each of the support structures 2228, 2230, 2242 are formed of a conductive material (e.g., metal).

[0170] FIG. 23 is a graph depicting frequency response characteristics of the illustrative singlequadruplet filter 2100 shown in FIGS. 22A and 22B, according to one or more embodiments. Referring to FIG. 23, the filter 2100 has a passband 1302 from about 700 MHz - 735 MHz, a first transmission zero 2304 above the passband at about 742 MHz, a second transmission zero 2306 above the passband at about 748 MHz, and a third transmission zero 2308 above the passband at about 770 MHz.

[0171] Various combinations of the inventive concept may be integrated to form more complex and real filter solutions. By way of example only and without limitation, FIG. 24 is a schematic diagram depicting an illustrative filter 2400 including two mixed couplings, according to one or more embodiments of the inventive concept. The filter 2400 aims to realize a configuration which is less sensitive to manufacturing tolerances compared to the filter 2100 shown in FIG. 21. This may be achieved by rearranging the resonators to reduce the capacitive coupling value in the FDC 2114 (see FIG. 21) and thereby increase the gap between adjacent resonators.

[0172] Referring to FIG. 24, the filter 2400 may be configured in a manner consistent with the illustrative filter 2100 shown in FIG. 21, except that the first cross -coupling between the first and third resonators 2102, 2106 has been removed and a second FDC 2402 has been added between the second and fourth resonators 2104, 2108. The second FDC 2402, like the first FDC 2114 between the first and fourth resonators 2102, 2108, may be implemented as a parallel LC circuit, with the capacitance C2 implemented using the capacitive coupling between respective folded resonator heads of the second and fourth resonators 2104, 2108, and the inductance L2 implemented using a conductive ridge between the second and fourth resonators 2104, 2108. Due to the parallel LC circuit configurations, the first and second FDCs 2114, 2402 may be referred to as mixed couplings.

[0173] The filter 2400 may also include one or more additional resonators 2404, 2406 and 2408. For example, a common resonator (CR) 2404 may be connected between the first input / output port 2110 and the first resonator 2102 of the quadruplet, and first resonators 2406 and 2408 of other triplet or quadruplet filters, respectively, which may be coupled (e.g., via corresponding main couplings) to the common resonator 2404. The common resonator 2404 may be used to couple an input RF signal supplied to the first input / output port 2110 to multiple signal paths in the filter 2100, as may be used in a multiplexer configuration.

[0174] FIG. 25 is a top plan view depicting at least a portion of a physical realization of the illustrative filter 2400 of FIG. 24, according to one or more embodiments of the inventive concept. Referring to FIG. 25, the filter 2400 includes the first resonator 2102, the second resonator 2104, the third resonator 2106, the fourth resonator 2108, the common resonator 2404, and one of the resonators 2406 forming an additional triplet or quadruplet filter coupled to the common resonator 2404. The first resonator 2102 includes a first resonator head 2502, the second resonator 2104 includes a second resonator head 2504, the third resonator 2106 includes a third resonator head 2506, and the fourth resonator 2108 includes a fourth resonator head 2508. Each of the first through fourth resonator heads 2502, 2504, 2506, 2508 may be implemented using the folded resonator head configuration consistent with the folded resonator head design shown in FIGS. 15A and 15B, although embodiments are not limited to any specific shape and / or dimensions of the folded resonator head.

[0175] As shown in FIG. 24, the filter 2400 includes a first main coupling between the first and second resonators 2102, 2104, a second main coupling between the second and third resonators 2104, 2106, and a third main coupling between the third and fourth resonators 2106, 2108. Each of the first through third main couplings may be implemented as inductive couplings. With continued reference to FIG. 25, the first main coupling may comprise a first conductive ridge 2510 arranged along a base of filter housing 2511, within which the resonators 2102, 2104, 2106, 2108, 2404, 2406 are disposed, and extending vertically between support structures on which the adjacent first and second resonator heads 2502, 2504 are supported. Similarly, the second main coupling may comprise a second conductive ridge 2512 arranged along the base of filter housing 2511 and extending vertically between support structures on which the adjacent second and third resonator heads 2504, 2506 are supported, and the third main coupling may comprise a third conductive ridge 2514 arranged along the base of filter housing 2511 and extending vertically between support structures on which the adjacent third and fourth resonator heads 2506, 2508 are supported. The amount of inductive coupling provided by the first through third conductive ridges 2510, 2512, 2514 will be a function of the shape and / or dimensions of the respective conductive ridges 2510, 2512, 2514. Furthermore, it is to be appreciated that the conductive ridges 2510, 2512, 2514 need not be straight, but may, for example, be curved, as in the case of the second conductive ridge 2512 in this example embodiment. The curved arrangement of the second conductive ridge 2512 may facilitate manufacturing of the filter 2400 and may also allow for an increase in inductive coupling between the second and third resonators 2104, 2106 if needed.

