Low ripple balanced high temperature superconducting bandpass filter based on three-section coupled-line cascade

Abstract

The application relates to the technical field of radio frequency microwave passive devices, and particularly discloses a low-ripple balanced high-temperature superconducting band-pass filter based on three-section coupled line cascades, which comprises a dielectric substrate, high-temperature superconducting film layers arranged on the upper and lower surfaces of the dielectric substrate and a high-temperature superconducting film grounding plate, a filter circuit is arranged on the high-temperature superconducting film layer, the filter circuit comprises an input feed line, an output feed line, a cascaded transmission network and a common-mode suppression network; the cascaded transmission network comprises N parallel coupled line units and N+1 parallel branch line units; the common-mode suppression network comprises a transverse transmission line and an open-circuit stub, the transverse transmission line is connected to the center positions of a first parallel branch line unit and a second parallel branch line unit at both ends, one end of the open-circuit stub is connected to the center position of an N+1 parallel branch line unit, and the other end extends outward, the structure can be flexibly expanded, can provide additional common-mode design freedom, and does not affect the differential-mode transmission characteristics.

Description

Technical Field

[0001] This application relates to the field of radio frequency microwave passive device technology, and in particular to a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines. Background Technology

[0002] With the development of wireless communication, radar detection, and satellite systems towards broadband, high sensitivity, and strong anti-interference capabilities, balanced circuits are widely used due to their inherent ability to suppress common-mode interference. Among them, balanced bandpass filters, as key components in communication systems, are required to achieve wideband suppression of common-mode (CM) signals while obtaining good differential-mode (DM) passband characteristics, in order to improve the system's signal-to-noise ratio and transmission stability.

[0003] Existing balanced bandpass filters are often implemented based on coupled-line structures, branch-line structures, or combinations thereof. However, to achieve broadband response and high common-mode rejection performance, such filters typically require the introduction of multiple stages of coupled lines, complex interleaved transmission paths, or additional notch filters into the structure. As the filter order and structural complexity increase, not only does the overall circuit size increase, but design freedom is also limited, making it difficult to simultaneously achieve low insertion loss in the differential-mode passband and a wide bandwidth in the common-mode stopband.

[0004] In addition, although high-temperature superconducting (HTS) materials can significantly reduce insertion loss, their extremely low surface resistance makes the structure very sensitive to small size changes. Traditional structures often cannot provide enough degrees of freedom to independently control the differential and common-mode responses while maintaining a compact structure. Summary of the Invention

[0005] This application aims to propose a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines, in order to at least solve the technical problem in the prior art that as the filter order and structural complexity increase, the overall circuit size of the bandpass filter will increase, and the design freedom will be limited, making it difficult to simultaneously achieve low insertion loss in the differential mode passband and wide bandwidth in the common mode stopband.

[0006] To achieve the above objectives, one aspect of this application provides a low-ripple balanced high-temperature superconducting bandpass filter based on a three-segment cascaded coupling line, comprising: Dielectric substrate; A high-temperature superconducting thin film layer is stacked on the upper surface of the dielectric substrate; A high-temperature superconducting thin film ground plane is stacked on the lower surface of the dielectric substrate; A filter circuit is constructed on the high-temperature superconducting thin film layer. The filter circuit includes a pair of input feed lines, a pair of output feed lines, a cascaded transmission network, and a common-mode rejection network. The cascaded transmission network includes N parallel coupling line units and N+1 parallel branch line units cascaded alternately along the signal transmission direction, where N is an integer greater than or equal to 2; the i-th parallel coupling line unit is connected between the i-th parallel branch line unit and the (i+1)-th parallel branch line unit, i = 1, ..., N; The common-mode suppression network includes a lateral transmission line and an open-circuit stub. The two ends of the lateral transmission line are respectively connected to the center positions of the first parallel branch line unit and the second parallel branch line unit along the length direction. One end of the open-circuit stub is connected to the center position of the (N+1)th parallel branch line unit along the length direction, and the other end extends outward away from the (N+1)th parallel branch line unit. The lateral transmission line constitutes a common-mode adjustment structure, and the open-circuit stub constitutes a final-stage common-mode suppression structure.

[0007] In some embodiments, the filter circuit is symmetrical about the central horizontal plane, each of the parallel coupling line units includes an upper coupling line and a lower coupling line that are symmetrical about the central horizontal plane, and each of the parallel branch line units is a resonant structure symmetrical about the central horizontal plane; The central horizontal plane is located between the two output feed lines and / or the two output feed lines, and is perpendicular to the plane formed by the projection of the high-temperature superconducting thin film layer along its thickness direction.

[0008] In some embodiments, the transverse transmission line is located between the first parallel branch line unit and the second parallel branch line unit, and coincides with the straight line formed by the projection of the central horizontal plane along the thickness direction of the high-temperature superconducting thin film layer.

