Wide-stop-band suppression high-temperature superconducting low-pass filter

Abstract

The invention relates to the technical field of low-pass filters, and discloses a wide-stop-band suppression high-temperature superconducting low-pass filter which comprises an interdigital hairpin low-pass filter and a narrow-band band-stop filter which are sequentially connected in series. The interdigital hairpin low-pass filter comprises at least one interdigital hairpin resonator, and the interdigital hairpin resonator introduces an interdigital microstrip line into a pair of coupling transmission lines of a hairpin type resonance unit so as to introduce a transmission zero point at the edges of a pass band and a stop band; the narrowband band-stop filter comprises at least one half-wavelength stepped impedance resonator, and the half-wavelength stepped impedance resonator loads the half-wavelength resonator to the main transmission line in an electromagnetic coupling mode so as to introduce a series transmission zero point. According to the invention, effective interference suppression in broadband band-pass and a specific frequency band is realized, and meanwhile, relatively small insertion loss and good sideband suppression are kept.

Description

Technical Field

[0001] The present invention relates to the technical field of low-pass filters, and particularly to a wide-stopband-suppressed high-temperature superconducting low-pass filter. Background Art

[0002] With the continuous progress of modern wireless communication technologies, radio frequency and microwave front-end devices require various types of filters to improve system performance. A low-pass filter can separate quasi-DC or low intermediate-frequency signals from high-frequency signals, and plays a crucial role in suppressing harmonics, spurious frequencies, and high-frequency interference in the stopband, as Figure 1 shown. Although existing filter technologies have been relatively mature, with the continuous emergence of new materials, the progress of microelectronic processing technologies, and the rapid replacement of communication systems, the performance requirements for low-pass filters are constantly increasing, specifically manifested as requirements for smaller volume and weight, steeper out-of-band suppression characteristics, and lower insertion loss.

[0003] To improve the sideband suppression characteristics and wide-stopband characteristics of low-pass filters, commonly used methods in the academic community currently mainly include introducing notch structures into filters and applying improved resonator units, etc. Existing technologies use a high-order filter structure integrated with multiple hairpin-shaped resonators to implement a compact elliptic function low-pass filter. To improve the stopband performance, two resonator units operating at high harmonic frequencies can be integrated on the right side of the filter to suppress parasitic passbands. However, the relationship between the out-of-band suppression depth and the filter order is not linear. After reaching a certain order, adding units cannot further improve the suppression effect, but the in-band loss deteriorates. Existing technologies introduce transmission zeros by loading composite open stubs on the main transmission line with high impedance to implement a miniaturized wide-stopband low-pass filter, but its stopband suppression degree and selectivity are insufficient. Existing technologies propose a wide-stopband low-pass filter based on an integrated fan-shaped radial stub (TRS) resonator unit, and this resonator unit can achieve a wider band-stop characteristic and steeper sideband selectivity. Existing technologies introduce transmission zeros by using the structure of a double-layer coupled resonator (DLCR) with high quality factor and low insertion loss to achieve stronger band-edge selectivity and stopband attenuation. Therefore, compared with introducing notch structures into filters, the method of applying improved resonator units provides a wider intrinsic stopband and is more simple and efficient, and is more suitable for low-cost and high-performance applications.

[0004] On the other hand, the band-stop function can provide strong interference suppression ability within a specific frequency band, especially suitable for situations where there is strong interference or adjacent channel interference within certain frequency bands. Particularly in radar and navigation systems, narrow-band band-stop low-pass filters can effectively filter out specific frequency band noise interference, whether intentional or unintentional, ensuring signal integrity and significantly improving the anti-interference ability of the system in a complex electromagnetic environment. In the past few years, the technical idea of using the band-stop characteristics of devices to achieve interference signal suppression has been emerging. Existing technologies have significantly improved the Q value of band-stop filters by using the defected ground structure (DGS) of U-grooves and V-grooves, providing steeper stop-band characteristics and flat pass-band characteristics, which are suitable for suppressing point-frequency interference or spurious signals. However, they will also introduce additional radiation losses. Additionally, existing technologies have designed a low-pass band-pass filter with a stop-band by using multi-mode resonant units for the filter, but it is inevitable that adjusting the stop-band position will affect the frequency position of the pass-band and the return loss performance. Existing technologies have designed a broadband band-pass filter with a reconfigurable in-band notch, which can change the notch position by adjusting the bias voltage of varactor diodes. It is used to suppress narrow-band interference within the band, but the notch depth of this filter is limited, and the stop-band suppression degree is also relatively poor. Existing technologies have proposed a design of an on-chip band-stop filter based on the defected ground structure, using a capacitively loaded T-shaped defected ground resonator to introduce two transmission poles, achieving a higher Q value and high roll-off stop-band suppression. On the other hand, using a complex coupling structure to introduce transmission zeros within the stop-band edge is also a common idea, which can achieve the purpose of optimizing the roll-off characteristics of the band-stop band edge. In summary, limited by parasitic effects, material losses, and process accuracy, band-stop filters usually have difficulty achieving a high quality factor (Q value), which will lead to a decrease in the selectivity of the filter, unable to provide sufficient attenuation within an extremely narrow bandwidth, and may also affect the pass-band characteristics. Therefore, it is difficult to directly integrate into practical systems.

