Acoustic filter with improved reflectivity

Abstract

A filter circuit comprises a main operating unit (MU) arranged in a series signal line, the main operating unit providing most of the filtering functions of the filter circuit. A last microacoustic series resonator (RL) as the last element of the main operating unit in the series signal line S ) tends to excite spurious modes caused by the final series capacitor (CE) connected between the last element and the antenna terminal (AT) S )inhibition.

Description

[0001] describe

[0002] The present invention provides a method for acoustic filters with improved reflectivity, which method allows efficient multiplexing of these acoustic filters that may be affected by spurious modes.

[0003] In so-called TC-SAW filters (temperature compensated SAW filters), the resonators are usually covered by a dielectric layer to reduce the sensitivity of the filter to temperature changes. In addition to the main mode propagating in such filters, a stack with such a structure also propagates another spurious mode. The excitation of this secondary mode (also called plate mode) depends on the characteristics of the dielectric layer. In some cases, in order to meet the specifications of the temperature coefficient of the filter, it is necessary to deposit a thick dielectric layer, which causes a strong excitation of the plate mode.

[0004] The location of these interference modes may conflict with other LTE bands. As more applications use new band combinations, the selectivity and reflectivity required from a single filter becomes increasingly challenging. As a result, the presence of spurious modes becomes an increasing problem.

[0005] One known way to deal with spurious modes is to move them out of the frequency band they interfere in. This can be done by modifying the resonator that causes the unwanted mode.

[0006] However, the displacement of interference modes impairs the performance of the resonator that induced them. The excitation of interference modes is related to the quality factor of the resonator. Reducing the excitation of these modes leads to a decrease in the quality factor, thereby reducing performance.

[0007] Furthermore, additional poles can be provided by inserting or modifying the shunts to create resonances at the frequencies of the spurious modes. Due to the finite Q factors of practical passive components, such shunt resonances are not particularly selective in dealing with spurious modes. In other words, a pole in the transfer function will usually have a negative impact on other frequency bands as well.

[0008] It is therefore an object of the present invention to provide a filter circuit that suppresses spurious modes, particularly spurious modes (ie, plate modes, bulk waves, etc.) in multi-layer stacked acoustic wave filters and resonators (SAW / BAW).

[0009] This and other problems are solved by a filter circuit according to claim 1 .

[0010] Further embodiments and advantageous features can be taken from the further claims.

[0011] The filter circuit providing the passband includes a series signal line connecting the first terminal and the antenna terminal.

[0012] The main operating unit is arranged in the series signal line and provides most of the filtering functions of the filter circuit, thereby creating a passband. The microacoustic series resonator is the last element arranged in the series signal line and is part of the main operating unit. Finally, the final series capacitor is connected by circuit between the last element and the antenna terminal as an additional element.

[0013] Optionally, one or more parallel branches may connect corresponding nodes in the series signal line to a ground terminal. A parallel resonator may be arranged in one of the parallel branches.

[0014] The proposed filter circuit successfully suppresses spurious modes (eg, plate modes and bulk modes) that may be excited due to the stacked design of the final resonator.

[0015] The inventors have found that the main disturbing effect of the spurious mode is produced by the last stage of the filter circuit (which may be the last element next to the antenna terminals). In the present case, the last series resonator is prone to produce this mode.

[0016] These modes excited at frequencies far from the main mode frequency of the resonator have a negative impact on the reflectivity at the antenna terminals and thus on the selectivity of the filter circuit. The key to good multiplexing is that the reflectivity at the antenna does not have any spikes from any unwanted spurious modes. This objective is achieved by the proposed filter circuit, which ends with a final series capacitor right next to the antenna terminals.

[0017] The filter circuit can be formed on a substrate that acts only as a carrier, or provides a functional material like a piezoelectric layer. According to one embodiment, the final series capacitor can be an external discrete component. Alternatively, the final series capacitor can be an integrated capacitor on the substrate. This can be a resonator that functions only with its static capacitance and has a resonant frequency outside the passband.

