CN115765665A - Branch knot loading resonator-based balun filter - Google Patents

Abstract

The invention relates to the technical field of filters, and discloses a balun filter based on a stub loading resonator, which comprises a first coupling microstrip line, a second coupling microstrip line, a third coupling microstrip line, a fourth coupling microstrip line, a fifth coupling microstrip line, a first microstrip resonator, a second microstrip resonator, a first open-circuit stub, a second open-circuit stub, a first capacitor piece and a ground plate; the fourth coupling microstrip line is coupled with the first microstrip resonator in parallel, and the first open-circuit branch is connected with the fourth coupling microstrip line; the fifth coupling microstrip line is coupled with the second microstrip resonator in parallel, and the second open-circuit branch is connected with the fifth coupling microstrip line; the grounding plate is provided with a defected ground structure. The microstrip resonator and the circuit generate multimode resonance, and the open-circuit branch and the microstrip resonator increase the selectivity of the filter passband edge; the defected ground structure can reduce the volume of the filter and widen the use frequency; the lengths and the widths of the microstrip resonators and the open-circuit branches are adjusted, and the bandwidth of the filter can be adjusted.

Description

Balun filter based on branch loading resonator

Technical Field

The invention relates to the technical field of filters, in particular to a balun filter based on a stub loading resonator.

Background

With the continuous update and upgrading of modern communication systems, the rapid development of wireless communication technology puts forward more strict requirements on radio frequency front-end circuit components, and high performance, miniaturization, low manufacturing cost and the like become important indexes for evaluating the components at present. In a modern wireless transceiving system, a circuit structure is an unbalanced circuit consisting of a floor and an excitation end, a terminal is a dual-polarized antenna element needing balanced port excitation, and a filter is needed for frequency screening before the antenna element transmits information or after the antenna element receives signals. Under the condition, the balun filter is generated by operation, and the balun and the filter are combined into a device, so that the size is reduced, the cost is saved, and meanwhile, the signal loss and the damage risk caused by the arrangement of an intermediate circuit are avoided.

In the balance-unbalance conversion, microstrip line coupling is a common means, but the microstrip line coupling has the problems of large required area and narrow used bandwidth, and can not meet different bandwidths required by a filter in different application scenarios.

Disclosure of Invention

The purpose of the invention is: the balun filter based on the stub loading resonator is provided, and the problems that the balun filter in the prior art is small in size, narrow in bandwidth and incapable of adjusting the bandwidth are solved.

In order to achieve the above object, the present invention provides a balun filter based on a stub loaded resonator, which includes an input port, a first output port, a second output port, a first coupled microstrip line, a second coupled microstrip line, a third coupled microstrip line, a fourth coupled microstrip line, a fifth coupled microstrip line, a first microstrip resonator, a second microstrip resonator, a first open-circuit stub, a second open-circuit stub, a first capacitor plate, and a ground plate;

the first coupling microstrip line is connected with the input port, a half-wavelength front section of the first coupling microstrip line and a quarter-wavelength section of the second coupling microstrip line are coupled in parallel to form an in-phase coupling circuit, a half-wavelength rear section of the first coupling microstrip line and a quarter-wavelength section of the third coupling microstrip line are coupled in parallel to form a differential phase coupling circuit, and the in-phase coupling circuit, the differential phase coupling circuit and the ground plate form a balun;

the second coupling microstrip line is connected with the fourth coupling microstrip line, the fourth coupling microstrip line is coupled with the first microstrip resonator in parallel, the first open-circuit branch is connected with the fourth coupling microstrip line, and the fourth coupling microstrip line is communicated with the second output port;

the third coupling microstrip line is connected with the fifth coupling microstrip line, the fifth coupling microstrip line is coupled with the second microstrip resonator in parallel, the second open-circuit branch is connected with the fifth coupling microstrip line, and the fifth coupling microstrip line is communicated with the first output port;

the grounding plate is provided with a defected ground structure, the first capacitor plate and the grounding plate form a first capacitor element, and the fourth coupling microstrip line and the fifth coupling microstrip line are both connected with the first capacitor plate.