[0005] High-temperature superconducting materials (such as YBCO) have a surface resistance close to zero. Therefore, filters designed based on high-temperature superconducting materials usually have an extremely high Q value, enabling extremely low insertion losses within their operating frequency bands. At the same time, their excellent out-of-band suppression characteristics can not only be used to construct wide-stop-band low-pass filters but also form an extremely narrow stop-band at specific frequency points to filter out specific narrow-band interference signals.

[0006] In superconducting radar communication systems, miniaturization design is one of the key requirements. Although high-temperature superconducting materials have excellent electrical characteristics, their processing costs are relatively high. Therefore, how to reduce the volume and cost while ensuring the filtering performance has become an important research direction. Summary of the Invention

[0007] In view of the above deficiencies in the prior art, the present invention provides a wide-stopband-suppression high-temperature superconducting low-pass filter. Through the integrated design of a wide-stopband low-pass filter and a narrowband band-stop filter, it can not only achieve wideband suppression but also accurately filter out specific-frequency interference. This design provides an efficient and compact solution for high-precision signal processing in complex electromagnetic environments, contributing to the miniaturization and high-performance development of superconducting radar communication systems.

[0008] In order to achieve the above-mentioned invention object, the technical solution adopted by the present invention is as follows:

[0009] A wide-stopband-suppression high-temperature superconducting low-pass filter, comprising:

[0010] A interdigital hairpin low-pass filter and a narrowband band-stop filter connected in series in sequence;

[0011] The interdigital hairpin low-pass filter includes at least one interdigital hairpin resonator, and the interdigital hairpin resonator introduces interdigital microstrip lines into a pair of coupled transmission lines of the hairpin-type resonator unit to introduce transmission zeros at the edges of the passband and the stopband.

[0012] The narrowband band-stop filter includes at least one half-wavelength stepped-impedance resonator, and the half-wavelength stepped-impedance resonator loads the half-wavelength resonator onto the main transmission line in a magnetic coupling manner to introduce series transmission zeros.

[0013] Optionally, the interdigital hairpin resonator adjusts the position of the transmission zero by adjusting the coupling capacitance of the interdigital microstrip line or the lead width of the hairpin-type resonator unit.

[0014] Optionally, the interdigital microstrip line in the interdigital hairpin resonator includes a first LC resonator unit and a second LC resonator unit respectively connected to a pair of coupled transmission lines of the clip-type resonator unit, and interdigital fingers are arranged at intervals between the first LC resonator unit and the second LC resonator unit.

[0015] Optionally, the width of the interdigital fingers of the interdigital microstrip line is less than the substrate thickness, and the interdigital spacing is equal to the interdigital finger width.

[0016] Optionally, the interdigital hairpin low-pass filter is composed of a plurality of cascaded interdigital hairpin resonators to form a symmetric structure.

[0017] Optionally, the interdigital hairpin low-pass filter is composed of four cascaded interdigital hairpin resonators to form a fourth-order symmetric low-pass filter.

[0018] Optionally, the half-wavelength stepped-impedance resonator determines the center frequency through the interference signal frequency band and controls the narrowband suppression frequency by adjusting the size and spacing of the resonator.

[0019] Optionally, the narrowband band-stop filter is formed by cascading multiple half-wavelength stepped impedance resonators to form a symmetric structure.

[0020] Optionally, the narrowband band-stop filter is formed by cascading four half-wavelength stepped impedance resonators to form a fourth-order symmetric narrowband band-stop filter.

[0021] Optionally, the interdigital hairpin low-pass filter and the narrowband band-stop filter are cascaded to form an L-shaped structure.

[0022] The present invention has the following beneficial effects:

[0023] The present invention combines the advantages of high-temperature superconducting materials and the dual suppression effect of cascading two filters, achieving effective interference suppression in a wide frequency band passband and a specific frequency band, while maintaining a small insertion loss and good sideband suppression. Description of the Drawings

[0024] Figure 1 Schematic diagram of the filter response required for the low-pass filter when the interference signal f1 is outside the passband;

[0025] Figure 2 Schematic diagram of a high-temperature superconducting low-pass filter with wide stopband suppression of the present invention;

[0026] Figure 3 Schematic diagram of the filter response required for the low-pass filter when the interference signal f2 is within the passband;

[0027] Figure 4 Schematic diagram of the application of a high-temperature superconducting low-pass filter with wide stopband suppression of the present invention;

[0028] Figure 5 Schematic diagram of the structure of the interdigital hairpin low-pass filter and its equivalent circuit;

