CN112952319B - Microstrip dual-passband filter with independently controllable passband based on zero-degree feed structure - Google Patents
Microstrip dual-passband filter with independently controllable passband based on zero-degree feed structure Download PDFInfo
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Abstract
The invention discloses a microstrip dual-passband filter with independently controllable passband based on a zero-degree feed structure. The whole structure comprises a first open-circuit stub loading dual-mode resonator 1 and a second open-circuit stub loading dual-mode resonator 2 which are in capacitive coupling at the ends, a lambda/2 stepped impedance resonator 3, a lambda/2 uniform impedance resonator 4, an input circuit 5 and an output circuit 6, wherein the lambda/2 stepped impedance resonator 3 and the lambda/2 uniform impedance resonator 4 are embedded in the first open-circuit stub loading dual-mode resonator 1 and the second open-circuit stub loading dual-mode resonator 2, so that the circuit structure is compact.
Description
Technical Field
The invention relates to the field of wireless communication and filter design, in particular to a filter design method which is compatible with different communication standards and can support working at different frequency bands.
Background
In recent decades, with the development and evolution of 1G communication technology to 5G communication technology, a single communication device needs to be capable of operating in compliance with and supporting multiple communication standards, such as GSM, WIFI, WiMAX, CDMA, and LTE standards. To achieve this, the most conventional solution is that the rf front end of the communication device is composed of a plurality of rf circuits, each of which contains independent filters, amplifiers, etc. with different center frequencies and bandwidths. Filters with different center frequencies are selected for communication using the radio frequency switch. While this scheme can support simultaneous operation of multiple communication standards, it adds to the size, power consumption, and cost of the communication system. To overcome the weakness of this solution, a multi-frequency rf circuit composed of multi-frequency devices such as a multi-frequency filter and a multi-frequency amplifier is developed. The multi-frequency device can work in a plurality of frequency bands simultaneously and is a single-port input and single-port output device. Therefore, the scheme can effectively reduce the volume, power consumption and cost of the communication system and has wide application prospect. Multifrequency filters are one of the key components of a multifrequency communication system. The performance of the communication system is directly affected and determined by the performance of the communication system. Therefore, the multifrequency filter is widely concerned and emphasized by the academic and industrial circles at home and abroad. The research on the high-performance multifrequency filter can effectively improve the performance of a communication system and reduce the volume and the cost of the system.
Currently, commonly used dual-passband filter design methods include a single filter combination design method, a multi-mode resonator method, and a multi-mode single resonator method. In recent years, a dual-band filter with compact size, high frequency selectivity, high band isolation and independently controllable passband bandwidth gradually becomes a research hotspot of the dual-band filter.
Xiu Yin Zhang et al propose a microstrip dual-passband filter with controllable passband based on a zero-degree feed structure. The center frequencies of the two pass bands are 1.6GHz and 2.45GHz respectively, and the insertion losses are 1.46dB and 1.16dB respectively. The filter comprises two coupled open-circuit stub-loaded dual-mode resonators. However, the filter cannot realize independent control of the bandwidths of the two pass bands, and the filter only comprises three transmission zeros in the stop band, so that the pass band isolation degree of the filter is to be improved.
Chi-Feng Chen et al designs a four-order microstrip dual-band filter based on a zero-degree feed structure by using a pair of stepped impedance resonators and two embedded half-and-half wavelength resonators. Although the filter can generate seven transmission zeros at the stop band, the filter cannot achieve independently controllable pass band bandwidths due to the close distance between the two embedded pairs of half-wavelength resonators.
At present, few reports on microstrip dual-band filters with compact size, high frequency selectivity and passband isolation and independently controllable passband are reported in the literature. The invention discloses a three-order cross-coupling dual-band filter, which is realized by using a micro-strip process.
Disclosure of Invention
The invention provides a microstrip dual-passband filter with independently controllable passband based on a zero-degree feed structure. As shown in fig. 1, the whole structure includes a first open-circuit stub-loaded dual-mode resonator 1 and a second open-circuit stub-loaded dual-mode resonator 2, a λ/2 stepped impedance resonator 3, a λ/2 uniform impedance resonator 4, an input circuit 5, and an output circuit 6, which are symmetrically end-capacitively coupled, where the λ/2 stepped impedance resonator 3 and the λ/2 uniform impedance resonator 4 are embedded inside the first open-circuit stub-loaded dual-mode resonator 1 and the second open-circuit stub-loaded dual-mode resonator 2, so as to make the circuit structure compact. Because the first open-circuit stub-loaded dual-mode resonator 1 and the second open-circuit stub-loaded dual-mode resonator 2 are capacitively coupled, the stop bands of the two pass bands can generate a transmission zero. The filter adopts a zero-degree feed structure, and can generate three additional transmission zeros in a stop band. The bandwidths of the two passbands can be adjusted independently. The filter is simulated by full-wave electromagnetic simulation software HFSS and processed into a real object according to the whole circuit structure of the filter.