Preferably, the first microstrip resonator and the second microstrip resonator are both L-shaped structures, and the first microstrip resonator and the second microstrip resonator are symmetrically arranged.

Preferably, the first open-circuit branch is vertically connected to the fourth coupling microstrip line, the second open-circuit branch is vertically connected to the fifth coupling microstrip line, and the first open-circuit branch and the second open-circuit branch are arranged in a central symmetry manner.

Preferably, the filter further includes a first inductance element and a second inductance element, the first inductance element is connected between the first capacitor plate and the fifth coupling microstrip line, the second inductance element is connected between the first capacitor plate and the fourth coupling microstrip line, and the first inductance element, the second inductance element and the first capacitance element form a filter circuit.

Preferably, the first inductance element and the second inductance element are both U-shaped strip lines, and the first inductance element and the second inductance element are symmetrically arranged.

Preferably, the self-feedback switch further comprises a second capacitor plate and a third capacitor plate, the fourth coupling microstrip line and the second output port, and the second inductive element and the fourth coupling microstrip line are connected through the second capacitor plate, the fifth coupling microstrip line and the first output port, and the fifth coupling microstrip line and the first inductive element are connected through the third capacitor plate, and the second capacitor plate and the third capacitor plate form a second capacitor element and form a self-feedback circuit.

Preferably, the second capacitor plate and the third capacitor plate are both square capacitor plates.

Preferably, the antenna comprises a plurality of dielectric substrates arranged in a stacked manner, wherein a conductor layer is arranged between each two adjacent dielectric substrates, the conductor layers are defined as a first conductor layer, a second conductor layer, a third conductor layer, a fourth conductor layer, a fifth conductor layer and a sixth conductor layer from top to bottom, the third capacitor plate and the first output port are arranged on the first conductor layer, the second capacitor plate and the second output port are arranged on the second conductor layer, the first inductance element is arranged on the third conductor layer, the second inductance element is arranged on the fourth conductor layer, the input port, the first coupling microstrip line, the second coupling microstrip line, the third coupling microstrip line, the fourth coupling microstrip line, the fifth coupling microstrip line, the first microstrip resonator, the second microstrip resonator, the first open-circuit branch, the second open-circuit branch and the first capacitor plate are all arranged on the fifth conductor layer, the ground plate is arranged on the sixth conductor layer, a ground port connected with the sixth ground plate is also arranged on the sixth conductor layer, and four ground ports are all arranged at intervals along the edge of the ground plate;

a first conductive column is arranged between the first conductor layer and the third conductor layer in a penetrating manner, a second conductive column is arranged between the first conductor layer and the fifth conductor layer in a penetrating manner, a third conductive column is arranged between the second conductor layer and the fourth conductor layer in a penetrating manner, a fourth conductive column is arranged between the second conductor layer and the fifth conductor layer in a penetrating manner, a fifth conductive column is connected between the third conductor layer and the fifth conductor layer, the third capacitive piece is connected with the first inductive element through the first conductive column, the third capacitive piece is connected with the fifth coupling microstrip line through the second conductive column, the second capacitive piece is connected with the second inductive element through the third conductive column, the second capacitive piece is connected with the fourth coupling microstrip line through the fourth conductive column, and the first capacitive piece, the first inductive element and the second inductive element are connected through the fifth conductive column.

Preferably, the dielectric substrate is manufactured and molded by adopting a low-temperature co-fired ceramic process.

Preferably, the defected ground structure is a dumbbell-shaped grid.