[0029] Figure 6 When C s = 250 pF, the frequency-domain response S of the interdigital hairpin resonator 21 versus the parameter L s Schematic diagram of the relationship;

[0030] Figure 7 When Ls = 30 nH, the frequency-domain response S of the interdigital hairpin resonator 21 versus the parameter Cs;

[0031] Figure 8 Schematic diagram of the equivalent circuit of the fourth-order high-temperature superconducting low-pass filter;

[0032] Figure 9 Schematic diagram of the circuit plane layout of the fourth-order high-temperature superconducting low-pass filter;

[0033] Figure 10 Schematic diagram of the frequency response of a fourth-order high-temperature superconducting low-pass filter;

[0034] Figure 11 Schematic diagram of the current density distribution of a fourth-order high-temperature superconducting low-pass filter at the passband frequency (f1 = 1.5 GHz);

[0035] Figure 12 Schematic diagram of the current density distribution of a fourth-order high-temperature superconducting low-pass filter near the first transmission zero (f Tz1 = 1.76 GHz);

[0036] Figure 13 Schematic diagram of the current density distribution of a fourth-order high-temperature superconducting low-pass filter near the second transmission zero (f Tz2 = 1.97 GHz);

[0037] Figure 14 Physical processing diagram of a fourth-order high-temperature superconducting low-pass filter;

[0038] Figure 15 Schematic diagram of the measured frequency response of a fourth-order high-temperature superconducting low-pass filter;

[0039] Figure 16 Schematic diagram of the coupling model between a half-wavelength resonator and the main transmission line and its equivalent circuit;

[0040] Figure 17 For the reactance slope parameter Schematic diagram of the variation curve with l1;

[0041] Figure 18 For the reactance slope parameter With the variation of h 1a and h 1b Schematic diagram of the variation curve;

[0042] Figure 19 Schematic diagram of the equivalent topology of a fourth-order narrowband band-stop filter;

[0043] Figure 20 Schematic diagram of the circuit plane layout of a fourth-order narrowband band-stop filter;

[0044] Figure 21 Schematic diagram of the frequency response simulation of a fourth-order high-temperature superconducting narrowband band-stop filter;

[0045] Figure 22 Schematic diagram of the current density distribution in the stop band of a fourth-order band-stop filter when f1 = 1.4178;

[0046] Figure 23 Schematic diagram of the current density distribution in the pass band of a fourth-order band-stop filter when f2 = 1.4 GHz;

[0047] Figure 24 It is a physical diagram of a high-temperature superconducting narrow-band band-stop filter;

[0048] Figure 25 It is a schematic diagram of the measured frequency response of a high-temperature superconducting narrow-band band-stop filter;

[0049] Figure 26 It is a schematic diagram of the simulation result of a wide-band-stop suppression high-temperature superconducting low-pass filter;

[0050] Figure 27 It is a physical diagram of a wide-band-stop suppression high-temperature superconducting low-pass filter;

[0051] Figure 28 It is a comparison diagram of the measurement and simulation results of a wide-band-stop suppression high-temperature superconducting low-pass filter. Specific embodiments

[0052] The specific embodiments of the present invention will be described below to facilitate those skilled in the art of the present technology to understand the present invention. However, it should be clear that the present invention is not limited to the scope of the specific embodiments. For those of ordinary skill in the art of the present technology, as long as various changes are within the spirit and scope of the present invention defined and determined by the appended claims, these changes are obvious, and all inventions and creations using the concept of the present invention are within the scope of protection.

[0053] As Figure 2 shown, a wide-band-stop suppression high-temperature superconducting low-pass filter provided by an embodiment of the present invention includes:

[0054] An interdigital hairpin low-pass filter and a narrow-band band-stop filter connected in series in sequence;

[0055] The interdigital hairpin low-pass filter includes at least one interdigital hairpin resonator. The interdigital hairpin resonator introduces interdigital microstrip lines into a pair of coupled transmission lines of the hairpin-type resonator unit to introduce transmission zeros at the edges of the passband and the stopband;

[0056] The narrow-band band-stop filter includes at least one half-wavelength stepped impedance resonator. The half-wavelength stepped impedance resonator loads the half-wavelength resonator onto the main transmission line in a magnetic coupling form to introduce series transmission zeros.

[0057] The present invention makes full use of the technical advantages of high-temperature superconducting materials and introduces a narrow-band stop characteristic in the passband on the basis of low-pass filtering to accurately suppress interference signals in a specific frequency band f2, as Figure 3 shown.

[0058] The present invention adopts an integrated structure of a low-pass filter and a band-stop filter for suppressing specific interference, as Figure 4As shown. Through the integrated design of a wide-stopband low-pass filter and a narrowband band-stop filter, the present invention proposes a low-pass filter that can not only achieve wideband suppression but also precisely filter out interference at specific frequencies. This design provides an efficient and compact solution for high-precision signal processing in complex electromagnetic environments, contributing to the miniaturization and high-performance development of superconducting radar communication systems.