Preferably, the first open-circuit stub-loaded dual-mode resonator 1 is composed of a λ/2 transmission line resonator 11 having a length of 2(L1+ L2) + W2 and a width of W1 and an open-circuit stub 12 having a length of L3 and a width of W2, the second open-circuit stub-loaded dual-mode resonator 2 is composed of a λ/2 transmission line resonator 21 having a length of 2(L1+ L2) + W2 and a width of W1 and an open-circuit stub 22 having a length of L3 and a width of W2, the coupling distances of the first open-circuit stub-loaded dual-mode resonator 1 and the second open-circuit stub-loaded dual-mode resonator 2 are S1 and are symmetrical to each other, the length of the λ/2 ladder impedance resonator 3 is L4+2(L5+ L6), the length of the λ/2 uniform impedance resonator 4 is L7, the λ/2 ladder impedance resonator 3 and the first open-circuit stub-loaded dual-mode resonator are G2, the coupling gaps of the lambda/2 stepped impedance resonator 3 and the second open-circuit stub loaded dual-mode resonator 2 are respectively G2, the coupling gaps of the lambda/2 uniform impedance resonator 4 and the first open-circuit stub loaded dual-mode resonator 1 are respectively G3, the coupling gaps of the lambda/2 uniform impedance resonator 4 and the second open-circuit stub loaded dual-mode resonator 2 are respectively G3, the width of a feeder line 51 of the input circuit 5 is W01, the length and the width of a high-impedance open-circuit stub 52 loaded on the feeder line 51 are respectively L01+ L02 and W02, the width of a feeder line 61 of the output circuit 6 is W01, and the length and the width of a high-impedance open-circuit stub 62 loaded on the feeder line 61 are respectively L01+ L02 and W02; specific physical structure parameters are W01-1.5 mm, W02-0.37 mm, W1-0.5 mm, W2-1 mm, W3-0.5 mm, W4-3 mm, L01-15.44 mm, L02-3.9 mm, L1-17.52 mm, L2-9.64 mm, L3-7.7 mm, L4-13.55 mm, L5-10 mm, L6-1.47 mm, L7-31.3 mm, S1-0.76 mm, G1-0.16 mm, G2-0.62 mm, G3-0.54 mm, D-1.23 mm, and overall size of x 21.73 mm.
Preferably, the lambda/2 stepped impedance resonator 3 and the lambda/2 uniform impedance resonator 4 are embedded in the two open-circuit stub-loaded dual-mode resonators, so that the circuit structure is compact; the first open-circuit stub loaded dual-mode resonator 1 and the second open-circuit stub loaded dual-mode resonator 2 are capacitively coupled, and a transmission zero point can be generated at the stop band of each passband; the filter adopts a zero-degree feed structure, and can generate three additional transmission zeros in a stop band; the bandwidths of the two passbands can be adjusted independently.
Preferably, the first odd mode 13 of the first open-circuit stub-loaded dual-mode resonator 1 and the first odd mode 23 of the second open-circuit stub-loaded dual-mode resonator 2 are coupled with the lambda/2 stepped impedance resonator 3 positioned above to form a passband a; the first even mode 14 of the first open-circuit stub loaded dual-mode resonator 1 and the first even mode 24 of the second open-circuit stub loaded dual-mode resonator 2 are coupled with the lambda/2 uniform impedance resonator 4 positioned below to form a passband B; the first odd mode 13 of the first open-circuit stub loaded dual-mode resonator 1 and the first odd mode 23 of the second open-circuit stub loaded dual-mode resonator 2, the lambda/2 stepped impedance resonator 3 and the first odd mode 23 of the second open-circuit stub loaded dual-mode resonator 2, the first even mode 14 and the lambda/2 uniform impedance resonator 4 of the first open-circuit stub loaded dual-mode resonator 1, and the coupling between the lambda/2 uniform impedance resonator 4 and the first even mode 24 of the second open-circuit stub loaded dual-mode resonator 2 are direct coupling; the coupling between the first odd mode 13 of the first open-circuit stub loaded dual-mode resonator 1 and the first odd mode 23 of the second open-circuit stub loaded dual-mode resonator 2, the first even mode 14 of the first open-circuit stub loaded dual-mode resonator 1 and the first even mode 24 of the second open-circuit stub loaded dual-mode resonator 2 is cross coupling, so that a transmission zero can be generated in the stop bands of the pass band A and the pass band B.