[0176] Where the amount of capacitive coupling between adjacent pairs of resonators forming the main couplings is too high, the filter 2400 may include conductive posts between the respective adjacent pairs of resonators to reduce the amount of capacitive coupling, consistent to the conductive posts previously described in conjunction with the filter 2100 shown in FIGS. 22A and 22B. For example, the filter 2400 may include a first conductive post 2516 disposed between the first and second resonator heads 2502, 2504, a second conductive post 2518 disposed between the second and third resonator heads 2504, 2506, and a third conductive post 2520 disposed between the third and fourth resonator heads 2506, 2508. Each of the conductive posts 2516, 2518, 2520 may be implemented as turning screws having a variable electrical and mechanical length.

[0177] The filter 2400 further includes first and second FDCs 2114, 2402 (see FIG. 24), which may be mixed couplings. The first FDC 2114 includes a first capacitance (Cl in FIG. 24), implemented as capacitive coupling between the first and fourth folded resonator heads 2502, 2508, and a first inductance (LI in FIG. 24), which may be implemented using a fourth conductive ridge 2522 disposed between the first and fourth resonators 2102, 2108. The fourth conductive ridge 2522 may be arranged perpendicular to an imaginary line drawn between centers of the first and fourth resonator heads 2502, 2508 for providing weak inductive coupling. If the amount of capacitive coupling between the first and fourth resonators 2102, 2108 is too high, a fourth conductive post 2524 may be disposed between the first and fourth resonators 2102, 2108, in a manner consistent with the conductive post 2238 disposed between the first and fourth resonators 2102, 2108 in the filter 2100 of FIGS. 22A and 22B.

[0178] The second FDC 2402 includes a second capacitance (C2 in FIG. 24), implemented as capacitive coupling between the second and fourth folded resonator heads 2504, 2508, and a second inductance (L2 in FIG. 24), which may be implemented using a fifth conductive ridge 2526 disposed between the second and fourth resonators 2104, 2108. The fifth conductive ridge 2526 may be arranged parallel to an imaginary line drawn between centers of the second and fourth resonator heads 2504, 2508 for providing stronger inductive coupling. If the amount of capacitive coupling between the second and fourth resonators 2104, 2108 is too high, a fifth conductive post 2528 may be disposed between the second and fourth resonators 2104, 2108.

[0179] FIG. 26 is a graph illustrating simulated frequency response characteristics of the illustrative filter 2400 shown in FIG. 25 compared to simulated frequency response characteristics of a classical quadruplet filter topology, according to one or more embodiments. In FIG. 26, waveform 2602 depicts frequency response characteristics of the illustrative filter 2400 and waveform 2604 depicts frequency response characteristics of a filter utilizing a classical quadruplet topology. Referring to FIG. 26, both the filter 2400 and the classical quadruplet filter share a similar passband 2606 from about 700 MHz - 735 MHz. However, while the classical quadruplet filter (waveform 2604) has two transmission zeros above the passband 2606 (at about 760 MHz and 780 MHz), the filter 2400 (waveform 2602) has three transmission zeros above the passband 2606; a first transmission zero 2610 at about 750 MHz, a second transmission zero 2612 at about 757 MHz, and a third transmission zero 2614 at about795 MHz. The three transmission zeros 2610, 2612, 2614 above the passband 2606 provide increased selectivity compared to the classical quadruplet filter topology.

[0180] A similar frequency response characteristic may be achieved by changing a sequence of the resonators in the filter layout. By way of example only and without limitation, FIG. 27 is a top plan view depicting at least a portion of an illustrative filter 2700, according to another embodiment of the inventive concept. The filter 2700 is a physical realization of the illustrative filter topology shown in FIG. 24 and is consistent with the filter 2400 shown in FIG. 25, except that a sequence of the resonators forming the quadruplet in the filter 2700 is changed compared to the filter 2400 of FIG. 25. Specifically, the sequence of the second and fourth resonators 2104, 2108 has been swapped in the filter 2700. The filter 2700, like the filter 2400 of FIG. 25, includes a first FDC 2114 disposed between the first and fourth resonators 2102, 2108 and a second FDC 2402 disposed between the second and fourth resonators 2104, 2108 (see FIG. 24); only the position of the second and fourth resonators 2104, 2108 in the filter housing 2711 has changed.

