A microstrip filter with high spurious passband suppression
By cascading interdigital low-pass filters and hairpin bandpass filters, optimizing frequency matching and impedance design, the problems of insufficient parasitic passband suppression and return loss in microstrip filters are solved, achieving efficient parasitic passband suppression and good return loss characteristics.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- UNIV OF ELECTRONICS SCI & TECH OF CHINA
- Filing Date
- 2026-05-12
- Publication Date
- 2026-06-19
AI Technical Summary
Existing microstrip filters have parasitic passbands at high-frequency harmonics, which cause leakage of out-of-band interference signals, affecting the system's anti-interference capability and signal purity. At the same time, the in-band return loss and impedance matching performance are insufficient.
By cascading the first and second interdigital low-pass filters and the hairpin-type bandpass filter, the cutoff frequency of the low-pass filter and the operating frequency band of the bandpass filter are optimized. A 50 Ω characteristic impedance microstrip line is used to achieve matching connection, avoid impedance abrupt change, and form an integrated structure.
It achieves effective suppression of high-frequency parasitic passbands, with a parasitic passband suppression degree of over 60 dB and an in-band return loss better than 18 dB, significantly improving out-of-band suppression and return loss performance.
Smart Images
Figure CN122246446A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of microwave passive circuit technology, specifically to a microstrip filter with high suppression of parasitic passbands. Background Technology
[0002] Microstrip bandpass filters are key components in modern microwave communication systems, widely used in satellite communication, radar, and 5G millimeter-wave communication for frequency selection and interference suppression. Traditional hairpin-type bandpass filters, employing a quarter-wavelength circuit structure, typically exhibit parasitic passbands at high-frequency harmonics. These parasitic passbands cause out-of-band interference signals to leak to the output, severely impacting the system's anti-interference capability and signal purity. Furthermore, in the filtering selection after broadband mixing, they also output a large number of multi-order components. Therefore, achieving high suppression of the inherent parasitic passbands of hairpin-type bandpass filters while ensuring passband insertion loss and return loss performance has become one of the critical problems urgently needing to be solved in the field of microwave filter design.
[0003] Patent CN112072227A, entitled "A Harmonic Suppression Microstrip Bandpass Filter," proposes a microstrip structure design consisting of two open-circuit transmission lines, multiple T-type transmission line units, and multiple coupled transmission line units. (See appendix to its specification.) Figure 3 As can be seen, under the operating conditions of a center frequency of 26.5 GHz and a bandwidth of 1 GHz, its suppression of the second harmonic frequency range is greater than 40 dB, but the suppression of some frequency bands in the stopband is less than 20 dB.
[0004] Patent No. ZL201910508851X discloses "An Improved Hairpin Filter and Its Operation Method," which can shift the parasitic passband to a higher frequency band and effectively suppress the first parasitic passband of the traditional hairpin structure. However, although the comb-type cross-coupling structure it employs can generate transmission zeros near the passband, its suppression effect on the far-end parasitic passband is limited. (See appendix to the specification.) Figure 5 It can be seen that, under the operating conditions of a center frequency of 23 GHz and a bandwidth of 2 GHz, the filter's suppression of parasitic passband is only 20 dB, and the in-band return loss is about 10 dB, indicating poor passband impedance matching performance.
[0005] Patent No. ZL202010757323.0 discloses "A Hairpin Bandpass Filter," proposing a wide stopband filter design: a quarter-wavelength open-circuit microstrip line is loaded onto the resonant units at the input and output ends, and metal vias are provided on both sides of the input feed line. This structure can effectively suppress high-frequency parasitic passbands outside the passband, possesses wide stopband characteristics, and also has good frequency selectivity and low insertion loss performance. (See attached specification.) Figure 4Simulation results show that, under operating conditions of a center frequency of 3.5 GHz and a bandwidth of 200 MHz, the filter can achieve a high-frequency parasitic passband suppression of over 45 dB. However, this scheme has an in-band return loss of less than 10 dB, resulting in poor passband matching performance; furthermore, the relevant performance results are based solely on simulation analysis and lack physical test data for verification.