[0059] In an alternative embodiment of the present invention, the interdigital hairpin resonator adjusts the position of the transmission zero by adjusting the coupling capacitance of the interdigital microstrip line or the lead inductance of the hairpin-shaped resonant unit.

[0060] Among them, the interdigital microstrip line in the interdigital hairpin resonator includes a first LC resonant unit and a second LC resonant unit respectively connected to a pair of coupled transmission lines of the clamp-shaped resonant unit, and interdigital fingers are arranged at intervals between the first LC resonant unit and the LC resonant unit board.

[0061] The width of the interdigital fingers of the interdigital microstrip line is less than the substrate thickness, and the interdigital spacing is equal to the interdigital width.

[0062] The interdigital hairpin low-pass filter is composed of a plurality of cascaded interdigital hairpin resonators to form a symmetric structure.

[0063] The interdigital hairpin low-pass filter is composed of four cascaded interdigital hairpin resonators to form a fourth-order symmetric low-pass filter.

[0064] This embodiment proposes a novel compact low-pass filter structure that can additionally introduce transmission zeros. By introducing a novel interdigital hairpin-shaped resonant unit composed of an interdigital structure between two taps, its circuit structure and equivalent circuit are as Figure 5 shown.

[0065] Assume that this structure is lossless and all discontinuity effects can be ignored. Among them, C s represents the coupling capacitance generated by the interdigital structure, and C p represents the capacitance to ground of the interdigital structure. The capacitive elements introduced by the interdigital structure have the advantages of small size, low cost, and broadband characteristics. To achieve the maximum capacitance density, it is necessary to satisfy the conditions that the substrate thickness is much larger than the finger width g 1c , and the spacing g 1b and the finger width g 1c have equal numerical values. So let g 1b = g 1c = 50 μm. At this time, the coupled interdigital capacitance C s and the capacitance to ground C p can be estimated by the following formula:

[0066] C s = 0.04 × g 1a (ε r+1)[0.11(n - 3)+0.252]

[0067]

[0068] where g 1a represents the length of the interdigital fingers, n represents the number of interdigital fingers, ε r represents the relative dielectric constant of the substrate material, and h represents the dielectric layer thickness of the substrate.

[0069] Due to the symmetry of the equivalent circuit, the overall characteristics of the low - pass filter structure can be obtained through even - odd mode analysis. The analysis shows that:

[0070] Y even = sC p

[0071]

[0072] where s is the complex frequency variable, and Y odd and Y even are the input admittances in the even - mode and odd - mode respectively.

[0073] Thus, the transmission coefficient S 21 and the input impedance Z in of the proposed interdigital hairpin - type low - pass filter structure can be calculated.

[0074]

[0075] The frequency f TZ of the transmission zero Tz can be determined by S 21 = 0. Further, it can be obtained that the condition Z even = Z odd needs to be satisfied at this time. Thus, the expression of the frequency f Tz of the transmission zero Tz is as follows:

[0076]

[0077] It can be seen from the above formula that the position of the frequency of the transmission zero Tz is mainly determined by C s and L s together, and is not affected by the capacitance C p to the ground.

[0078] As Figure 6 shown, when the value of L s increases from 25 nH to 30 nH, the position of the transmission zero Tz will be adjusted from 2.05 GHz to 1.68 GHz. It can be seen that the steepness of the low - pass filter structure at the band edge is improved, while the passband of the low - pass structure is hardly affected. Similarly, by adjusting the inductance C s, it is also possible to control the position of the transmission zero point TZ to achieve the same effect, such as Figure 7 shown. When the value of C s increases from 200 pF to 300 pF, the position of the transmission zero point Tz will move from 2.01 GHz to 1.68 GHz. In addition, it can be found in the figure that if the interdigital structure is not introduced, that is, when the coupling capacitor C p = 0, the transmission zero point Tz will disappear. From the above analysis, it can be concluded that by improving the structure of the low-pass filter unit to additionally introduce a transmission zero point, and then by reasonably selecting the values of C s and Ls, the position of the transmission zero point Tz can be flexibly adjusted, and finally the purpose of improving the frequency selectivity of the filter and expanding the stopband can be achieved.

[0079] Based on the above discussion, a fourth-order symmetric low-pass filter (LPF) composed of four integrated low-pass filter units was designed, simulated and manufactured. Its equivalent schematic diagram and layout are as shown in Figure 8 and Figure 9 . The proposed filter was simulated by IE3D. The thickness h of the substrate was set to 0.5 mm, and the relative permittivity ε r was set to 9.8. The positions of its feed lines and the dimensional parameters between the low-pass filter units are as follows: d1 = d2 = 3.144 mm, l1 = 0.798 mm, l2 = 0.91 mm, l3 = 0.798 mm, k1 = k4 = 0.757 mm, k2 = k3 = 0.308 mm, w1 = w5 = 0.66 mm, w2 = w4 = 2.308 mm, w2 = 2.05 mm.