Compared with the prior art, the balun filter based on the branch loading resonator has the beneficial effects that: on the basis that a balun is formed by a first coupling microstrip line, a second coupling microstrip line, a third coupling microstrip line and a ground plate, a first open-circuit branch is connected to a fourth coupling microstrip line, a second open-circuit branch is connected to a fifth coupling microstrip line, and when the electrical length of the open-circuit branch is equal to 1/4 wavelength of an out-of-band frequency suppression point, a signal is guided into the ground plate due to the quarter-wavelength microstrip line principle, so that the selectivity of the edge of a pass band of the filter is increased; in addition, the fourth coupling microstrip line is coupled with the first microstrip resonator in parallel, the fifth coupling microstrip line is coupled with the second microstrip resonator in parallel, the microstrip resonators generate multimode resonance with the circuit, a plurality of transmission zeros are generated, and the selectivity of the edge of the pass band of the filter is increased; the defected ground structure can change the distributed inductance and the distributed capacitance of the transmission line and cause multimode resonance, so that slow wave characteristics are obtained, the length of the needed microstrip line is reduced, the size of the filter is reduced, and the use frequency can be widened; in different use environments, the out-of-band rejection frequency of the open-circuit stub and the microstrip resonator can be adjusted by adjusting the lengths and the widths of the first microstrip resonator, the second microstrip resonator, the fourth coupled microstrip line, the fifth coupled microstrip line and the open-circuit stub, so that the bandwidth of the filter can be adjusted, and the filter can be used in different application scenes.

Drawings

FIG. 1 is a schematic structural diagram of a balun filter based on stub-loaded resonators according to the present invention;

fig. 2 is a schematic structural diagram of a first conductor layer of the balun filter based on stub-loaded resonators of fig. 1;

fig. 3 is a schematic structural diagram of a second conductor layer of the balun filter based on stub-loaded resonators of fig. 1;

fig. 4 is a schematic structural diagram of a third conductor layer of the balun filter based on stub-loaded resonators of fig. 1;

fig. 5 is a schematic structural diagram of a fourth conductor layer of the balun filter based on the stub loaded resonator of fig. 1;

fig. 6 is a schematic structural diagram of a fifth conductor layer of the balun filter based on the stub loaded resonator of fig. 1;

fig. 7 is a schematic structural diagram of a sixth conductor layer of the balun filter based on the stub-loaded resonator of fig. 1.

In the figure, 1, a dielectric substrate, 2, a first conductor layer, 21, a first output port, 211, a third capacitor plate, 3, a second conductor layer, 32, a second output port, 321, a second capacitor plate, 4, a third conductor layer, 401, a first inductance element, 5, a fourth conductor layer, 501, a second inductance element, 6, a fifth conductor layer, 63, an input port, 631, a first coupling microstrip line, 632, a second coupling microstrip line, 633, a third coupling microstrip line, 634, a fourth coupling microstrip line, 635, a fifth coupling microstrip line, 636, a first open-circuit stub, 637, a second open-circuit stub, 638, a first microstrip resonator, 639, a second microstrip resonator, 601, a first capacitor plate, 7, a sixth conductor layer, 74, a ground plate, 741, a ground port, 701, a defected ground structure, L1, a first conductive pillar, L2, a second conductive pillar, L3, a third conductive pillar, L4, a fourth conductive pillar, L5, and a fifth conductive pillar.

Detailed Description

The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention, but are not intended to limit the scope of the invention.

As shown in fig. 1 to 7, the balun filter based on a stub-loaded resonator of the present invention includes an input port 63, a first output port 21, a second output port 32, a first coupling microstrip line 631, a second coupling microstrip line 632, a third coupling microstrip line 633, a fourth coupling microstrip line 634, a fifth coupling microstrip line 635, a first microstrip resonator 638, a second microstrip resonator 639, a first open-circuit stub 636, a second open-circuit stub 637, a first capacitor plate 601, and a ground plate 74.