[0006] Hang Yan et al. designed a hairpin bandpass filter with cross-coupling and periodic groove structure. Under operating conditions of a center frequency of 4.9 GHz and a bandwidth of 200 MHz, the filter exhibits a parasitic passband suppression of only 20 dB, and an in-band return loss of only 10 dB. However, its passband matching performance is poor, and no further experimental data supports this assessment. Reference: H. Yan, X. Wu and Y. Hu, "Cross-Coupling Hairpin Bandpass Filter with Periodic Grooves," IEEE 5th International Conference on Electronic Information and Communication Technology (ICEICT), Hefei, China, 2022, pp.910-912. Summary of the Invention
[0007] To address the shortcomings of existing microstrip filters, such as insufficient parasitic passband suppression and in-band return loss, this invention proposes a microstrip filter with high parasitic passband suppression. By optimizing the structure and characteristic parameters of the low-pass and band-pass filters, and using a 50 Ω characteristic impedance microstrip line to achieve matched connection of multiple filter stages, effective suppression of high-frequency parasitic passbands is achieved without introducing additional circuit complexity, while also taking into account low insertion loss and good return loss characteristics.
[0008] The technical solution adopted in this invention is as follows:
[0009] A microstrip filter with high suppression of parasitic passbands, comprising a dielectric substrate;
[0010] At least one set of microstrip filter components is integrated on the front side of the dielectric substrate; the microstrip filter components include a first interdigitated low-pass filter, a second interdigitated low-pass filter, a hairpin bandpass filter, a first microstrip line, and a second microstrip line; wherein:
[0011] The first interdigital low-pass filter is connected to one end of the hairpin bandpass filter via the first microstrip;
[0012] The second interdigital low-pass filter is connected to the other end of the hairpin-type bandpass filter via the second microstrip to form an integrated cascaded structure.
[0013] A metal grounding layer is provided on the back side of the dielectric substrate.
[0014] Furthermore, the first interdigital low-pass filter includes an input microstrip line, a multi-order interdigital filter unit, and an output connection terminal;
[0015] The input microstrip line is connected to the outside and is used to receive signals; the multi-stage interdigital filter units are cascaded sequentially along the signal transmission direction, with the first-stage interdigital filter unit connected to the input microstrip line and the last-stage interdigital filter unit connected to the first microstrip line via the input connection terminal.
[0016] The second multi-order interdigital low-pass filter has the same structural dimensions as the first multi-order interdigital low-pass filter. The only difference is that the input microstrip line of the first-order interdigital filter unit in the second multi-order interdigital low-pass filter is connected to the second microstrip line, and the output connection terminal is the output of the entire device.
[0017] Furthermore, the multi-order interdigital filtering units in the first and second multi-order interdigital low-pass filters are composed of multiple sets of staggered metal interdigital branches, with a preset coupling spacing between adjacent sets of metal interdigital branches to form a capacitive coupling path.
[0018] Furthermore, the output connection of the first interdigital low-pass filter is matched with the impedance of the first microstrip line, and the widths of the two are consistent, so as to achieve a gapless coplanar direct connection between the two and avoid reflection caused by impedance abrupt change.
[0019] Furthermore, in the multiple sets of staggered metal interdigitated branches, the spacing between adjacent metal interdigitated branches and the length of the metal interdigitated branches can be flexibly adjusted according to the filtering requirements to adapt to the cutoff frequency requirements of different frequency bands.
[0020] Furthermore, the hairpin-type bandpass filter includes an input connection terminal, multiple hairpin resonators, and an output microstrip line; the multiple hairpin resonators are arranged sequentially along the length of the dielectric substrate, and adjacent hairpin resonators are arranged in opposite directions, so that adjacent resonators form parallel coupling through their parallel closed ends spaced at a predetermined distance, thereby realizing signal transmission within a predetermined passband; wherein:
[0021] The input connection of the first-order hairpin resonator is directly and coplanarly connected to the first microstrip line without any gap.
[0022] The last-order hairpin resonator is connected to the output stripline, and the output microstrip line is directly connected to the second microstrip line in the same plane.
[0023] Furthermore, the hairpin resonator is a half-wavelength hairpin resonator structure, and its resonant length, arm spacing, and coupling spacing are adjusted according to the center frequency and bandwidth requirements of the entire device.
[0024] The positions of the input connection terminal and the feed point of the output microstrip line are adjusted along the resonator axis to achieve coordinated matching between the operating frequency of the bandpass filter and the cutoff frequency of the low-pass filter.