[0080] Figure 10 shows the frequency response of a fourth-order high-temperature superconducting (HTS) low-pass filter. By using two low-pass filter units with different sizes, two transmission zero points (T Z ) are introduced at different frequencies. The zero point at the near end of the passband significantly enhances the frequency selectivity of the filter, making it have a steeper skirt at the junction of the passband and the stopband, and improving the ability to suppress interference signals; the zero point at the far end effectively expands the stopband range, suppresses harmonic interference in non-target frequency bands, and further improves the signal-to-noise ratio of the system. The filter is designed with a symmetric structure. The high-order structure improves the frequency response stability of the filter, reduces the asymmetry effect, avoids frequency offset or distortion, and simplifies the design and debugging process. The symmetric design also reduces the insertion loss and reflection coefficient, improves the impedance matching of the system, and enhances the transmission efficiency. In addition, the symmetric structure effectively suppresses the generation of parasitic modes, improves the spectral purity and electrical performance of the filter. Through this design, the consistency and reliability of the filter in production and manufacturing are also significantly improved, ensuring excellent performance and stability in high-precision communication systems.

[0081] To further verify the above analysis, in this embodiment, an excitation is applied to port 1 to generate the simulation results of the current density distribution of the low-pass filter at different typical frequencies. From Figure 11 it can be seen that within the passband, the current smoothly transmits from the input end to the output end and is evenly distributed along the planar circuit path, indicating that the filter has good transmission characteristics in this frequency band. However, as the frequency increases to the transmission zeros f Tz1 and f Tz2 , a significant change occurs. As shown in Figure 12 and Figure 13 , at these two transmission zero frequencies, due to the action of the interdigital hairpin-shaped resonant unit, the current is gradually guided to the ground instead of passing through the entire circuit. This indicates that the filter realizes an effective signal suppression function at these frequency points, preventing the current from continuing to transmit to the output end. Such a design effectively enhances the out-of-band rejection performance of the filter and simultaneously demonstrates the distribution characteristics of the current density with frequency change, which is highly consistent with the theoretical analysis results. This simulation not only verifies the feasibility of the design but also provides a strong basis for further optimizing the circuit.

[0082] This filter is fabricated on an MgO wafer with a diameter of 2 inches and a thickness of 0.5 mm, and the YBCO thin film is covered on both sides of the wafer. This material can make full use of the excellent dielectric properties of the MgO substrate and the superconducting properties of the YBCO thin film, ensuring the low-loss performance of the filter in high-frequency applications. Figure 14 The physical picture of the fabricated HTS filter is shown. The performance test of the filter is carried out in a cryogenic cooler, the test temperature is maintained at 77K, and a vector network analyzer (VNA) is used to measure it. In the low-temperature environment, the YBCO thin film enters the superconducting state, thereby significantly reducing the insertion loss and internal loss of the filter. The scattering parameters (S-parameters) obtained in the measured results are highly consistent with the simulation results, as shown in Figure 15 . This result indicates that the designed filter performs in actual manufacturing and testing as expected theoretically, verifying the advantages of its high-temperature superconducting material and the feasibility and stability of the structural design. Two finite transmission zeros are observed on the transmission curve, located at f Tz1 = 1.76 GHz and f Tz2 = 1.97 GHz respectively, and the signal suppression reaches -79.6 dB and -84.6 dB respectively. With the introduction of these two zeros, the rejection ability of the stopband of this low-pass filter is greatly improved, exceeding 50 dB. At the same time, this LPF shows good passband performance in the range from DC to 1.54 GHz, with an insertion loss less than 0.1 dB and a return loss higher than 20 dB. In addition, the overall size of this low-pass filter is: 15.2 mm × 4.2 mm, having the advantage of a compact area.

[0083] In summary, compared with the conventional hairpin-shaped resonator unit, interdigital microstrip lines are introduced into a pair of low-impedance coupled transmission lines of this unit, which creates a new coupling path between two non-coupled low-impedance structures and can significantly increase the coupling capacitance of the resonator unit. This structure can introduce transmission zeros near the edges of the passband and stopband, and reduce the resonance frequency by adjusting the size of the coupling capacitance or the lead inductance, thereby adjusting the position of the transmission zeros, further achieving the improvement of the frequency selectivity and out-of-band rejection effect of the filter, expanding the stopband while simplifying the design structure, achieving a compact size, and reducing the manufacturing cost. Finally, this filter unit is designed, manufactured, and tested separately. The measurement results show good performance.

[0084] In an alternative embodiment of the present invention, the half-wavelength stepped impedance resonator determines the center frequency through the interference signal frequency band, and controls the narrowband rejection frequency by adjusting the size and spacing of the resonator.