[0009] In some embodiments, the electrical length of the parallel coupling line unit at the differential mode center frequency is one-quarter of the waveguide wavelength, and the electrical length of each of the parallel branch line units is one-half of the waveguide wavelength; and the equivalent electrical length of the transverse transmission line and the open stub is one-quarter of the waveguide wavelength or an odd multiple of the waveguide wavelength.

[0010] In some embodiments, under differential mode excitation, the potentials at both ends of the transverse transmission line are equal and equivalent to an open circuit due to the symmetry of the filter circuit; under common mode excitation, the transverse transmission line serves as a signal transmission path between the first parallel branch line unit and the second parallel branch line unit, and forms a common mode transmission zero in a predetermined frequency band. The differential mode excitation state is an input state in which electrical signals with equal amplitude and opposite phase are input through the pair of input feed lines, and the common mode excitation state is an input state in which electrical signals with equal amplitude and same phase are input through the pair of input feed lines.

[0011] In some embodiments, under the differential mode excitation, the connection point of the open stub is in a virtual ground state; under the common mode excitation, the open stub serves as a resonant unit loaded at the end of the N+1th parallel branch unit and forms at least one transmission zero within the common mode stopband to extend the common mode rejection bandwidth.

[0012] In some embodiments, when the filter is third-order, the cascaded transmission network includes a first parallel branch line unit, a first parallel coupling line unit, a second parallel branch line unit, a second parallel coupling line unit, a third parallel branch line unit, a third parallel coupling line unit, and a fourth parallel branch line unit, which are sequentially and alternately cascaded along the signal transmission direction. The transverse transmission line is connected between the first parallel branch line unit and the second parallel branch line unit, and the open-circuit stub is connected to the center of the fourth parallel branch line unit.

[0013] In some embodiments, the pair of input feed lines includes an upper input feed line and a lower input feed line, the pair of output feed lines includes an upper output feed line and a lower output feed line, the first parallel coupling line unit includes a first upper coupling line and a first lower coupling line, the second parallel coupling line unit includes a second upper coupling line and a second lower coupling line, and the third parallel coupling line unit includes a third upper coupling line and a third lower coupling line. Wherein, one end of the first upper coupling line is connected to the upper input feed line, and the other end is cascaded with the second upper coupling line; the end of the second upper coupling line away from the first input feed line is cascaded with the third upper coupling line; and the end of the third upper coupling line away from the second upper coupling line is connected to the upper output feed line. One end of the first lower coupling line is connected to the lower input feed line, and the other end is cascaded with the second lower coupling line. The end of the second lower coupling line away from the first input feed line is cascaded with the third lower coupling line. The end of the third lower coupling line away from the second lower coupling line is connected to the lower output feed line.

[0014] In some embodiments, the two ends of the first parallel branch line unit are respectively connected to the junctions of the upper input feed line and the first upper coupling line, and the lower input feed line and the first lower coupling line; The two ends of the second parallel branch line unit are respectively connected to the intersection of the first upper coupling line and the second upper coupling line, and the first lower coupling line and the second lower coupling line; The two ends of the third parallel branch line unit are respectively connected to the intersection of the second upper coupling line and the third upper coupling line, and the second lower coupling line and the third lower coupling line; The fourth parallel branch line unit is connected at both ends to the junctions of the third upper coupling line and the upper output feed line, and the third lower coupling line and the lower output feed line, respectively.

[0015] In some embodiments, the dielectric substrate is a magnesium oxide substrate, and the high-temperature superconducting thin film layer is a yttrium barium copper thin film layer.

[0016] The one or more technical solutions provided in the first aspect of this application have at least the following technical effects or advantages: The filter provided in this application adopts a chain-like alternating cascade structure of N parallel coupling lines and N+1 branch lines, where N is an integer. The value of can be flexibly adjusted according to the actual application requirements for the filter order. By simply increasing or decreasing the number of cascaded parallel coupling line units and parallel branch line units, filters of different orders can be quickly constructed, adapting to the design of filters of any order and possessing flexible structural scalability. Under differential-mode excitation, the transverse transmission line is equivalent to an open circuit due to circuit symmetry, which does not affect the transmission characteristics of the differential-mode passband. At the same time, under common-mode excitation, it can introduce an additional signal path and form a common-mode zero in the passband, realizing independent control of differential-mode and common-mode responses. The open-circuit stub of the final stage can form a transmission zero under common-mode excitation. Combined with the role of the transverse transmission line, it effectively expands the common-mode rejection bandwidth and improves the filter's ability to suppress common-mode interference. Furthermore, the overall structure does not require the introduction of complex interleaved transmission paths or large additional notch units. Combined with the low surface resistance characteristics of the high-temperature superconducting thin film layer, it ensures broadband differential-mode low-loss transmission and broadband common-mode rejection while maintaining circuit compactness, reducing the sensitivity of high-temperature superconducting materials to size changes, and improving structural stability and design flexibility.