[0025] Furthermore, the cutoff frequencies of the first and second interdigital low-pass filters are higher than the upper edge frequency of the passband of the hairpin bandpass filter, but lower than its parasitic passband frequency, in order to effectively suppress higher harmonics and parasitic passbands.
[0026] Furthermore, the input microstrip lines of the first and second interdigital low-pass filters, the output microstrip line of the hairpin bandpass filter, and the first and second microstrip lines are all 50 Ω characteristic impedance microstrip transmission lines to achieve impedance matching with the standard interface of the radio frequency system.
[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:
[0028] 1. To address the problem of insufficient parasitic passband suppression depth in existing microstrip filters, a cascaded design of first and second interdigital low-pass filters and hairpin-type bandpass filters was implemented. By co-optimizing the cutoff frequency of the low-pass filter and the operating frequency band of the bandpass filter, effective suppression of the parasitic passband of the traditional hairpin-type structure was achieved. The theoretical parasitic passband suppression degree can reach more than 60 dB, significantly improving the out-of-band suppression performance over a wide frequency range.
[0029] 2. To address the problem of deteriorated in-band return loss caused by impedance mismatch at cascaded ports in existing microstrip filters, this invention coplanarly integrates the first and second interdigital low-pass filters with a hairpin-line bandpass filter on the same dielectric substrate. While ensuring good return loss performance (better than 20 dB) for individual filters, the integrated structure design is achieved through coplanar direct connection between the input and output ends, effectively avoiding the introduction of additional matching transmission lines. At the same time, the filter connection ports adopt a continuous impedance design with equal width, effectively reducing impedance abrupt changes, thereby reducing in-band reflection loss and improving return loss performance. Attached Figure Description
[0030] Figure 1 This is a schematic diagram of the structure of a single hairpin linear bandpass filter in this invention;
[0031] Figure 2 S-parameter simulation curves for a single hairpin linear bandpass filter (with obvious parasitic passband);
[0032] Figure 3 This is a schematic diagram of the structure of a single interdigital low-pass filter in this invention;
[0033] Figure 4 The S-parameter simulation curves of a single interdigital low-pass filter (with obvious high-frequency suppression characteristics);
[0034] Figure 5 This is a schematic diagram of the overall structure of the first and second interdigital low-pass filters and the hairpin bandpass filter cascaded in this invention.
[0035] Figure 6 The simulated S-parameter curves of the cascaded filter in Example 1 show that it can effectively suppress parasitic passband and has good return loss within the passband.
[0036] Figure 7 This is a comparison of the S-parameter simulation curves before and after parasitic passband suppression in Example 1;
[0037] Figure 8 This is a physical diagram of the cascaded filter in Example 1;
[0038] Figure 9 The measured S-parameter curves of the cascaded filter in Example 1 are shown.
[0039] Figure 10 The above is the S-parameter simulation curve of a single hairpin linear bandpass filter when its return loss performance deteriorates, as shown in Example 2.
[0040] Figure 11 The above is the S-parameter simulation curve of the single interdigital low-pass filter when the return loss performance degrades in Example 2.
[0041] Figure 12 The simulated S-parameter curves of the cascaded filter when the return loss of both the low-pass filter and the band-pass filter deteriorates simultaneously, as shown in Example 2.
[0042] Figure 13 The above is the S-parameter simulation curve corresponding to the transition microstrip line length Lt = 0.685 mm in Example 3;
[0043] Figure 14 The above is the S-parameter simulation curve corresponding to the transition microstrip line length Lt==1.085 mm in Example 3;
[0044] Figure 15 The above is the S-parameter simulation curve corresponding to the transition microstrip line length Lt=1.485 mm in Example 3.
[0045] Figure labels: 1 is the bandpass input port, 2 is the bandpass output port, 3 is the hairpin bandpass filter, 4 is the low-pass input port, 5 is the low-pass output port, 6 is the interdigital low-pass filter, and 7 is the transition microstrip line. Detailed Implementation
[0046] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments.
[0047] This embodiment takes a bandpass filter with a center frequency f0 = 26.1 GHz and a bandwidth of 1.2 GHz as an example. It requires that the parasitic passband suppression be greater than 50 dB in the 35~50 GHz frequency range, and the return loss in the passband after cascading be better than 18 dB.