Preferably, the input circuit 5 and the output circuit 6 adopt a zero-degree feeding structure to perform slot feeding on the first open-circuit stub-loaded dual-mode resonator 1 and the second open-circuit stub-loaded dual-mode resonator 2, and the feeding structure can introduce three transmission zeros in the stop band; the position of the transmission zero point generated by the zero-degree feed structure is related to the position D of the feeder line; as the feeder position D increases, the positions of the first, third and fifth transmission zeroes TZ1, TZ3, TZ5 change, while the positions of the second and fourth transmission zeroes TZ2, TZ4 remain nearly unchanged; the transmission zeroes TZ1, TZ3, and TZ5 are created by a zero-degree feed structure, and the transmission zeroes TZ2, TZ4 are created by a cross-coupled structure.
Preferably, when the filter operates at two different passband center frequencies, signals will travel through different paths; when the filter works in a passband A, current is distributed on two arms of the first open-circuit stub loaded dual-mode resonator 1 and the second open-circuit stub loaded dual-mode resonator 2 and the lambda/2 stepped impedance resonator 3 above the two arms; when the filter works in a passband B, current is distributed on the two arms of the first open-circuit stub loaded dual-mode resonator 1 and the second open-circuit stub loaded dual-mode resonator 2, the open-circuit stubs of the two arms, and the lambda/2 uniform impedance resonators 4 below the two arms.
Preferably, a coupling gap between the λ/2 stepped-impedance resonator 3 and the first open-circuit stub-loaded dual-mode resonator 1 and the second open-circuit stub-loaded dual-mode resonator 2 is G2, a coupling gap between the λ/2 uniform-impedance resonator 4 and the first open-circuit stub-loaded dual-mode resonator 1 and the second open-circuit stub-loaded dual-mode resonator 2 is G3, and the bandwidth of the pass band a and the bandwidth of the pass band B can be controlled by independently adjusting the sizes of G2 and G3; when G2 is increased, the bandwidth of the pass band A is reduced, and the bandwidth of the pass band B is not affected; when G3 is increased, the bandwidth of the pass band B is reduced, and the bandwidth of the pass band A is not affected; the bandwidths of the two passbands can be adjusted independently.
Preferably, the operating frequencies are 2GHz and 3.5 GHz.
The filter provided by the invention uses a zero-degree feed structure and has completely new circuit structure characteristics.
Drawings
FIG. 1 is a structure of a microstrip dual bandpass filter;
FIG. 2 is a topology of a microstrip dual bandpass filter;
fig. 3 shows the variation of the transmission response of the filter with the feed position D;
FIG. 4 simulated current distribution of the filter at 2GHz and 3.5 GHz;
FIG. 5(a) the transmission response of the filter as a function of G2;
FIG. 5(b) the transmission response of the filter as a function of G3;
FIG. 6 is a schematic diagram of a microstrip dual bandpass filter;
FIG. 7 is a simulated layout structure of the filter;
simulation results for the filter of FIG. 8;
FIG. 9 simulation test results for the filter;
Detailed Description
The center frequency of the filter involved in the invention is 9GHz, the relative bandwidth is 5%, the return loss is better than 30dB, and the transmission zero point is 9.6 GHz. From the above indices, the coupling coefficient (M) of the filter can be determined by the coupling matrix theory12＝M23＝0.0637，M130.0425) and external quality factor (Q)e10.69) and specific physical dimensions.
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.
The microstrip dual-passband filter based on the zero-degree feed structure works at 2GHz and 3.5GHz, and performance indexes of the microstrip dual-passband filter comprise center frequency, relative bandwidth, return loss, transmission zero position and the like.