[0018] The last element in the signal line next to the final capacitor is the last series resonator. According to a further embodiment, a further node is arranged in the series signal line between the last element and the final series capacitor. A further parallel branch is circuit-connected between the further node and the ground terminal as a further additional element and comprises an impedance element selected from a further parallel resonator, a parallel coil and a parallel capacitor.

[0019] A parallel capacitor may be arranged in a further parallel branch, and a series coil may be connected in parallel with the final series capacitor.

[0020] According to another embodiment, the phase shift circuit is connected between the main operating unit and the antenna terminal. The circuit comprises a last series resonator, a shunt coil arranged in a last shunt branch and a final series capacitance in the series signal line.

[0021] According to yet another embodiment, the main operating unit ends with a last node, and the last parallel branch is connected to the last node. The parallel microacoustic resonator is arranged in the parallel branch. The last node is directly connected to the last series capacitor, and the last series capacitor is directly connected to the antenna terminal.

[0022] The main operating unit, which provides most of the filtering functions, may include a ladder arrangement of series and parallel resonators. The resonators in the ladder arrangement may include SAW or BAW resonators.

[0023] Alternatively, the main operating unit may be embodied in any other technology.In one example, the main operating unit comprises a DMS filter connected in series to a microacoustic series resonator.

[0024] In all embodiments, the last series resonator may be a temperature compensated thin film SAW resonator or a BAW resonator.

[0025] In an advantageous complex application, a first operating unit comprising the proposed filter circuit and a second microacoustic operating unit selected from a passband filter circuit or a notch filter are connected to the same antenna terminal in a permanent or switchable manner. The first or second operating unit comprises a microacoustic resonator in close proximity to the antenna terminal. Since this resonator in close proximity to the antenna terminal is prone to generate spurious modes in the operating band of the respective other unit, a final series capacitor is included to suppress this spurious mode.

[0026] Hereinafter, the present invention is explained in more detail with reference to specific embodiments and related schematic drawings.

[0027] Figure 1 The admittance curve of the filter circuit with the main mode and the plate mode as spurious modes is shown.

[0028] Figure 2 At A the reflectivity of the antenna around the frequency of the spurious mode is shown, and at B the passband of another operating unit that lies within the frequency range of the spurious mode is shown.

[0029] Figure 3 A first embodiment of the invention is shown, which has a ladder-shaped arrangement of resonators as the main operating unit.

[0030] Figure 4 The general structure of a first embodiment is shown, wherein the main operating unit can be any filter circuit ending with a last series resonator on the antenna side of the circuit.

[0031] Figure 5 Another general structure of a further first embodiment is shown, wherein the main operating unit can be any filter circuit ending with a last parallel branch and a parallel resonator on the antenna side of the circuit.

[0032] Figure 6 Another embodiment is shown, in which Figure 3 The main operating unit selected similarly for an embodiment of the present invention may be any filter circuit ending with a last parallel branch and a parallel impedance element on the antenna side of the circuit.

[0033] Figure 7 An embodiment is shown in which the main operating unit is a ladder arrangement of resonators ending with a last parallel branch and a parallel resonator arranged in the arrangement.

[0034] Figure 8 A to Figure 8 D shows an embodiment where the main operating unit is a ladder arrangement of resonators ending with a last series resonator and different additional elements are added including a last series capacitor.

[0035] Fig. 9 An embodiment is shown in which the first operating unit and the second operating unit are coupled to the same antenna terminal and one of the first operating unit and the second operating unit ends with a last series resonator.

[0036] Figure 1 : In this example, the B1 / B3 quad multiplexer is connected to the B40 filter at the same antenna. The spurious modes originating from the B3 TX filter negatively affect the performance of the B40 filter. Figure 2 As shown in A, a severe spike in the antenna reflectivity can be seen. Figure 2 This results in a dip in the passband of the B40 filter as shown in Figure B. The solid line shows the B3TX filter affected by the spurious mode, while the dashed line is the same filter without the spurious mode effect.