[0085] Among them, the narrowband band-stop filter is composed of a plurality of cascaded half-wavelength stepped impedance resonators to form a symmetric structure.

[0086] The narrowband band-stop filter is composed of four cascaded half-wavelength stepped impedance resonators to form a fourth-order symmetric narrowband band-stop filter.

[0087] A simple and practical method for designing a narrowband band-stop filter is based on the reactance slope parameter of the resonator. The coupling model between the half-wavelength resonator and the main transmission line is as Figure 16 shown, and is achieved by loading the half-wavelength (0.5λ g0 ) resonator onto the main transmission line in the form of magnetic coupling. Among them, in the equivalent circuit, L i , C i are used as parallel resonance units, and L l represents the equivalent inductance of the main coupling line, and its numerical parameter resonates at the center frequency f0 of the target band-stop filter. Z0 represents the characteristic impedance, and θ 1a = θ 1b = π / 4. According to the different coupling distances l1, h 1a and h 1b between the half-wavelength resonator and the main transmission line, the electromagnetic simulation software (IE3D) is used to simulate the frequency response of a single resonator coupled to the main line. The thickness h of the substrate is set to 0.5 mm, and the relative dielectric constant ε r is set to 9.8. Its transmission parameters are given by the following formula:

[0088]

[0089] Among them, k represents the reactance slope parameter, Z0 represents the characteristic impedance, and Δω 3dB represents the 3 dB bandwidth. It can be seen from the above formula that when |S21 When | = ∞, the circuit is in resonance state, and the resonance unit makes the transmission open circuit and causes a large signal attenuation. At this time:

[0090]

[0091] Therefore, the normalized reactance slope parameter can be extracted according to the following equation:

[0092]

[0093] where f0 represents the center frequency, and Δf 3dB = Δω 3dB / 2π represents the 3dB frequency range.

[0094] In addition, narrowband band-stop filters usually also require frequency tuning to compensate for manufacturing tolerances. In this case, the value of the coupling spacing of each resonator can be slightly adjusted.

[0095] As Figure 17 shown, as the value of l1 continues to increase, the reactance slope parameter will also continue to increase. As Figure 18 shows the variation curve of the reactance slope parameter with h 1a and h 1b changing, it can be seen that as h increases, the reactance slope parameter will decrease. At the same time, as h increases, its variation curve gradually becomes flat.

[0096] Using the Chebyshev low-pass prototype to design a narrowband band-stop filter, transforming the low-pass prototype into a filter with band-stop characteristics can be achieved through frequency mapping, and its circuit topology is as Figure 19 shown. The spacing between resonators is set to a quarter of the guided wavelength λ g0 . S and L represent the source end and the load end respectively. The reactance slope parameter of the parallel resonance unit can be calculated from the component values of the low-pass prototype and the center frequency and relative bandwidth of the band-stop filter, as shown in Table 1.

[0097] Table 1. Impedance slope parameters of the fourth-order narrowband band-stop filter

[0098]

[0099] Based on the above discussion, a fourth-order narrowband band-stop filter is designed, as Figure 20As shown. The IE3D software is used to model the device and optimize the performance of the narrowband band-stop filter. Its dimensional parameters are as follows: s1 = 1.631 mm, s2 = 1.451 mm, s3 = 1.63 mm, l1 = l4 = 1.634 mm, l2 = l3 = 1.333 mm; h 1a = h 4a = 5.99 mm, h 1b = h 4b = 5.867 mm, h 2a = h 3a = 2.962 mm; h 2b = h 3b = 2.47 mm. The simulation results of the narrowband band-stop filter are as Figure 21 shown.

[0100] To further verify the above analysis, in this embodiment, an excitation is applied to port 1, and the simulation results of the current density distribution of the narrowband filter at different typical frequencies are generated. From Figure 22 it can be seen that within the stopband, the current is gradually guided to the ground instead of passing through the entire circuit. As Figure 23 shown, within the passband, the current smoothly transmits from the input end to the output end and is evenly distributed along the planar circuit path, indicating that the filter has good transmission characteristics in this frequency band.

[0101] Similarly, the filter is fabricated on a MgO wafer with a diameter of 2 inches and a thickness of 0.5 mm. Figure 24 shows a physical diagram of the fabricated HTS filter, and a vector network analyzer (VNA) is used to measure it. The scattering parameters (S-parameters) obtained from the measured results are in good agreement with the simulation results, as Figure 25 shown. It is observed on the transmission curve that at the center frequency of 1.406 GHz, the stopband attenuation is 59.4 dB, and the corresponding relative bandwidth is 0.7%. Two transmission zeros appear beside the stopband, located at 1.392 GHz and 1.434 GHz respectively. In addition, the overall size of the narrowband band-stop filter is: 19.07 mm × 8.33 mm, and the structure is compact.

[0102] In an alternative embodiment of the present invention, the interdigital hairpin low-pass filter and the narrowband band-stop filter are cascaded to form an L-shaped structure.