[0181] FIG. 28 is a graph illustrating simulated frequency response characteristics of the illustrative filter 2700 shown in FIG. 27 compared to simulated frequency response characteristics of a classical quadruplet filter topology, according to one or more embodiments. In FIG. 28, waveform 2802 depicts frequency response characteristics of the illustrative filter 2700 and waveform 2804 depicts frequency response characteristics of a filter utilizing a classical quadruplet topology. As shown in FIG. 28, the three transmission zeros above the passband provided by the filter 2700 result in improved selectivity compared to the classical quadruplet filter topology. In this example, at 751.4 MHz, the filter 2700 achieves 63 dB of attenuation. In comparison, the classical quadruplet topology achieves the same level of attenuation (63 dB) at 757.5 MHz. Thus, the filter 2700 achieves greater than a 6 MHz increase in rejection using the FDC topology according to embodiments of the inventive concept.

[0182] In a multiplexer filter application, as shown, by way of example only, in FIGS. 24, 25 and 27, the attenuation shape of the filter can deteriorate if spurious couplings between first resonators of transmit (TX) and receive (RX) sections are not properly managed during the integration of the section under analysis within the multiplexer. A standard approach to cancel the contribution of spurious couplings is through the use of a suspended stripline that introducesa coupling whose sign is opposite the spurious coupling. One or more embodiments of the inventive concept provide an approach which eliminates the need for the suspended stripline, thereby reducing cost.

[0183] FIG. 29A is a top plan view depicting at least a portion of an illustrative fdter 2900, according to embodiments of the inventive concept. FIG. 29B is an enlarged view of a region A of the filter 2900 shown in FIG. 29A, with a resonator head of the common resonator removed and including isolation ridges, according to one or more embodiments. Referring to FIGS. 29A and 29B, the filter 2900, which may be used in a multiplexer application, includes a common resonator 2902 comprising a folded resonator head 2904 and a support structure 2906 supporting the resonator head 2904. The resonator head 2904 and the support structure 2906 may be implemented, for example, in a manner consistent with the folded resonator head 1602 and support structure 1604 shown in FIG. 16.

[0184] As shown in FIG. 29B, the filter 2900 may include first and second isolation ridges 2910 and 2912, respectively, arranged along a base of the filter housing and extending vertically between the support structure 2906 and corresponding proximate support structures 2914 and 2916 in the filter 2900. The first and second isolation ridges 2910, 2912 are configured to cancel spurious coupling. By confining the magnetic field of the first resonator of the section, the first and second isolation ridges 2910, 2912 may cancel or reduce spurious coupling by avoiding propagation of the magnetic field towards the first resonator of the filter section employing frequency dependent coupling according to embodiments of the inventive concept.

[0185] FIG. 30 is a graph depicting frequency response of the filter 2900 before introducing corrections for spurious coupling, according to one or more embodiments. Waveform 2902 represents the output of the filter 2900. It is not possible to push the circled portion 2904 of the filter output response waveform 2902 below the attenuation limit by using tuning elements only on resonators and desired couplings. FIG. 30 shows suppression of spurious coupling which may be achieved using the isolation ridges shown in FIG. 29B.

[0186] It is to be appreciated that the folded resonator head configuration may offer less than an optimal solution to obtain a resonant cavity with a high Q-factor, if needed. However, this apparent drawback is at least partially recovered by realizing an FDC filter topology according to embodiments of the inventive concept that enhances selectivity without adding resonators. Akey advantage of this approach is its compactness, which makes this solution well-suited for BTS filters, where insertion loss is not a critical parameter, and for some antenna filters where the transmit (TX) passband is far from the receive (RX) passband. In one or more embodiments, a separation range comparable to the TX / RX passbands width is suitable. It is also possible to realize TX zeros below the passband without using any probes, although in that case the distance among resonators would decrease. By way of example only, FIGS. 31 A and 3 IB are graphs depicting insertion loss of example filters incorporating frequency-dependent coupling, according to embodiments of the inventive concept.

[0187] Although some specific embodiments of the present disclosure have been described in detail through examples, those skilled in the art should understand that the above examples are only for illustration rather than for limiting the scope of the present disclosure. The embodiments disclosed herein can be combined arbitrarily without departing from the spirit and scope of the present disclosure. Those skilled in the art should also understand that various modifications can be made to the embodiments without departing from the scope and spirit of the present disclosure. The scope of the present disclosure is defined by the attached claims.