[0037] According to the present invention, the real challenge is to find a way to completely suppress the spurious modes, rather than trying to move them out of the main band. Figure 1 In the example shown, a resonator designed to have its plate mode in Band 3 (uplink) has its plate mode in Band 1 (downlink). If such a resonator is used for a multiplexed application (Band 1 + Band 3), the overall performance will be significantly affected due to the general cross isolation between the two operating bands.

[0038] One could try to move the plate mode in frequency, but this would result in violating other constraints (eg, in the case of carrier aggregation with Band 40).

[0039] From a circuit level point of view, the nature of the unwanted modes is fairly irrelevant. Here, the focus is more on how the unwanted modes from filter "A" can negatively impact the second filter "B" connected to the same antenna terminals. The answer to this question can be found in the reflectivity of the antenna, as Figure 2 A. Poor reflectivity in a particular frequency range (e.g., spikes in unwanted modes) means that there is energy leakage in the other filter. This effect can be thought of as a very significant frequency localized loss caused by one filter to the other.

[0040] As will become clearer after the following sections, the purpose of the present invention is not only to cover unwanted modes from SAW filters, but more generally, seeks to cover a wider range of situations (e.g. spurious modes in BAW filters or other filtering technologies such as DMS filters when connected to microacoustic resonator circuits).

[0041] The idea comes from the observation that the spikes in reflectivity are mainly caused by resonators close to the antenna. When moving along the ladder topology from the antenna to the first terminal (i.e., the other port), the effects of the other resonators quickly disappear. This can be mathematically demonstrated by separating each stage of the filter (e.g., parallel / series resonator + series / parallel resonator) into a cascade of multiple 2-port networks.

[0042] Since the spike is only / mainly caused by the last stage on the antenna side of the filter, the idea is to add an additional stage or replace one or two elements of the existing last stage using one of the following solutions:

[0043] - add a passive capacitor (can be a series or parallel resonator), and keep the original resonator (if its unwanted mode does not cause any trouble),

[0044] - add a stage with two passive capacitors (a series capacitor and a parallel capacitor) and keep the original resonator (if its unwanted modes do not create any trouble),

[0045] - Add passive capacitors and resonators that don't create any trouble,

[0046] -Replace the resonator causing the unwanted mode with a passive capacitor. This can be a series or parallel resonator, which must be replaced by a corresponding series or parallel capacitor.

[0047] Figure 3 A filter circuit is shown, in which the main operating unit MU is the original filter unit, and its last series resonator RL SLocated on the antenna terminal AT side. The main operating unit MU includes a trapezoidal arrangement of series resonators RS arranged in the series signal line SL between the antenna terminal AT and the first terminal T1. A parallel branch with a parallel resonator RP arranged therein connects a node N in the series signal line SL to a ground terminal. A final series capacitor CE S is added and inserted into the last series resonator RL S and the antenna terminal AT in a filter circuit.

[0048] Figure 4 A more general embodiment using the same idea is shown. The main operating unit MU providing most of the passband filtering functionality can be presented with any technology that can provide the target passband. Preferably, acoustic filters such as SAW, BAW or DMS are used. However, LC filters made of discrete or integrated passive components are also possible. The main operating unit MU if connected to the last series resonator RL S If the circuit is connected in series, it may cause spikes, which will interfere with other frequency bands. Therefore, here we also connect the antenna terminal AT and the last series resonator RL S The circuit is connected in series with the final capacitor CE S To suppress spurious modes.

[0049] Figure 5 Another embodiment is shown, in which any master operating unit MU ends with a last node NL and has a last parallel resonator RL P The parallel branch of is connected to the last node. The last parallel resonator RL P It is easy to excite the spurious mode, which is finally connected to the series capacitor CE. S inhibition.

[0050] according to Figure 6 The embodiment shown uses a series resonator RL S The same main operating unit MU ends. A parallel impedance element IE is added as a new shunt element in the filter circuit and a final series capacitor CE is added between the last series resonator and the antenna terminal S The final series capacitance added can be an external capacitor or a capacitor in a filter circuit, such as a resonator with a resonance frequency far from the used frequency band.