[0048] Example 1: Cascading performance when individual filters all have excellent return loss
[0049] like Figure 1 and Figure 2 As shown, the hairpin-type bandpass filter includes an input connection terminal 1, an output microstrip line 2, and a fifth-order hairpin resonator 3. Both the input microstrip line 1 and the output microstrip line 2 are 50 Ω characteristic impedance microstrip lines with a width of W1. The fifth-order hairpin resonator 3 is a half-wavelength hairpin resonator with arm lengths L1, L2, and L3, a width of W2, a distance of W3 between the two arms, and coupling distances of S12 and S23 between adjacent resonators. This bandpass filter operates at a center frequency... At a frequency of 26.1 GHz and a bandwidth of 1.2 GHz, the simulated return loss within the passband is better than 21 dB, but a parasitic passband exists in the 35–50 GHz frequency range. The structural parameters of the hairpin bandpass filter are shown in Table 1.
[0050] Table 1 Structural parameters of hairpin line bandpass filters
[0051]
[0052] like Figure 3 and Figure 4 As shown, the interdigital low-pass filter includes an input microstrip line 4, an output connector 5, and a 5th-order interdigital filter unit 6. Both the input microstrip line 4 and the output connector 6 are 50 Ω characteristic impedance microstrip lines with a width of W1. The 5th-order interdigital filter unit 6 consists of five sets of staggered metal interdigital stubs, with lengths L4, L5, and L6 and widths W4, W5, and W6, respectively. This low-pass filter has a cutoff frequency of 29 GHz, and its in-band return loss is better than 25 dB when operating alone. Above 32 GHz, its out-of-band rejection is better than 30 dB, effectively suppressing the parasitic passband of the bandpass filter in the 35–50 GHz frequency range. The structural parameters of the interdigital low-pass filter are shown in Table 2.
[0053] Table 2 Structural parameters of interdigital low-pass filters
[0054]
[0055] In this embodiment, two interdigital low-pass filters 6 and one hairpin-line band-pass filter 3 are cascaded together via a transition microstrip line 7 of length Lt to form a microstrip filter assembly. The cascaded assembly forms a microstrip filter module as shown below. Figure 5 The structure shown is illustrated in the physical diagram. Figure 8 Specifically: the microstrip filter assembly includes a first interdigital low-pass filter, a second interdigital low-pass filter, a hairpin-line bandpass filter, a first microstrip line, and a second microstrip line; wherein: the first interdigital low-pass filter is connected to one end of the hairpin-line bandpass filter via the first microstrip line; the second interdigital low-pass filter is connected to the other end of the hairpin-line bandpass filter via the second microstrip line, thus forming an integrated cascaded structure; a metal ground layer is provided on the back side of the dielectric substrate.
[0056] Simulation tests were performed on the microstrip filter components. (See attached document.) Figure 6 and Figure 7 It can be seen that the microstrip filter component has a return loss of better than 18 dB in the 25.5~26.7 GHz passband and a parasitic passband suppression of better than 60 dB in the 35~50 GHz frequency range. Its out-of-band suppression capability is better than that of the previous patent and references.
[0057] Figure 9 The measured S-parameter curves are shown. The experimental results indicate that the filter exhibits a return loss better than 20 dB in the 25.5–26.7 GHz passband and a parasitic passband suppression better than 49 dB in the 35–40 GHz band. Both passband and out-of-band suppression performance are experimentally verified, demonstrating that the method described in this invention can effectively suppress the parasitic passband of the filter. Furthermore, there is a certain difference between the simulated and measured out-of-band suppression indices. This is mainly because the overall structure of the filter is relatively small. Under high out-of-band suppression conditions, weak radiated leakage signals at the test port can easily cause additional responses at other ports through spatial coupling / cavity coupling / filter assembly differences, thus affecting the measured out-of-band suppression performance.