[0017] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a first-view schematic diagram of the topology of a third-order embodiment of a low-ripple balanced high-temperature superconducting bandpass filter based on a three-segment cascaded coupling line according to an embodiment of this application. Figure 2 This is a second-view schematic diagram of the topology of a third-order embodiment of a low-ripple balanced high-temperature superconducting bandpass filter based on a cascaded three-segment coupled line, according to an embodiment of this application. Figure 3 This is a transmission line model of a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines, provided according to an embodiment of this application. Figure 4 This is the differential-mode equivalent circuit of a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupled lines, according to an embodiment of this application. Figure 5 This is the common-mode equivalent circuit of a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines, according to an embodiment of this application. Figure 6 This is a first-stage common-mode equivalent circuit with band-stop response of a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupled lines, according to an embodiment of this application. Figure 7 This is the final stage common-mode equivalent circuit with bandpass response of a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupled lines, according to an embodiment of this application. Figure 8 This is a topology diagram of a third-order embodiment of a low-ripple balanced high-temperature superconducting bandpass filter based on a three-segment cascaded coupling line, according to an embodiment of this application. Figure 9 The figure shows the electromagnetic simulation results of a third-order embodiment of a low-ripple balanced high-temperature superconducting bandpass filter based on a three-segment coupled line cascade, according to an embodiment of this application.

[0020] Figure label: 10. Dielectric substrate; 20. High-temperature superconducting thin film ground plane; 30. High-temperature superconducting thin film layer; 40. Filter circuit; 41. First input feed line; 42. Second input feed line; 43. First output feed line; 44. Second output feed line; 45. Lateral transmission line; 46. Open circuit stub; 401. First upper coupling line; 402. First lower coupling line; 403. Second upper coupling line; 404. Second lower coupling line; 405. Third upper coupling line; 406. Third lower coupling line; 407. Fourth upper coupling line; 406. Fourth lower coupling line; 411. First parallel branch line unit; 412. Second parallel branch line unit; 413. Third parallel branch line unit; 414. Fourth parallel branch line unit. Detailed Implementation

[0021] The embodiments of this application are described in detail below. The embodiments described with reference to the accompanying drawings are exemplary. It should be understood that the specific embodiments described herein are merely for explaining this application and are not intended to limit this application.

[0022] It should be noted that when a component is said to be "fixed to" another component, it can be directly attached to the other component or there may be an intervening component. When a component is said to be "connected to" another component, it can be directly connected to the other component or there may be an intervening component.

[0023] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0024] Please see Figures 1 to 3 This embodiment provides a low-ripple balanced high-temperature superconducting bandpass filter based on three-segment cascaded coupling lines. The filter includes a dielectric substrate, a high-temperature superconducting thin film layer, and a high-temperature superconducting thin film ground plane. Specifically, the high-temperature superconducting thin film layer is stacked on the upper surface of the dielectric substrate, and the high-temperature superconducting thin film ground plane layer is stacked on the lower surface of the dielectric substrate. The projection contours of the high-temperature superconducting thin film layer and the high-temperature superconducting thin film ground plane along the thickness direction are equal to those of the dielectric substrate, that is, the high-temperature superconducting thin film layer and the high-temperature superconducting thin film ground plane completely cover the dielectric substrate. The connection method between the high-temperature superconducting thin film layer and the dielectric substrate can be obtained from the prior art and will not be described in detail here.

[0025] Furthermore, the filter circuit includes a pair of input feed lines, a pair of output feed lines, a cascaded transmission network, and a common-mode rejection network. The input feed lines are differential input feed line structures, and the output feed lines are differential output feed line structures. The cascaded transmission network and the common-mode rejection network are located between the input and output feed lines. The cascaded transmission network includes N parallel coupling line units and N+1 parallel branch line units cascaded alternately along the signal transmission direction, where N is an integer greater than or equal to 2. The i-th parallel coupling line unit is connected to the i-th parallel branch line unit. Between the branch line unit and the (i+1)th parallel branch line unit, i = 1, ..., N; the common-mode rejection network includes a transverse transmission line and an open-circuit stub. The two ends of the transverse transmission line are connected to the center positions along the length direction of the first parallel branch line unit and the second parallel branch line unit, respectively. One end of the open-circuit stub is connected to the center position along the length direction of the (N+1)th parallel branch line unit, and the other end extends outward away from the (N+1)th parallel branch line unit. The transverse transmission line constitutes a common-mode adjustment structure, and the open-circuit stub constitutes a final-stage common-mode rejection structure.

[0026] It should be noted that, in combination Figure 2 The entire filter circuit is symmetrically arranged about a central horizontal plane, which extends along the signal transmission direction, lies between the two differential input feed lines, and is perpendicular to the plane containing the high-temperature superconducting thin film layer. The signal transmission direction is from one end of the differential input feed line structure to one end of the differential output feed line structure, i.e., from left to right in the figure. A pair of differential input feed lines are arranged parallel and symmetrically at the edge region of the input end of the high-temperature superconducting thin film layer, with a constant distance between them throughout. The input end is used to receive differential signals, and the output end is electrically connected to the starting end of the cascaded transmission network. A pair of differential output feed lines are symmetrical to the differential input feed line structure, arranged parallel and at equal intervals at the edge region of the output end of the high-temperature superconducting thin film layer. The input end is electrically connected to the end of the cascaded transmission network, and the output end is used to output the filtered differential signal. The parallel coupling line unit includes an upper coupling line and a lower coupling line, and the upper coupling line and the lower coupling line are arranged symmetrically.