[0051] In this way, the following effects can be achieved:

[0052] - The matching of the filter is practically unchanged. The same topology can be used and only one more stage is required. This may result in only small losses due to the higher number of components, but at the same time there is more freedom in the trade-off between selectivity / in-band attenuation. Adding stages generally helps to improve selectivity, but on the other hand increases losses (and therefore insertion losses). If the selectivity level achieved before the introduction of this additional stage was sufficient, the filter can be reoptimized to restore the small additional insertion losses. The small additional losses can in principle be compensated by redesigning the filter.

[0053] -Finally the series resonator RL S (and its plate mode) is no longer the last element of the filter circuit. At the antenna terminal AT, the incident wave will first see the final series capacitor CE S , so they do not excite any plate modes.

[0054] It is worth mentioning that the plate mode will still be excited, but its intensity will be less pronounced because at the end of the series resonator RL S There is an additional stage between the antenna.

[0055] Furthermore, the proposed method has a negative impact on other frequency bands that can be used with the filter circuit and that can be connected to the same antenna terminal AT.

[0056] Again, for clarity, the series connection elements may be external or may be integrated directly on the chip. Figure 7 In the embodiment of the invention, the main operating unit MU ends with a parallel branch on the antenna side and a parallel resonator arranged therein. The final series capacitance is denoted as a capacitor, since its main purpose is to act as a capacitor. However, it is also possible to use an additional series resonator instead of a capacitor. This method is effective in suppressing spurious modes from the last series resonator, but also reduces the strength of the parallel connection.

[0057] like Figure 8 Schematically shown, in the case of a main operating unit ending with a final series resonator at the antenna, different damping stages can be added:

[0058] A) can be connected to the main operating unit MU and the final series capacitor CE S An additional parallel resonator RP is introduced between A In this way, the last series resonator causing the spurious mode is no longer closest to the antenna, its spurious mode is attenuated, and the filter maintains the same ladder topology, just with one more stage.

[0059] B) In a variation of case A), an additional parallel capacitor CP is used A Instead of adding a parallel resonator RP AThis is useful when the last resonator, the last series and the last shunt resonator all cause unwanted mode problems in the counter band. Other more complex networks are also allowed. In these topologies, two (or more) elements are added in series to the filter to cause a phase shift of the filter at the antenna terminals.

[0060] C) In this case, the main operating unit MU is connected to the final series capacitor CE S Introduce additional parallel coil IP between A If we consider the last series resonator RS L The static capacitance of the device realizes a high-pass T phase shifter (series capacitor, shunt coil, series capacitor).

[0061] D) In ​​the last case, including the additional series coil IS A The final series capacitor CE connected in parallel S The parallel resonant circuit of the filter creates a resonance in the frequency range of the unwanted spurious mode. A final series capacitor close to the filter helps to restore the match.

[0062] Fig. 9 An embodiment is shown in which the first operating unit OU1 and the second operating unit OU2 are coupled to the same antenna terminal, and one of the first operating unit and the second operating unit is connected with the last series resonator RL S The first operation unit OU can be as above according to Figure 4 The second operating unit OU2 may be any filter or notch circuit that operates in a frequency band consistent with the frequency of the spurious mode that the last series resonator is easily excited. S , this mode is suppressed and the interference in the operating frequency band of the second operating unit is minimized.

[0063] All presented filter circuits show the ability to suppress spurious modes by replacing the components (or even two components if necessary) of the last stage of a ladder topology filter or another filtering technique / topology with passive components that do not present any spurious modes.

[0064] exist Fig. 9 The best application of the presented filter circuit in a system is shown in FIG.

[0065] A system consists of at least two different acoustic operating units (eg BAW- / SAW filters, notch filters), which are temporarily or permanently connected in a system (eg module).

[0066] - One operating unit generates spurious modes that affect the performance of a second unit (eg, filter, duplexer of a quad multiplexer) in a critical frequency range.

[0067] - A spurious mode is very critical if it is excited in an acoustic operating unit close to the connection of two operating units (eg antenna terminals in a quad multiplexer).