[0058] Example 2: Cascading performance when the return loss of individual filters deteriorates
[0059] The component structure and material selection used in this embodiment are completely consistent with those in Embodiment 1, with only minor adjustments to some structural dimensional parameters. The purpose is to study the impact of the deterioration of the return loss performance of a single filter on the overall performance of the cascaded filter. Specifically, the tap feed position of the hairpin-type bandpass filter was adjusted from 0 mm to 0.08 mm; for the interdigital low-pass filter, its structural parameters were adjusted as follows: L4 was adjusted from 0.08 mm to 0.1 mm, L5 remained unchanged, and L6 was adjusted from 0.2 mm to 0.24 mm; simultaneously, W4 was adjusted from 0.05 mm to 0.08 mm, W5 remained unchanged, and W6 was adjusted from 0.06 mm to 0.05 mm. After parameter adjustments, the return loss of the hairpin-type microstrip bandpass filter 3 in the passband is approximately 13 dB, and the return loss of the interdigital microstrip low-pass filter 6 in the passband is approximately 14 dB.
[0060] Figure 10 The S-parameter simulation curves of the clip-on bandpass filter under this return loss specification are shown. Figure 11 The simulated S-parameter curves of the interdigital low-pass filter under this return loss specification are shown. The simulation results show that, compared to Example 1, both filters exhibit significant degradation in return loss within the passband, indicating a decrease in their matching performance.
[0061] Modeling and simulation were performed according to the cascaded method described in Example 1, resulting in the simulated S-parameter curves of the overall filter component, as shown below. Figure 12 As shown in the figure. Simulation results show that the cascaded filter component has a minimum return loss of only 8 dB in the 25.5~26.7 GHz passband, which is significantly worse than that of Example 1.
[0062] A comparative analysis of Examples 2 and 1 reveals that the return loss level of a single filter directly affects the overall matching performance of the cascaded filter. When the impedance matching performance of a single filter deteriorates, the port reflection coefficient in the cascaded structure increases, leading to a significant deterioration in the overall return loss performance. Therefore, ensuring that each single filter possesses good return loss characteristics is a key technology for achieving good overall impedance matching of the cascaded filter assembly, and is also one of the innovative aspects of this invention. Based on previous simulations and engineering design experience, it is recommended that the return loss of each single filter be better than 20 dB.
[0063] Example 3: Individual filters all exhibit good return loss; the influence of transition microstrip line length on cascade performance.
[0064] The single filter performance indicators, component structure, and material selection used in this embodiment are the same as those in Embodiment 1. Among them, the hairpin-type microstrip bandpass filter 3 has a return loss of better than 21 dB in the passband, and the interdigital microstrip lowpass filter 6 has a return loss of better than 25 dB in the passband.
[0065] Based on this, only the length Lt of the transition microstrip line 7 was adjusted, with values of 0.685 mm, 1.085 mm, and 1.485 mm, respectively. After modeling and simulating the cascaded structure according to the method described in Example 1, simulation analysis was performed on the return loss of the overall component in the passband (25.5~26.2 GHz) and the out-of-band suppression in the frequency band (35~50 GHz). The results are as follows. Figures 13 to 15 , and as shown in Table 3.
[0066] Table 3 Comparison of simulation results for different transition microstrip line lengths Lt
[0067]
[0068] Depend on Figures 13 to 15 As shown in Table 3, the simulation results demonstrate that, based on the good matching performance of the individual filters, the cascaded components exhibit good robustness to variations in the transition microstrip line length Lt. When Lt varies over a wide range, the overall performance fluctuation of the cascaded components is small, maintaining a high level of technical specifications. In particular, when Lt takes a specific fit length value (Lt = 1.085 mm in this embodiment), the cascaded structure achieves an optimal balance between good in-band matching performance and out-of-band suppression, with a passband return loss better than 18.8 dB and an out-of-band suppression better than 62 dB.
[0069] Example 3 verifies that the transition microstrip line length Lt is one of the key parameters for adjusting the performance of cascaded components. When Lt varies over a wide range, it has little impact on the in-band matching performance and out-of-band rejection of the cascaded components. Therefore, in actual design and optimization, the optimal physical size can be determined by performing parameter scanning and simulation analysis on the transition microstrip line length Lt, enabling the cascaded system to achieve optimal values for in-band impedance matching and out-of-band rejection.
[0070] In summary, this invention addresses the technical problems of insufficient parasitic passband suppression and in-band return loss performance in existing microstrip filters by proposing an integrated cascaded design method for an interdigital microstrip low-pass filter and a hairpin-line microstrip band-pass filter. While ensuring good return loss characteristics for individual filters, it effectively achieves good impedance matching for the overall cascaded filter components. Furthermore, by reasonably setting the cutoff frequency of the low-pass filter and optimizing the cascaded transition structure, without introducing additional complex circuitry, experimental results show that it effectively suppresses most of the inherent parasitic passband of the hairpin-line band-pass filter by more than 50 dB, while ensuring a passband return loss better than 18 dB, thus balancing the design requirements of high stopband suppression and excellent in-band matching characteristics.