[0027] It should also be noted that the extension direction of all parallel coupled line units is consistent with the signal transmission direction, and the extension direction of all parallel branch line units is perpendicular to the signal transmission direction, forming an alternating chain structure of horizontal branching and vertical coupling. In this structure, the value of N can be flexibly adjusted according to the filter's design requirements for bandwidth and roll-off coefficient. By increasing or decreasing the number of cascaded parallel coupling line units and parallel branch line units, it can quickly adapt to filter designs of any order of 2nd order or higher without changing the overall structural topology. The lateral transmission line is set in the starting section of the cascaded transmission network near the differential input feed line, specifically between the first parallel branch line unit and the second parallel branch line unit. Its extension direction is in the same direction as the signal transmission direction, and its two ends are respectively fixed to the central area of ​​the first parallel branch line unit and the second parallel branch line unit along their own extension length direction. The open-circuit stub is set in the end of the cascaded transmission network near the differential output feed line. One end is fixed to the central area of ​​the last parallel branch line unit along its own extension length direction, and the other end can extend outward perpendicular to the parallel branch line unit to form an open-circuit structure without grounding. The lateral transmission line constitutes a common-mode adjustment structure, and the open-circuit stub constitutes a final-stage common-mode rejection structure.

[0028] The filter provided in this embodiment adopts a chain-like alternating cascade structure of N parallel coupling lines and N+1 branch lines, where N is an integer. The value of can be flexibly adjusted according to the actual application requirements for the filter order. By simply increasing or decreasing the number of cascaded parallel coupling line units and parallel branch line units, filters of different orders can be quickly constructed, adapting to the design of filters of any order and possessing flexible structural scalability. Under differential-mode excitation, the transverse transmission line is equivalent to an open circuit due to circuit symmetry, which does not affect the transmission characteristics of the differential-mode passband. At the same time, under common-mode excitation, it can introduce an additional signal path and form a common-mode zero in the passband, realizing independent control of differential-mode and common-mode responses. The open-circuit stub of the final stage can form a transmission zero under common-mode excitation. Combined with the role of the transverse transmission line, it effectively expands the common-mode rejection bandwidth and improves the filter's ability to suppress common-mode interference. Furthermore, the overall structure does not require the introduction of complex interleaved transmission paths or large additional notch units. Combined with the low surface resistance characteristics of the high-temperature superconducting thin film layer, it ensures broadband differential-mode low-loss transmission and broadband common-mode rejection while maintaining circuit compactness, reducing the sensitivity of high-temperature superconducting materials to size changes, and improving structural stability and design flexibility.

[0029] Optionally, the dielectric substrate is a magnesium oxide substrate, and the high-temperature superconducting thin film layer is a yttrium barium copper thin film layer. The magnesium oxide substrate has excellent high-frequency insulation performance, low dielectric loss, and strong thermal stability. Its thickness is set according to the operating frequency band requirements of the filter. The extremely low surface resistance of the yttrium barium copper superconducting thin film can significantly reduce the ohmic loss of signal transmission in differential input feed lines, differential output feed lines, cascaded transmission networks, and common-mode rejection networks, and is suitable for the high-frequency signal coupling and transmission requirements of parallel coupling line units and parallel branch line units.

[0030] In some embodiments, continue reading Figures 1 to 3 The filter circuit is symmetrical about the central horizontal plane. Each parallel coupling line unit includes an upper coupling line and a lower coupling line symmetrical about the central horizontal plane. Each parallel branch line unit is a resonant structure symmetrical about the central horizontal plane. The central horizontal plane is located between the two output feed lines and / or between the two output feed lines, and is perpendicular to the plane formed by the projection of the high-temperature superconducting thin film layer along its thickness direction. Specifically, the upper and lower coupling lines are arranged symmetrically about the central horizontal plane, with their extension directions consistent with the signal transmission direction. The distance between them remains constant along their entire length, forming a tightly coupled microstrip line structure. The length and linewidth of the two coupling lines are the same. The extension direction of each parallel branch line unit is perpendicular to the signal transmission direction, and the midpoint of the resonant line segment falls exactly on the central horizontal plane, constituting the center of symmetry of the unit. The two ends of the parallel branch line unit are connected to the upper and lower coupling lines respectively, forming a symmetrical cascaded link.