The physical structure parameters of the filter are shown in fig. 1. The first open-circuit stub-loaded dual-mode resonator 1 is composed of a λ/2 transmission line resonator 11 with the length of 2(L1+ L2) + W2 and the width of W1 and an open-circuit stub 12 with the length of L3 and the width of W2, the second open-circuit stub-loaded dual-mode resonator 2 is composed of a λ/2 transmission line resonator 21 with the length of 2(L1+ L2) + W2 and the width of W1 and an open-circuit stub 22 with the length of L3 and the width of W2, the coupling distances of the first open-circuit stub-loaded dual-mode resonator 1 and the second open-circuit stub-loaded dual-mode resonator 2 are S1 and are symmetrical to each other, the length of the λ/2 stepped impedance resonator 3 is L4+2(L5+ L6), the length of the λ/2 uniform impedance resonator 4 is L7, the λ/2 stepped impedance resonator 3 and the first open-circuit stub-loaded dual-mode resonator 1 are coupled gaps G2, the coupling gaps of the lambda/2 stepped impedance resonator 3 and the second open-circuit stub loaded dual-mode resonator 2 are respectively G2, the coupling gaps of the lambda/2 uniform impedance resonator 4 and the first open-circuit stub loaded dual-mode resonator 1 are respectively G3, the coupling gaps of the lambda/2 uniform impedance resonator 4 and the second open-circuit stub loaded dual-mode resonator 2 are respectively G3, the width of a feeder line 51 of the input circuit 5 is W01, the lengths and the widths of the high-impedance open-circuit stubs 52 loaded on the feeder line 51 are respectively L01+ L02 and W02, the width of a feeder line 61 of the output circuit 6 is W01, and the lengths and the widths of the high-impedance open-circuit stubs 62 loaded on the feeder line 61 are respectively L01+ L02 and W02.
The coupling topological structure of the filter is shown in fig. 2, a first odd mode 13 of a first open-circuit stub loaded dual-mode resonator 1 and a first odd mode 23 of a second open-circuit stub loaded dual-mode resonator 2 are coupled with a lambda/2 stepped impedance resonator 3 positioned above to form a pass band A; the first even mode 14 of the first open-circuit stub loaded dual-mode resonator 1 and the first even mode 24 of the second open-circuit stub loaded dual-mode resonator 2 are coupled with the lambda/2 uniform impedance resonator 4 positioned below to form a passband B; the first odd mode 13 of the first open-circuit stub loaded dual-mode resonator 1 and the first odd mode 23 of the second open-circuit stub loaded dual-mode resonator 2, the lambda/2 stepped impedance resonator 3 and the first odd mode 23 of the second open-circuit stub loaded dual-mode resonator 2, the first even mode 14 of the first open-circuit stub loaded dual-mode resonator 1 and the lambda/2 uniform impedance resonator 4, and the coupling between the lambda/2 uniform impedance resonator 4 and the first even mode 24 of the second open-circuit stub loaded dual-mode resonator 2 are direct coupling; the coupling between the first odd mode 13 of the first open-circuit stub loaded dual-mode resonator 1 and the first odd mode 23 of the second open-circuit stub loaded dual-mode resonator 2, the first even mode 14 of the first open-circuit stub loaded dual-mode resonator 1 and the first even mode 24 of the second open-circuit stub loaded dual-mode resonator 2 is cross coupling, so that a transmission zero can be generated in the stop bands of the pass band A and the pass band B.
The structure of the filter is shown in fig. 1, the input and output feeder lines adopt a zero-degree feed structure to perform slot feed on the first open-circuit branch-loaded dual-mode resonator 1 and the second open-circuit branch-loaded dual-mode resonator 2, and the feed structure can introduce three transmission zeros in a stop band. The position of the transmission zero produced by the zero degree feed structure is related to the feed line position D, as shown in fig. 3. As the feeder position D increases, the positions of the first, third and fifth transmission zeroes TZ1, TZ3, TZ5 change, while the positions of the second and fourth transmission zeroes TZ2, TZ4 remain nearly unchanged. The transmission zeroes TZ1, TZ3 and TZ5 are thus created by the zero-degree feed structure, while the transmission zeroes TZ2, TZ4 are created by the cross-coupled structure.
When the filter is operating at two different passband center frequencies, the signal will travel through different paths as shown in fig. 4. When the filter works in a passband A, current is distributed on two arms of the first open-circuit stub loaded dual-mode resonator 1 and the second open-circuit stub loaded dual-mode resonator 2 and the lambda/2 stepped impedance resonator 3 above the two arms; when the filter works in a passband B, current is distributed on the two arms of the first open-circuit stub loaded dual-mode resonator 1 and the second open-circuit stub loaded dual-mode resonator 2, the open-circuit stubs of the two arms, and the lambda/2 uniform impedance resonators 4 below the two arms.