The first coupling microstrip line 631 is connected to the input port 63, and a front half-wavelength section of the first coupling microstrip line 631 is coupled in parallel with a quarter-wavelength section of the second coupling microstrip line 632 to form a same-phase coupling circuit; the rear half-wavelength section of the first coupling microstrip line 631 is coupled in parallel with the quarter-wavelength section of the third coupling microstrip line 633, and a differential phase-coupling circuit is formed because the coupling direction of the first coupling microstrip line is opposite to that of the in-phase coupling circuit; the in-phase coupling circuit, the differential phase coupling circuit and the ground plate 74 form a balun. In the present embodiment, the front half-wavelength section and the rear half-wavelength section of the first coupling microstrip line 631 are high-impedance microstrip lines.

The second coupling microstrip line 632 is connected to the fourth coupling microstrip line 634, the fourth coupling microstrip line 634 is coupled to the first microstrip resonator 638 in parallel, the first open-circuit branch 636 is connected to the fourth coupling microstrip line 634, and the fourth coupling microstrip line 634 is communicated with the second output port 32. The third coupling microstrip line 633 is connected to the fifth coupling microstrip line 635, the fifth coupling microstrip line 635 is coupled to the second microstrip resonator 639 in parallel, the second open-circuit stub 637 is connected to the fifth coupling microstrip line 635, and the fifth coupling microstrip line 635 is communicated with the first output port 21.

The resonator and the circuit generate multimode resonance to generate a plurality of transmission zeros to increase the selectivity of the passband edge of the filter. In this embodiment, the out-of-band rejection frequency points of the first microstrip resonator 638 and the second microstrip resonator 639 are set to be 4.6GHz and 6.7GHz.

The open stub in the stub-loaded resonator based balun filter is not connected to the ground plate 74 and is therefore an open stub. When the electrical length of the open-circuit branch is equal to 1/4 wavelength of the out-of-band frequency rejection point, signals are guided into the floor due to the quarter-wavelength microstrip line principle, and the selectivity of the filter passband edge is increased. In this embodiment, the out-of-band rejection frequency point is set to 6.7GHz.

The ground plate 74 is provided with a defected ground structure 701, the first capacitor patch 601 and the ground plate 74 form a first capacitor, and the fourth coupling microstrip line 634 and the fifth coupling microstrip line 635 are both connected to the first capacitor patch 601. The defected ground structure 701 is a grid structure etched on a grounding metal plate of a microstrip line, and can change distributed inductance and distributed capacitance of a transmission line and cause multimode resonance, so that slow-wave characteristics are obtained.

On the basis that the first coupling microstrip line 631, the second coupling microstrip line 632, the third coupling microstrip line 633 and the ground plate 74 form a balun, the fourth coupling microstrip line 634 is connected with the first open-circuit stub 636, the fifth coupling microstrip line 635 is connected with the second open-circuit stub 637, and when the electrical length of the open-circuit stub is equal to 1/4 wavelength of an out-of-band frequency suppression point, due to the quarter-wavelength microstrip line principle, a signal is guided into the ground plate, so that the selectivity of the passband edge of the filter is increased; in addition, the fourth coupling microstrip line 634 is coupled with the first microstrip resonator 638 in parallel, the fifth coupling microstrip line 635 is coupled with the second microstrip resonator 639 in parallel, the microstrip resonators and the circuit generate multimode resonance to generate a plurality of transmission zeros, and the selectivity of the passband edge of the filter is increased; the defected ground structure 701 can change distributed inductance and distributed capacitance of a transmission line and cause multimode resonance, so that slow wave characteristics are obtained, the length of a needed microstrip line is shortened, the size of the filter is reduced, and the use frequency can be widened; in different use environments, the out-of-band rejection frequency of the open-circuit stub and the microstrip resonator can be adjusted by adjusting the lengths and widths of the first microstrip resonator 638, the second microstrip resonator 639, the fourth coupling microstrip line 634, the fifth coupling microstrip line 635 and the open-circuit stub, so that the bandwidth of the filter can be adjusted, and the filter can be used in different application scenarios.

Preferably, the first microstrip resonator 638 and the second microstrip resonator 639 are both L-shaped structures, and the first microstrip resonator 638 and the second microstrip resonator 639 are symmetrically arranged.