[0031] It should be understood that this symmetrical structural design works synergistically with the low-loss characteristics of the high-temperature superconducting thin film. Under differential-mode excitation, the input reverse equal-amplitude signal is transmitted symmetrically along the upper and lower coupling lines. Since the circuit structure on both sides of the central horizontal plane is completely symmetrical, the potentials at both ends of the transverse transmission line remain equal due to the symmetrical characteristics, which is equivalent to an open circuit. The connection point of the open-circuit stub is in a virtual ground state due to the symmetrical distribution. Neither of these factors will interfere with the transmission path of the differential-mode signal, ensuring the flatness and low insertion loss characteristics of the differential-mode passband. Under common-mode excitation, when the co-directional equal-amplitude common-mode signal is transmitted along the upper and lower coupling lines, a reverse induced current will be generated due to the symmetrical structure. This current cancels out the common-mode signal itself. At the same time, the symmetrical layout of the transverse transmission line and the open-circuit stub ensures that the transmission phase of the common-mode signal is consistent on both paths. The interference between the additional transmission path and the main path is more significant, and the suppression effect of the common-mode transmission zero point is more stable, thereby improving the consistency of common-mode suppression and broadband coverage.

[0032] In some embodiments, see Figure 2 and Figure 3 The transverse transmission line is located between the first parallel branch line unit and the second parallel branch line unit, and coincides with the straight line formed by the projection of the central horizontal plane along the thickness direction of the high-temperature superconducting thin film layer. Specifically, for the straight line formed by the projection of the central horizontal plane along the thickness direction of the high-temperature superconducting thin film layer, please refer to... Figure 3 The dashed line SS' in the model, which is the straight line formed by the projection, is located at the position of the dashed line SS' in the transmission line model of the low ripple balanced high temperature superconducting bandpass filter based on three cascaded coupled lines. The transverse transmission line is located at the line segment of the dashed line SS' between the first parallel branch line unit and the second parallel branch line unit, and its two ends are respectively connected to the center points of the first parallel branch line unit and the second parallel branch line unit.

[0033] In some embodiments, please refer to Figure 3 The electrical length of the parallel coupled line unit at the differential mode center frequency is one-quarter of the waveguide wavelength, and the electrical length of each parallel branch line unit is one-half of the waveguide wavelength; and the equivalent electrical length of the transverse transmission line and the open stub is one-quarter of the waveguide wavelength or an odd multiple of the waveguide wavelength.

[0034] Furthermore, the lateral transmission line constitutes a common-mode adjustment structure. Under differential-mode excitation, due to circuit symmetry, the potentials at both ends of the lateral transmission line are equal, which is equivalent to an open circuit and basically does not affect the differential-mode passband characteristics. Under common-mode excitation, the lateral transmission line serves as a signal transmission path between the first parallel branch line unit and the second parallel branch line unit, introducing an additional common-mode signal path in the common-mode response and generating a common-mode transmission zero in a predetermined frequency band. The differential-mode excitation state is the input state of electrical signals with equal amplitude and opposite phase through a pair of input feed lines, and the common-mode excitation state is the input state of electrical signals with equal amplitude and same phase through a pair of input feed lines.

[0035] Furthermore, the open-circuit stub forms the final stage common-mode rejection structure. Under differential-mode excitation, the connection point of the open-circuit stub is in a virtual ground state, and the influence of the open-circuit stub on the differential-mode signal is negligible. Under common-mode excitation, the open-circuit stub acts as a resonant unit loaded at the end of the N+1th parallel branch unit, introducing at least one transmission zero in the common-mode stopband to extend the common-mode rejection bandwidth.

[0036] It should be noted that under differential-mode excitation, the differential-mode signal is transmitted in opposite directions with equal amplitude along the symmetrical upper and lower coupling lines. The electrical length of one-quarter of the waveguide wavelength of the parallel coupling line unit achieves the optimal coupling coefficient at the differential-mode center frequency, efficiently guiding the differential-mode signal transmission in the cascaded link and reducing signal reflection and energy loss. The electrical length of half the waveguide wavelength of each parallel branch line unit causes series resonance at the differential-mode center frequency, exhibiting low impedance characteristics and providing a stable transmission path for the differential-mode signal. At the same time, the symmetrical structure ensures a balanced current distribution, further optimizing the amplitude-frequency flatness of the differential-mode passband. Meanwhile, due to the symmetrical characteristics of the circuit, the potentials at both ends of the transverse transmission line are equal. Coupled with its one-quarter waveguide wavelength electrical length design, it is equivalent to an open circuit and will not interfere with the differential-mode signal transmission. Under differential-mode excitation, the connection point of the open-circuit stub is in a virtual ground state due to symmetry. Its equivalent electrical length of one-quarter waveguide wavelength gives it high impedance characteristics, which has no substantial impact on the differential-mode signal transmission, thus ensuring low insertion loss and wide bandwidth characteristics in the differential-mode passband.