Claims
1. A microstrip filter with high rejection of spurious passbands, characterized by: Including dielectric substrate; At least one set of microstrip filter components is integrated on the front side of the dielectric substrate; the microstrip filter components include a first interdigitated low-pass filter, a second interdigitated low-pass filter, a hairpin bandpass filter, a first microstrip line, and a second microstrip line; wherein: The first interdigital low-pass filter is connected to one end of the hairpin bandpass filter via the first microstrip; The second interdigital low-pass filter is connected to the other end of the hairpin bandpass filter via the second microstrip to form an integrated cascaded structure. A metal ground layer is provided on the back side of the dielectric substrate.
2. A microstrip filter with high parasitic passband suppression according to claim 1, characterized in that: The first interdigital low-pass filter includes an input microstrip line, a multi-order interdigital filter unit, and an output connection terminal; The input microstrip line is connected to the outside and is used to receive signals; the multi-stage interdigital filter units are cascaded sequentially along the signal transmission direction, with the first-stage interdigital filter unit connected to the input microstrip line and the last-stage interdigital filter unit connected to the first microstrip line via the input connection terminal. The second multi-order interdigital low-pass filter has the same structural dimensions as the first multi-order interdigital low-pass filter. The only difference is that the input microstrip line of the first-order interdigital filter unit in the second multi-order interdigital low-pass filter is connected to the second microstrip line, and the output connection terminal is the output of the entire device.
3. A microstrip filter with high parasitic passband suppression according to claim 2, characterized in that: The multi-order interdigital filter units in the first and second multi-order interdigital low-pass filters are composed of multiple sets of staggered metal interdigital branches, with a preset coupling spacing between adjacent sets of metal interdigital branches to form a capacitive coupling path.
4. A microstrip filter with high parasitic passband suppression according to claim 2, characterized in that: The output connection of the first interdigital low-pass filter is matched with the impedance of the first microstrip line, and the widths of the two are the same, so as to achieve a gapless coplanar direct connection between the two.
5. A microstrip filter with high parasitic passband suppression according to claim 3, characterized in that: In the multiple sets of staggered metal interdigitated branches, the spacing between adjacent metal interdigitated branches and the length of the metal interdigitated branches can be flexibly adjusted according to the filtering requirements to adapt to the cutoff frequency requirements of different frequency bands.
6. A microstrip filter with high parasitic passband suppression according to claim 1, characterized in that: The hairpin-type bandpass filter includes an input connection terminal, multiple hairpin resonators, and an output microstrip line. The multiple hairpin resonators are arranged sequentially along the length of the dielectric substrate, and adjacent hairpin resonators are arranged in opposite directions to ensure parallel coupling between adjacent resonators through their parallel closed ends spaced at a predetermined distance, thereby achieving signal transmission within a predetermined passband. The input connection of the first-order hairpin resonator is directly and coplanarly connected to the first microstrip line without any gap. The last-order hairpin resonator is connected to the output stripline, and the output microstrip line is directly connected to the second microstrip line in the same plane.
7. A microstrip filter with high parasitic passband suppression according to claim 6, characterized in that: The hairpin resonator is a half-wavelength hairpin resonator structure, and its resonant length, arm spacing and coupling spacing are adjusted according to the center frequency and bandwidth requirements of the entire device. The positions of the input connection terminal and the feed point of the output microstrip line are adjusted along the resonator axis.
8. A microstrip filter with high parasitic passband suppression according to claim 1, characterized in that: The cutoff frequencies of the first and second interdigital low-pass filters are higher than the upper edge frequency of the passband of the hairpin bandpass filter, but lower than its parasitic passband frequency.
9. A microstrip filter with high parasitic passband suppression according to claim 1, characterized in that: The input microstrip lines of the first and second interdigital low-pass filters, the output microstrip line of the hairpin bandpass filter, and the first and second microstrip lines are all 50 Ω characteristic impedance microstrip transmission lines.