[0068] - Progressive treatment of spurious modes may require reworking of the electromagnetic environment of the last subunit of the main operating unit (last IDT in case of SAW filter) or / and the connection point between the last subunit and the two operating units.

[0069] - Rework may include partial replacement (parallel capacitors, resonators with different spacing) or complete replacement of the last subunit with alternative electrical components (e.g. coils, capacitors, damping circuits, etc.) that typically do not generate any spurious modes or at least are not within the critical frequency range.

[0070] The invention has been explained with reference to a limited number of embodiments, but the invention is not limited to those specific embodiments. The full scope of the invention is defined by the claims.

[0071] List of terms and reference symbols used

[0072] AT Antenna Terminal

[0073] CES Final series capacitor

[0074] CPA Additional parallel capacitance

[0075] EL Final Components

[0076] IPA additional parallel coil

[0077] ISA Additional series coil

[0078] MU Main Operation Unit

[0079] N-Node

[0080] NL Last Node

[0081] OU1, OU2 operating units

[0082] RL P The final parallel resonator

[0083] RL S The final series resonator

[0084] RP Parallel Resonator

[0085] RP AAdditional parallel resonator

[0086] RS Series Resonator

[0087] SLS serial signal line

[0088] T, T' terminal

[0089] T1 first terminal

Claims

1. A filter circuit, providing a passband, the filter circuit comprising: -Serial signal line (SL S ), connecting the first terminal (T1) and the antenna terminal (AT), a main operating unit (MU), arranged in the series signal line, providing the majority of the filtering functions of the filter circuit, - Microacoustic Last Series Resonator (RL S ), which is the last element in the series signal line and is part of the main operating unit, - Final series capacitance (CE S ), being circuit-connected between the last element and the antenna terminal, wherein the main operating unit ends with a last node, and a last parallel branch is connected to the last node, and a parallel microacoustic resonator is arranged in the last parallel branch, Wherein the last node is directly connected to the final series capacitor, and the final series capacitor is directly connected to the antenna terminal.

2. The filter circuit according to claim 1, in, The filter circuit is formed on a substrate, Therein, the final series capacitor is presented as a discrete component outside the substrate or as an integrated capacitor on the substrate, or is a resonator that acts only with its static capacitance and has a resonance frequency outside the passband.

3. The filter circuit according to claim 1 or 2, -in, A further node (N) is arranged in the series signal line between the last element and the final series capacitor, - wherein a further parallel branch is electrically connected between the further node and a ground terminal, the further parallel branch comprising an impedance element (IE) selected from a further parallel resonator, a parallel coil and a parallel capacitor.

4. The filter circuit according to claim 3, in, Additional parallel capacitance (CP A ) is arranged in the further parallel branch, Among them, the additional series coil (IS A ) and the final series capacitance (CE S ) are connected in parallel circuit.

5. The filter circuit according to claim 1 or 2, in, A phase shift circuit is connected between the main operating unit (MU) and the antenna terminal, the phase shift circuit comprising an additional parallel coil (IP) arranged in the last parallel branch. A ) and the final series capacitance in the series signal line.

6. The filter circuit according to claim 1 or 2, in, The main operating unit includes a ladder arrangement of series and parallel resonators.

7. The filter circuit according to claim 6, in, The ladder-arranged resonators include SAW resonators or BAW resonators.

8. The filter circuit according to claim 1 or 2, in, The main operating unit includes a DMS filter connected in series to the last series resonator of the microacoustics.

9. The filter circuit according to claim 1 or 2, in, The last series resonator is a temperature compensated thin film SAW resonator or a BAW resonator.

10. The filter circuit according to claim 1 or 2, in, a first operating unit including the filter circuit and a second operating unit selected from a passband filter circuit or a notch filter are connected to the same antenna terminal in a permanent or switchable manner, wherein one of the first operating unit or the second operating unit includes a microacoustic resonator adjacent to the antenna terminal, wherein the microacoustic resonator immediately adjacent to the antenna terminal is likely to generate a spurious mode in an operating frequency band of the other of the first operating unit or the second operating unit, Wherein, the final series capacitor suppresses the spurious mode.

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