[0037] It should also be noted that under common-mode excitation, the common-mode signal propagates in the same direction and with equal amplitude along the upper and lower coupling lines. At this time, the electrical length of the transverse transmission line, which is a quarter-wavelength (or an odd multiple) of the waveguide wavelength, no longer effectively creates an open circuit, but instead forms an additional transmission path for the common-mode signal. This path interferes with the main path of the cascaded transmission network at the differential-mode center frequency and adjacent frequency bands, precisely forming a common-mode transmission zero and suppressing the transmission of the common-mode signal within the passband. The final-stage open-circuit stub, designed based on a quarter-wavelength (or an odd multiple) of the waveguide wavelength, acts as a resonant unit under common-mode excitation, exhibiting low impedance characteristics and introducing an additional common-mode transmission zero within the common-mode stopband. The synergistic effect of these two common-mode zeros significantly widens the common-mode rejection bandwidth and enhances the suppression depth of common-mode interference.

[0038] It should be understood that the filters proposed in the above embodiments have scalable topologies and are applicable to arbitrary orders. N ( N ≥2). For ease of understanding and description, in one embodiment, a third-order filter (i.e., N Let's take 3 as an example to explain in detail: In some embodiments, when the filter is third-order, the cascaded transmission network includes a first parallel branch line unit, a first parallel coupling line unit, a second parallel branch line unit, a second parallel coupling line unit, a third parallel branch line unit, a third parallel coupling line unit, and a fourth parallel branch line unit, which are sequentially and alternately cascaded along the signal transmission direction; a transverse transmission line is connected between the first parallel branch line unit and the second parallel branch line unit, and an open-circuit stub is connected at the center of the fourth parallel branch line unit.

[0039] Furthermore, the pair of input feed lines includes an upper input feed line and a lower input feed line; the pair of output feed lines includes an upper output feed line and a lower output feed line; the first parallel coupling line unit includes a first upper coupling line and a first lower coupling line; the second parallel coupling line unit includes a second upper coupling line and a second lower coupling line; the third parallel coupling line unit includes a third upper coupling line and a third lower coupling line; wherein, one end of the first upper coupling line is connected to the upper input feed line, and the other end is cascaded with the second upper coupling line; the end of the second upper coupling line away from the first input feed line is cascaded with the third upper coupling line; the end of the third upper coupling line away from the second upper coupling line is connected to the upper output feed line; one end of the first lower coupling line is connected to the lower input feed line, and the other end is cascaded with the second lower coupling line; the end of the second lower coupling line away from the first input feed line is cascaded with the third lower coupling line; the end of the third lower coupling line away from the second lower coupling line is connected to the lower output feed line.

[0040] Furthermore, the first parallel branch line unit is connected at both ends to the junctions of the upper input feed line and the first upper coupling line, and the lower input feed line and the first lower coupling line, respectively; the second parallel branch line unit is connected at both ends to the junctions of the first upper coupling line and the second upper coupling line, and the first lower coupling line and the second lower coupling line, respectively; the third parallel branch line unit is connected at both ends to the junctions of the second upper coupling line and the third upper coupling line, and the second lower coupling line and the third lower coupling line, respectively; and the fourth parallel branch line unit is connected at both ends to the junctions of the third upper coupling line and the upper output feed line, and the third lower coupling line and the lower output feed line, respectively.

[0041] In some embodiments, please participate Figures 2 to 5 The transmission line model of the filter includes N coupling lines and N+1 branch lines; where Z ei and Z oi These are the even-mode and odd-mode characteristic impedances of the i-th coupled line, respectively, where i = 1, 2, ... , N. Z bi Let be the characteristic impedance of the i-th branch line, where i = 1, 2, ... N+1. The electrical lengths of all coupling lines and branch lines at the center frequency f0 are set to θ and 2θ, respectively. By properly designing these parameters, the differential-mode and common-mode responses of the filter can be optimized. When differential-mode excitation is applied to the input and output ports, the central horizontal symmetry plane exhibits ideal electric wall characteristics. The transverse transmission line connected at the center of the first and second branch lines has zero potential difference at its two ends, and therefore is equivalent to an open circuit in the differential-mode equivalent circuit, and does not participate in signal transmission. The open-circuit stub connected at the center of the fourth branch line has a virtual ground connection point, and therefore is short-circuited in the differential-mode equivalent circuit, and does not affect the main signal path.

[0042] Meanwhile, the entire circuit exhibits standard Nth-order Chebyshev bandpass filter characteristics in differential mode. Its equivalent circuit diagram is shown below. Figure 4 As shown. When common-mode excitation is applied to the input and output ports, the central horizontal symmetry plane exhibits ideal magnetic wall characteristics. At this time, the transverse transmission line acts as a practical signal path, connecting the first and second branch lines. This introduces a second common-mode signal transmission path besides the parallel coupling line. By rationally designing the impedance and length of the transverse transmission line, the common-mode signals passing through these two paths have similar amplitudes and opposite phases within the passband frequency range, resulting in destructive interference and generating common-mode transmission zeros. The open-circuit stub acts as a parallel resonant unit loaded on the final-stage branch line under common-mode conditions, which can introduce additional transmission zeros within the common-mode stopband, thereby significantly widening the common-mode rejection bandwidth. Its equivalent circuit diagram is shown below. Figure 5 As shown.