[0103] This embodiment designs a circuit structure integrating a low-pass filter (LPF) and a band-stop filter (BSF). The specific circuit layout is as Figure 2As shown. This integrated design aims to meet the interference suppression requirements of specific frequency bands in modern communication systems while maintaining the overall attenuation effect on broadband signals. The circuit realizes double filtering of interference signals by combining the wide-band suppression ability of the low-pass filter and the narrow-band suppression characteristics of the band-stop filter, thereby improving the signal-to-noise ratio and transmission quality of the communication link.

[0104] In the layout, the low-pass filter and the band-stop filter adopt a compact series structure, significantly reducing the parasitic effects and losses caused by wiring. The low-pass filter is designed with multiple resonant units to enhance the out-of-band suppression ability and uses microstrip lines to achieve good impedance matching. The resonant unit of the band-stop filter is integrated in the latter stage of the low-pass filter to achieve precise suppression of specific frequencies. The compact circuit layout not only reduces the physical space occupation but also minimizes the high-frequency signal transmission path, reducing the losses caused by parasitic capacitance and inductance. Through this reasonable structural design, the overall transmission characteristics of the filter have been significantly improved, especially in terms of the flatness of the frequency response, insertion loss, and suppression performance.

[0105] During the design process, the cut-off frequency of the low-pass filter and the center frequency of the band-stop filter are two key parameters. The cut-off frequency of the low-pass filter needs to meet the system's requirements for broadband signal suppression and maintain low loss near the operating frequency of the band-stop filter to ensure good transmission efficiency of the overall circuit in the passband. The center frequency of the band-stop filter is determined according to the interference signal frequency band in the actual application, and the depth suppression of specific frequencies is precisely controlled by optimizing the size and spacing of the resonant units. This design strategy not only effectively suppresses specific interference signals by the filter but also ensures the stability and consistency of the overall circuit performance.

[0106] In addition, special attention is paid to the influence of parasitic effects in the layout design. To minimize the interference of parasitic capacitance and inductance, the interconnecting lines between the low-pass filter and the band-stop filter are designed as short and direct as possible to reduce the losses and distortions of high-frequency signals. By optimizing the layout and resonant structure design, the integrated filter effectively suppresses high-frequency bands and narrow-band interference while maintaining low insertion loss, thus significantly improving the signal quality and anti-interference ability of the system.

[0107] To further improve the stability and reliability of the filter, the symmetry of the resonant unit is particularly optimized to reduce the influence of manufacturing deviations on performance. The symmetry design not only helps to reduce the influence of asymmetric effects on the filter characteristics but also increases the fault tolerance in the manufacturing process, improving the consistency and repeatability of the filter. In addition, the combination of the low-pass and band-stop filters enables the circuit to exhibit good temperature stability and mechanical stability under different operating environments, better meeting the requirements of modern communication systems for high-performance filtering.

[0108] In this embodiment, a YBCO / MgO / YBCO high-temperature superconducting thin film with a dielectric constant of 9.8 is used. One side of it is etched by high-precision lithography technology to produce a circuit mask as shown in Figure 26 , thus forming a superconducting circuit. The other side of the thin film is connected to a metal shielding box and grounded through a coupling circuit to ensure good shielding effect. A compact planar structure is adopted in the design process, making the size of the entire filter 31.85×17.28×0.50 mm 3 , as shown in Figure 27 , showing the specific structure of the physical circuit.

[0109] The performance measurement of the filter is carried out on an HTS test platform. The specific operation is to connect an insulating cable to the connector of the filter, then fix it on the copper plate in the Dewar flask, and cool the filter to the superconducting transition temperature of 77 K through a cryogenic controller. During this process, the S-parameters of the HTS filter are measured by connecting a vector network analyzer (VNA) to the RF input and output ports of the test system. Figure 28 shows the comparison between the frequency response of the HTS filter measured at low temperature and the electromagnetic (EM) simulation results. It can be seen from the test data that the -3dB cut-off frequency of this filter is 1.53 GHz, the transition bandwidth is 0.18 GHz, the out-of-band rejection is greater than 60 dB, the insertion loss is less than 0.2 dB, the return loss is better than 10 dB, and the center frequency f of the stopband at a specific frequency e is located at 1.42 GHz, with a relative bandwidth of 1.41% ultra-narrowband. These test results are basically consistent with the simulation results, showing good filtering characteristics.

[0110] Compared with the simulation results, the fractional bandwidth (FBW), return loss in the passband, band-edge rejection, isolation in the passband and other indicators of the high-temperature superconducting filter can all meet the expectations. This filter shows excellent performance under the measured conditions, especially the low insertion loss and high return loss in the superconducting state, ensuring its superior performance in practical applications. The comparison between the measurement results and the simulation results is shown in Table 2, further verifying the feasibility of the design and the high performance of the filter.