Wherein a coupling gap between the λ/2 stepped-impedance resonator 3 and the first and second open-circuit stub-loaded dual- mode resonators 1 and 2 is G2, a coupling gap between the λ/2 uniform-impedance resonator 4 and the first and second open-circuit stub-loaded dual- mode resonators 1 and 2 is G3, and the bandwidth of the pass band a and the pass band B can be controlled by independently adjusting the sizes of G2 and G3, as shown in fig. 5(a) and 5 (B). When G2 is increased, the bandwidth of the pass band A is reduced, and the bandwidth of the pass band B is not affected; as G3 increases, the bandwidth of pass band B decreases while the bandwidth of pass band a is unaffected. The bandwidths of the two passbands can be adjusted independently.
The design of the invention is processed by the traditional microstrip technology, and the finished product is shown in figure 6.
The filter provided by the invention has two pass bands, the center frequency of the pass band A is 2GHz, the return loss is better than 15dB, the 3dB relative bandwidth is designed to be 8.5%, the preset transmission zero point is positioned at 2.23GHz, and the non-normalized coupling matrix and the external quality factor of the pass band A can be determined to be as follows according to the coupling matrix theory:
Qe Ⅰ＝13.16
the center frequency of the pass band B is 3.5GHz, the return loss is better than 20dB, the 3dB relative bandwidth is designed to be 8.9%, the preset transmission zero point is positioned at 4.10GHz, and the non-normalized coupling matrix and the external quality factor of the pass band B are as follows:
Qe Ⅱ＝9.57
the initial physical size of the filter can be extracted from the given coupling matrix and the external figure of merit.
Figure 3 is a graph of the transmission response of the filter as a function of the feeder position D. Fig. 4 shows the current distribution of the filter at the center frequencies of pass band a and pass band B (2GHz and 3.5 GHz). Fig. 5(a) and 5(b) show the variation of the filter transmission response with coupling slots G2 and G3.
The filter according to the present invention has a structure as shown in fig. 1, where the physical size of the filter is W01-1.5 mm, W02-0.37 mm, W1-0.5 mm, W2-1 mm, W3-0.5 mm, W4-3 mm, L01-15.44 mm, L02-3.9 mm, L1-17.52 mm, L2-9.64 mm, L3-7.7 mm, L4-13.55 mm, L5-10 mm, L6-1.47 mm, L7-31.3 mm, S1-0.76 mm, G1-0.16 mm, G2-0.62 mm, G3-0.54 mm, and D-1 mm. The overall dimensions were 21.23mm by 36.73 mm.
The filter adopts a substrate material of Rogers5880, the relative dielectric constant is 2.2, and the thickness is 0.508 mm; the microstrip line is made of copper and has a thickness of 0.017 mm. The simulated layout structure of the filter is shown in fig. 7.
The data indexes of the results of software simulation and optimization according to the embodiment are shown in (1), and the simulation results are shown in fig. 8.
Fig. 6 is a schematic diagram of a filter. The vector network analyzer used for the test was Agilent E8363B, and the comparison of the test results with the simulation data is shown in fig. 9. The test result shows that the center frequencies of the two pass bands are respectively located at 1.95GHz and 3.47GHz, the relative bandwidths of 3dB are respectively 8% and 8.4%, the return loss is respectively better than 10dB and 20dB, the average insertion loss is less than 2dB, and the five transmission zeros are respectively located at 1.3GHz, 2.18GHz, 2.81GHz, 4.08GHz and 4.74GHz, so that the two pass bands have high frequency selectivity and pass band isolation. In general, the test performance of the physical device is good, and the deviation of the measurement and simulation results is mainly caused by processing errors.
Although the present invention has been described with reference to the preferred embodiments, it is not intended to limit the present invention, and those skilled in the art can make variations and modifications of the present invention without departing from the spirit and scope of the present invention by using the methods and technical contents disclosed above.