[0043] In some embodiments, such as Figure 6 As shown, this embodiment presents the first-stage common-mode equivalent circuit of a low-ripple balanced high-temperature superconducting bandpass filter with band-stop response based on a cascaded three-segment coupled line. To enhance common-mode rejection performance, additional common-mode design freedom needs to be introduced. In the first branch line (Z...) of the first stage... b1 ,θ) and the second branch line (Z) b2 Introduce a transmission line (Z) between θ and θ. t1 ,θ). Coupled line (Z) e1 Z o1 The first signal path (path I) is formed by the two branch lines (Z, θ), while the two branch lines (Z, θ) form the second signal path (path I). b1 ,θ and Z b2 ,θ) and transmission line (Z t1 The two common-mode noises (Z, θ) form a second signal path (path II). Under common-mode excitation, the common-mode noises of the two paths have equal amplitude and opposite phase in the differential-mode passband. Therefore, these two common-mode noises can cancel each other out in the differential-mode passband. To extend the frequency range of common-mode rejection, an open-circuit stub (Z, θ) is introduced at the end of the branch line. t2 ,θ), such as Figure 7 As shown. These two branch lines and one open-circuit stub can generate multiple transmission zeros in the upper and lower frequency bands of the common-mode response, thereby widening the common-mode noise suppression bandwidth. Transmission line (Z t1 ,θ) and open circuit stub (Z t2 θ) is only the common-mode design degree of freedom and has no effect on the differential-mode response.

[0044] In one example, see Figure 8 and Figure 9 To verify the performance of the above filter structure, a third-order filter with a center frequency of 4 GHz, a relative bandwidth of 105%, and a passband return loss of 30 dB was designed. Regarding the common-mode response, the insertion loss index across the entire frequency band is 17 dB. In this example, the design parameters are: Zb1 = 67.89 Ω, Zb2 = 24.49 Ω, Ze1 = 138.61 Ω, Zo1 = 47.06 Ω, Ze2 = 140.62 Ω, and Zo2 = 54.15 Ω. The characteristic impedance values ​​of the common-mode rejection unit are: Zt1 = 17.8 Ω and Zt2 = 23.8 Ω.

[0045] in, Figure 8The layout of a third-order embodiment of a low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines is shown in this embodiment. All dimensions in the figure are in millimeters (mm): W1 = 0.07, W2 = 0.05, W3 = 0.2, W4 = 2.58, W5 = 1.68, W6 = 1.61, G1 = 0.07, G2 = 0.1, L1 = 3.76, L2 = 3.78, L3 = 6.44, L4 = 14.60, L5 = 4.15, L6 = 2.18, L7 = 2.87, L8 = 8.4.

[0046] Furthermore, in combination Figure 9 , Figure 9 Electromagnetic simulation results of a third-order embodiment of the low-ripple balanced high-temperature superconducting bandpass filter based on a three-segment cascaded coupled line proposed in this embodiment are presented. The results show that the differential-mode performance achieves an ultra-wide passband of 1.89–6.1 GHz, and differential-mode stopbands of 0–1.5 GHz and 6.5–8 GHz. The return loss RL within the differential-mode passband is better than 23 dB, which is in high agreement with the ideal Chebyshev response, verifying that the common-mode rejection structure does not degrade the differential-mode performance. The common-mode performance maintains a common-mode rejection ratio greater than 10 dB across the entire 0–8 GHz frequency range. Particularly noteworthy is the average common-mode insertion loss of approximately 17 dB within the frequency range corresponding to the differential-mode passband, achieving excellent wideband common-mode rejection.

[0047] In the description of this application, it should be understood that the terms "center", "longitudinal", "lateral", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", "axial direction", "radial", "circumferential", etc., indicating the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on the invention.

[0048] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "illustrative embodiment," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example.

[0049] Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. The reference to "embodiment" herein means that a specific feature, structure, or characteristic described in connection with an embodiment can be included in at least one embodiment of this application. The appearance of this phrase in various places in the specification does not necessarily indicate the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0050] Although embodiments of this application have been shown and described, those skilled in the art will understand that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of this application, the scope of which is defined by the claims and their equivalents.

Claims

1. A low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines, characterized in that, include: Dielectric substrate; A high-temperature superconducting thin film layer is stacked on the upper surface of the dielectric substrate; A high-temperature superconducting thin film ground plane is stacked on the lower surface of the dielectric substrate; A filter circuit is constructed on the high-temperature superconducting thin film layer. The filter circuit includes a pair of input feed lines, a pair of output feed lines, a cascaded transmission network, and a common-mode rejection network. The cascaded transmission network includes N parallel coupling line units and N+1 parallel branch line units cascaded alternately along the signal transmission direction, where N is an integer greater than or equal to 2; the i-th parallel coupling line unit is connected between the i-th parallel branch line unit and the (i+1)-th parallel branch line unit, i = 1, ..., N; The common-mode suppression network includes a lateral transmission line and an open-circuit stub. The two ends of the lateral transmission line are respectively connected to the center positions of the first parallel branch line unit and the second parallel branch line unit along the length direction. One end of the open-circuit stub is connected to the center position of the (N+1)th parallel branch line unit along the length direction, and the other end extends outward away from the (N+1)th parallel branch line unit. The lateral transmission line constitutes a common-mode adjustment structure, and the open-circuit stub constitutes a final-stage common-mode suppression structure.

2. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 1, characterized in that, The filter circuit is symmetrical about the central horizontal plane, each of the parallel coupling line units includes an upper coupling line and a lower coupling line that are symmetrical about the central horizontal plane, and each of the parallel branch line units is a resonant structure that is symmetrical about the central horizontal plane; The central horizontal plane is located between the two output feed lines and / or the two output feed lines, and is perpendicular to the plane formed by the projection of the high-temperature superconducting thin film layer along its thickness direction.

3. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 2, characterized in that, The transverse transmission line is located between the first parallel branch line unit and the second parallel branch line unit, and coincides with the straight line formed by the projection of the central horizontal plane along the thickness direction of the high-temperature superconducting thin film layer.

4. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 2 or 3, characterized in that, The electrical length of the parallel coupling line unit at the differential mode center frequency is one-quarter of the waveguide wavelength, and the electrical length of each of the parallel branch line units is one-half of the waveguide wavelength; and the equivalent electrical length of the transverse transmission line and the open stub is one-quarter of the waveguide wavelength or an odd multiple of the waveguide wavelength.

5. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 2, characterized in that, Under differential mode excitation, due to the symmetry of the filter circuit, the potentials at both ends of the transverse transmission line are equal and it is equivalent to an open circuit; under common mode excitation, the transverse transmission line serves as a signal transmission path between the first parallel branch line unit and the second parallel branch line unit, and forms a common mode transmission zero in a predetermined frequency band. The differential mode excitation state is an input state in which electrical signals with equal amplitude and opposite phase are input through the pair of input feed lines, and the common mode excitation state is an input state in which electrical signals with equal amplitude and same phase are input through the pair of input feed lines.

6. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 5, characterized in that, Under the differential-mode excitation, the connection point of the open-circuit stub is in a virtual ground state; under the common-mode excitation, the open-circuit stub serves as a resonant unit loaded at the end of the N+1th parallel branch unit, and forms at least one transmission zero within the common-mode stopband to extend the common-mode rejection bandwidth.

7. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 1, characterized in that, When the filter is third-order, the cascaded transmission network includes a first parallel branch line unit, a first parallel coupling line unit, a second parallel branch line unit, a second parallel coupling line unit, a third parallel branch line unit, a third parallel coupling line unit, and a fourth parallel branch line unit, which are sequentially and alternately cascaded along the signal transmission direction. The transverse transmission line is connected between the first parallel branch line unit and the second parallel branch line unit, and the open-circuit stub is connected to the center of the fourth parallel branch line unit.

8. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 7, characterized in that, The pair of input feed lines includes an upper input feed line and a lower input feed line; the pair of output feed lines includes an upper output feed line and a lower output feed line; the first parallel coupling line unit includes a first upper coupling line and a first lower coupling line; the second parallel coupling line unit includes a second upper coupling line and a second lower coupling line; the third parallel coupling line unit includes a third upper coupling line and a third lower coupling line. Wherein, one end of the first upper coupling line is connected to the upper input feed line, and the other end is cascaded with the second upper coupling line; the end of the second upper coupling line away from the first input feed line is cascaded with the third upper coupling line; and the end of the third upper coupling line away from the second upper coupling line is connected to the upper output feed line. One end of the first lower coupling line is connected to the lower input feed line, and the other end is cascaded with the second lower coupling line. The end of the second lower coupling line away from the first input feed line is cascaded with the third lower coupling line. The end of the third lower coupling line away from the second lower coupling line is connected to the lower output feed line.

9. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 8, characterized in that, The first parallel branch line unit is connected at both ends to the junctions of the upper input feed line and the first upper coupling line, and the lower input feed line and the first lower coupling line, respectively. The two ends of the second parallel branch line unit are respectively connected to the intersection of the first upper coupling line and the second upper coupling line, and the first lower coupling line and the second lower coupling line; The two ends of the third parallel branch line unit are respectively connected to the intersection of the second upper coupling line and the third upper coupling line, and the second lower coupling line and the third lower coupling line; The fourth parallel branch line unit is connected at both ends to the junctions of the third upper coupling line and the upper output feed line, and the third lower coupling line and the lower output feed line, respectively.

10. The low-ripple balanced high-temperature superconducting bandpass filter based on three cascaded coupling lines according to claim 1, characterized in that, The dielectric substrate is a magnesium oxide substrate, and the high-temperature superconducting thin film layer is a yttrium barium copper thin film layer.

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