[0111] Table 2. Simulation and measurement responses of high-temperature superconducting low-pass filter with wide stopband rejection

[0112]

[0113] In summary, by combining high-precision lithography technology, compact physical design, and a strict low-temperature test environment, this filter exhibits excellent in-band filtering characteristics and out-of-band rejection capabilities, making it highly suitable for modern communication systems with extremely high signal quality requirements. The overall measurement results show that this HTS filter not only has significant advantages in terms of frequency selectivity, insertion loss, and isolation, but also the good consistency with the simulation data proves the reliability of its high-precision design.

[0114] The integrated high-temperature superconducting integrated low-pass filter designed in this invention combines the advantages of high-temperature superconducting materials and the double rejection effect of cascading two filters, achieving effective interference rejection in a wide frequency band passband and a specific frequency band, while maintaining a small insertion loss and good sideband rejection. Through a compact structural layout and precise resonance design, this filter shows significant advantages in terms of bandwidth utilization, anti-interference ability, and signal transmission quality. By optimizing the layout structure and reducing parasitic effects, the in-band stability and passband selectivity of the filter are effectively improved. The test results verify the high reliability and excellent performance of the designed circuit in practical applications, meeting the strict requirements for interference rejection and signal transmission quality in modern communication systems. This design not only verifies the application potential of high-temperature superconducting thin film materials in RF filters, but also provides an important theoretical basis and practical guidance for achieving better signal processing in future high-frequency communication systems.

[0115] Specific embodiments are used in this invention to elaborate on the principles and implementation methods of the invention. The description of the above embodiments is only used to help understand the method of the invention and its core idea; at the same time, for those of ordinary skill in the art, according to the idea of the invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be construed as a limitation to the invention.

[0116] Those of ordinary skill in the art will realize that the embodiments described here are for helping readers understand the principles of the invention, and it should be understood that the protection scope of the invention is not limited to such specific statements and embodiments. Those of ordinary skill in the art can make various other specific deformations and combinations that do not deviate from the essence of the invention based on the technical revelations disclosed in the invention, and these deformations and combinations are still within the protection scope of the invention.

Claims

1. A wide stopband suppression high temperature superconducting low pass filter, characterized in that: include: An interdigital hairpin low-pass filter and a narrow-band stop filter are sequentially connected in series; The interdigital hairpin low-pass filter comprises at least one interdigital hairpin resonator, which introduces an interdigital microstrip line into a pair of coupled transmission lines of a hairpin type resonant unit to introduce a transmission zero point at the edge of a passband and a stopband; The narrowband rejection filter comprises at least one half-wavelength stepped impedance resonator, and the half-wavelength stepped impedance resonator is loaded to the main transmission line in a magnetic coupling form to introduce a series transmission zero point.

2. A wide stopband suppression high temperature superconducting low pass filter according to claim 1, characterized in that: The interdigital hairpin resonator adjusts the transmission zero position by adjusting the coupling capacitance of the interdigital microstrip line or the lead inductance of the hairpin resonant unit.

3. A wide stopband suppression high temperature superconducting low pass filter according to claim 1, characterized in that: The interdigital microstrip line in the interdigital hairpin resonator comprises a first LC resonance unit and a second LC resonance unit respectively connected to a pair of coupled transmission lines of the clip-type resonance unit, and interdigital fingers are arranged between the first LC resonance unit and the LC resonance unit.

4. A wide stopband suppression high temperature superconducting low pass filter according to claim 3, characterized in that: The interdigital width of the interdigital microstrip line is smaller than the thickness of the substrate, and the interdigital spacing is equal to the interdigital width.

5. The wide stopband suppression high temperature superconducting low pass filter according to claim 1, characterized in that: The interdigital hairpin low-pass filter uses a cascaded plurality of interdigital hairpin resonators to form a symmetrical structure.

6. A wide stopband suppression high temperature superconducting low pass filter according to claim 1 or 5, characterized in that: The interdigital hairpin low-pass filter uses four cascaded interdigital hairpin resonators to form a fourth-order symmetrical low-pass filter.

7. The wide stopband suppression high temperature superconducting low pass filter according to claim 1, characterized in that: The half-wavelength stepped impedance resonator determines the center frequency according to the frequency band of the interference signal, and the narrowband suppression frequency is controlled by adjusting the size and spacing of the resonator.

8. The wide stopband suppression high temperature superconducting low pass filter according to claim 1, characterized in that: The narrowband rejection filter adopts a symmetrical structure formed by cascading multiple half-wavelength step impedance resonators.

9. A wide stopband suppression high temperature superconducting low pass filter according to claim 1 or 8, characterized in that: The narrowband rejection filter uses four cascaded half-wavelength step impedance resonators to form a fourth-order symmetrical narrowband rejection filter.

10. The wide stopband suppression high temperature superconducting low pass filter according to claim 1, characterized in that: The interdigital hairpin low-pass filter and the narrow-band stop filter are cascaded to form an L-type structure.

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