Claims (8)
1. The utility model provides a microstrip dual-passband filter that independent controllable of passband based on zero degree feed structure which characterized in that: the dual-mode resonator comprises a first open-circuit stub loading dual-mode resonator (1) and a second open-circuit stub loading dual-mode resonator (2), a lambda/2 stepped impedance resonator (3), a lambda/2 uniform impedance resonator (4), an input circuit (5) and an output circuit (6), wherein the symmetrical ends are capacitively coupled, and the lambda/2 stepped impedance resonator (3) and the lambda/2 uniform impedance resonator (4) are embedded in the first open-circuit stub loading dual-mode resonator (1) and the second open-circuit stub loading dual-mode resonator (2);
the first open-circuit stub loaded dual-mode resonator (1) is coupled with the second open-circuit stub loaded dual-mode resonator (2);
the first odd mode (13) of the first open-circuit stub-loaded dual-mode resonator and the first odd mode (23) of the second open-circuit stub-loaded dual-mode resonator are coupled with the lambda/2 stepped impedance resonator (3) positioned above;
a first even mode (14) of the first open-circuit stub loaded dual-mode resonator and a first even mode (24) of the second open-circuit stub loaded dual-mode resonator are coupled with a lambda/2 uniform impedance resonator (4) positioned below;
the input circuit (5) is coupled with the first open-circuit stub loading dual-mode resonator (1); the output circuit (6) is coupled with the second open-circuit stub-loaded dual-mode resonator (2).
2. The microstrip dual-passband filter based on the zero-degree feed structure and with independently controllable passband according to claim 1, characterized in that: the first open-circuit branch-node-loaded dual-mode resonator (1) is composed of a first lambda/2 transmission line resonator (11) with the length of 2(L1+ L2) + W2 and the width of W1, a first open-circuit branch (12) with the length of L3 and the width of W2, the second open-circuit branch-node-loaded dual-mode resonator (2) is composed of a second lambda/2 transmission line resonator (21) with the length of 2(L1+ L2) + W2 and the width of W1, and a second open-circuit branch (22) with the length of L3 and the width of W2, the coupling distance between the first open-circuit branch-node-loaded dual-mode resonator (1) and the second open-circuit branch-node-loaded dual-mode resonator (2) is S1 and is symmetrical to each other, the length of the lambda/2 stepped impedance resonator (3) is L4+2(L5+ L6), the uniform impedance resonator (4) is L7, the length of the lambda/2 is L462, and the first open-circuit branch-node-loaded dual-mode resonator (2), the coupling gap between the lambda/2 stepped impedance resonator (3) and the second open-circuit branch-node loaded dual-mode resonator (2) is G2, the coupling gap between the lambda/2 uniform impedance resonator (4) and the first open-circuit branch-node loaded dual-mode resonator (1) is G3, the coupling gap between the lambda/2 uniform impedance resonator (4) and the second open-circuit branch-node loaded dual-mode resonator (2) is G3, the input circuit (5) comprises a first feeder line (51) and a first high-impedance branch-node (52), the width of the first feeder line (51) is W01, the length and the width of the first high-impedance branch-node (52) loaded on the first feeder line (51) are L01+ L02 and W02 respectively, the output circuit (6) comprises a second feeder line (61) and a second high-impedance branch-node (62), the width of the second feeder line (61) is W01, and the length and the width of the second high-impedance branch-node (62) loaded on the second feeder line (61) are L35734 + L734 and W4934 respectively; specific physical structure parameters are W01=1.5mm, W02=0.37mm, W1=0.5mm, W2=1mm, W3=0.5mm, W4=3mm, L01=15.44mm, L02=3.9mm, L1=17.52mm, L2=9.64mm, L3=7.7mm, L4=13.55mm, L5=10mm, L6=1.47mm, L7=31.3mm, S1=0.76mm, G1=0.16mm, G2=0.62mm, G3=0.54mm, D =1mm, overall dimensions 21.23mm × 36.73 mm.
3. The microstrip dual-passband filter based on the zero-degree feed structure and with independently controllable passband according to claim 1, characterized in that: the lambda/2 stepped impedance resonator (3) and the lambda/2 uniform impedance resonator (4) are embedded in the two open-circuit branch-loaded dual-mode resonators, so that the circuit structure is compact; the first open-circuit stub loaded dual-mode resonator (1) and the second open-circuit stub loaded dual-mode resonator (2) are in capacitive coupling, and a transmission zero can be generated at the stop bands of the two pass bands; the filter adopts a zero-degree feed structure and can generate three additional transmission zeros in a stop band; the bandwidths of the two passbands are adjusted independently.