Claims

CLAIMSWhat is claimed is:

1. A radio frequency, RF, fdter, comprising: a fdter housing; a plurality of hybridly coupled resonators configured in a cascade arrangement in the filter housing, each of the plurality of hybridly coupled resonators including a conductive support structure extending vertically from a base of the filter housing and a folded resonator head supported on and electrically connected to the conductive support structure; and a three-dimensional structure for providing inductive coupling in the RF filter, the three- dimensional structure comprising at least one conductive ridge extending vertically from the base of the filter housing between and electrically connected to the conductive support structures of adjacent resonators of the plurality of hybridly coupled resonators, wherein the folded resonator head of each of the plurality of hybridly coupled resonators is configured to provide capacitive coupling in the RF filter.

2. The RF filter according to claim 1, further comprising at least one conductive post disposed between a pair of adjacent resonators among the plurality of hybridly coupled resonators, the conductive post configured to reduce an amount of capacitive coupling between folded resonator heads of the pair of adjacent resonators.

3. The RF filter according to either claim 1 or claim 2, wherein an amount of capacitive coupling in the RF filter is configured as a function of one or more dimensions of the respective folded resonator heads of the plurality of hybridly coupled resonators and / or a proximity of the at least one conductive post to the folded resonator heads of the pair of adjacent resonators.

4. The RF filter according to any one of the previous claims, wherein an amount of inductive coupling in the RF filter is configured as a function of a vertical height above the base of the filter housing of the at least one conductive ridge.

5. The RF filter according to any one of the previous claims, wherein each of at least a subset of the plurality of hybridly coupled resonators are disposed at a vertex of a polygon.

6. The RF filter according to any one of the previous claims, wherein the conductive support structure of each of at least a subset of the plurality of hybridly coupled resonators is configured having a constant diameter.

7. The RF filter according to any one of the previous claims, wherein the conductive support structure of each of at least a subset of the plurality of hybridly coupled resonators is configured having a diameter that varies as a function of a vertical distance from the base of the filter housing.

8. The RF filter according to any one of the previous claims, wherein the plurality of hybridly coupled resonators and the three-dimensional structure are configured to provide M transmission zeros, where N is an integer greater than one representing a number of hybridly coupled resonators in the RF filter.

9. A radio frequency, RF, filter, comprising: a filter housing; and a plurality of hybridly coupled resonators, each of the plurality of hybridly coupled resonators including a conductive support structure extending vertically from a base of the filter housing and a folded resonator head supported on and electrically connected to the conductive support structure, wherein at least one cross-coupling between non-adjacent resonators among the plurality of hybridly coupled resonators comprises a frequency-dependent coupling, FDC, the FDC configured as a parallel inductor-capacitor, LC, circuit, and wherein a capacitance of the FDC is configured as a function of a proximity of a pair of adjacent folded resonator heads among the respective folded resonator heads of the plurality of hybridly coupled resonators.

10. The RF filter according to claim 9, further comprising at least one conductive ridge extending vertically from the base of the filter housing between and electrically connected to the conductive support structures of adjacent resonators of the plurality of hybridly coupledresonators, the at least one conductive ridge configured to provide inductive coupling in the RF filter.

11. The RF filter according to either claim 9 or claim 10, wherein the plurality of hybridly coupled resonators and the at least one conductive ridge are configured to provide iV-1 transmission zeros, where N is an integer greater than one representing a number of hybridly coupled resonators in the RF filter.

12. The RF filter according to any one of claims 9 to 11, in particular claim 10 , wherein a vertical height above the base of the filter housing of the at least one conductive ridge is configured to provide a prescribed amount of inductive coupling in the RF filter.

13. The RF filter according to any one of claims 9 to 12, further comprising at least one conductive post disposed between a pair of adjacent resonators among the plurality of hybridly coupled resonators, the conductive post configured to reduce an amount of capacitive coupling between folded resonator heads of the pair of adjacent resonators.

14. The RF filter according to any one of claims 9 to 13, wherein each of at least a subset of the plurality of hybridly coupled resonators are disposed at a vertex of a polygon.

15. A radio frequency, RF, filter incorporating frequency-dependent coupling, FDC, the RF filter comprising: a plurality of resonators, each of the plurality of resonators including a folded resonator head and a conductive support structure supporting the folded resonator head; and at least one conductive ridge extending vertically between and electrically connected to the conductive support structures of adjacent resonators of the plurality of resonators, wherein an inductance in the FDC is provided by the at least one conductive ridge, and wherein a capacitance in the FDC is provided by capacitive coupling between adjacent folded resonator heads among the plurality of resonators.

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