4. The microstrip dual-passband filter based on the zero-degree feed structure and with independently controllable passband according to claim 1, characterized in that: a first odd mode (13) of the first open-circuit stub loaded dual-mode resonator (1) and a first odd mode (23) of the second open-circuit stub loaded dual-mode resonator (2) are coupled with the lambda/2 stepped impedance resonator (3) positioned above to form a passband A; a first even mode (14) of the first open-circuit stub loaded dual-mode resonator (1) and a first even mode (24) of the second open-circuit stub loaded dual-mode resonator (2) are coupled with a lambda/2 uniform impedance resonator (4) positioned below to form a passband B; the coupling between a first odd mode (13) of the first open-circuit stub loaded dual-mode resonator (1) and a lambda/2 stepped impedance resonator (3), between the lambda/2 stepped impedance resonator (3) and a first odd mode (23) of the second open-circuit stub loaded dual-mode resonator (2), between a first even mode (14) of the first open-circuit stub loaded dual-mode resonator (1) and a lambda/2 uniform impedance resonator (4), and between the lambda/2 uniform impedance resonator (4) and a first even mode (24) of the second open-circuit stub loaded dual-mode resonator (2) is direct coupling; the coupling between the first odd mode (13) of the first open-circuit stub loaded dual-mode resonator (1) and the first odd mode (23) of the second open-circuit stub loaded dual-mode resonator (2) and between the first even mode (14) of the first open-circuit stub loaded dual-mode resonator (1) and the first even mode (24) of the second open-circuit stub loaded dual-mode resonator (2) is cross coupling, so that the two couplings can generate a transmission zero point at the stop bands of a pass band A and a pass band B.
5. The microstrip dual-passband filter based on the zero-degree feed structure and with independently controllable passband according to claim 4, wherein: the input circuit (5) and the output circuit (6) adopt a zero-degree feed structure to carry out gap feed on the first open-circuit branch loading dual-mode resonator (1) and the second open-circuit branch loading dual-mode resonator (2), and the feed structure can introduce three transmission zeros in a stop band; the position of the transmission zero point generated by the zero-degree feed structure is related to the position D of the feeder line; as the feeder position D changes, the positions of the first, third and fifth transmission zeroes TZ1, TZ3, TZ5 change, while the positions of the second and fourth transmission zeroes TZ2, TZ4 remain nearly unchanged; the first transmission zero TZ1, the third transmission zero TZ3 and the fifth transmission zero TZ5 are generated by a zero-degree feeding structure, and the second transmission zero TZ2 and the fourth transmission zero TZ4 are generated by a cross-coupling structure.
6. The microstrip dual-passband filter based on the zero-degree feed structure and with independently controllable passband according to claim 1, characterized in that: when the filter works at two different passband center frequencies, signals can be transmitted through different paths; when the filter works in a passband A, current is distributed on two arms of the first open-circuit stub loaded dual-mode resonator (1) and the second open-circuit stub loaded dual-mode resonator (2) and the lambda/2 stepped impedance resonator (3) above the two arms; when the filter works in a passband B, current is distributed on two arms of the first open-circuit stub loaded dual-mode resonator (1) and the second open-circuit stub loaded dual-mode resonator (2), open-circuit stubs of the two arms, and lambda/2 uniform impedance resonators (4) below the two arms.
7. The microstrip dual-passband filter based on the zero-degree feed structure and with independently controllable passband according to claim 4, wherein: the coupling gap between the lambda/2 stepped impedance resonator (3) and the first open-circuit stub loaded dual-mode resonator (1) and the second open-circuit stub loaded dual-mode resonator (2) is G2, the coupling gap between the lambda/2 uniform impedance resonator (4) and the first open-circuit stub loaded dual-mode resonator (1) and the second open-circuit stub loaded dual-mode resonator (2) is G3, and the bandwidth sizes of a passband A and a passband B can be controlled by independently adjusting the sizes of G2 and G3; when G2 is increased, the bandwidth of the pass band A is reduced, and the bandwidth of the pass band B is not affected; when G3 is increased, the bandwidth of the pass band B is reduced, and the bandwidth of the pass band A is not affected; the bandwidths of the two passbands are adjusted independently.
8. The microstrip dual-passband filter based on the zero-degree feed structure and with independently controllable passband according to claim 1, characterized in that: the operating frequencies were 2GHz and 3.5 GHz.
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