The first microstrip resonator 638 and the second microstrip resonator 639 are symmetrically arranged, so that the frequency stability of the control line is facilitated.

Preferably, the first open-circuit stub 636 is vertically connected to the fourth coupling microstrip line 634, the second open-circuit stub 637 is vertically connected to the fifth coupling microstrip line 635, and the first open-circuit stub 636 and the second open-circuit stub 637 are arranged in a central symmetry manner.

Preferably, the inductor further comprises a first inductive element 401 and a second inductive element 501, the first inductive element 401 is connected between the first capacitor plate 601 and the fifth coupling microstrip line 635, the second inductive element 501 is connected between the first capacitor plate 601 and the fourth coupling microstrip line 634, and the first inductive element 401, the second inductive element 501 and the first capacitive element form a filter circuit.

Based on basic properties of capacitance and inductance, after debugging, the first inductance element 401, the second inductance element 501 and the first capacitance element may form a low-pass filter circuit, which has a filtering effect.

A filter circuit formed by a capacitor and an inductor has the defects of large volume and difficult bandwidth adjustment, but has good filter performance and good out-of-band rejection; the multimode resonance filter circuit formed by the balun and the microstrip resonator has the defects of poor filtering performance and high out-of-band rejection, but is small in size and flexible in design. The low-pass filter circuit is combined with the multimode resonance filter circuit, and the effect of adjusting the bandwidth can be achieved by matching with the flexible design of the multimode resonance filter circuit under the condition of certain filter performance of the capacitor and the inductor.

Preferably, the first inductive element 401 and the second inductive element 501 are both U-shaped strip lines, and the first inductive element 401 and the second inductive element 501 are symmetrically arranged.

Preferably, the self-feedback type self-inductance power-supply circuit further includes a second capacitor plate 321 and a third capacitor plate 211, the second capacitor plate 321 is connected between the fourth coupling microstrip line 634 and the second output port 32, and the second inductance element 501 is connected between the fourth coupling microstrip line 634 and the second output port 634, the third capacitor plate 321 is connected between the fifth coupling microstrip line 635 and the first output port 21, and the fifth coupling microstrip line 635 and the first inductance element 401, and the second capacitor plate 321 and the third capacitor plate 211 form a second capacitor element and form a self-feedback circuit.

The second capacitor plate 321 and the third capacitor plate 211 form a second capacitor element and form a self-feedback circuit, and after the self-feedback circuit is connected in series with the balun, the amplitude of signals at two output ports can be ensured to be more consistent, and the phases are close to and completely opposite.

Because the phase and amplitude of the differential signal output by the balun are not completely opposite, after the balun is connected with the self-feedback circuit in series, the differential signals on the two sides are respectively connected with the second capacitor plate 321 and the third capacitor plate 211 by using the time delay effect of the second capacitor element, and when the amplitudes of the differential signals on the two sides are not consistent or the phases are not completely opposite, the signal on the stronger side reinforces the signal on the weaker side due to the capacitor time delay, so that the amplitudes of the differential signals on the two sides are more consistent, and the phases are closer to being completely opposite.

Preferably, the second capacitor plate 321 and the third capacitor plate 211 are both square capacitor plates.

Preferably, the dielectric substrate comprises a plurality of dielectric substrates 1 arranged in a stacked manner, a conductive layer is arranged between two adjacent dielectric substrates 1, the conductive layers are defined as a first conductive layer 2, a second conductive layer 3, a third conductive layer 4, a fourth conductive layer 5, a fifth conductive layer 6 and a sixth conductive layer 7 from top to bottom, a third capacitive plate 211 and a first output port 21 are arranged on the first conductive layer 2, a second capacitive plate 321 and a second output port 32 are arranged on the second conductive layer 3, a first inductive element 401 is arranged on the third conductive layer 4, a second inductive element 501 is arranged on the fourth conductive layer 5, an input port 63, a first coupling microstrip line 631, a second coupling microstrip line 632, a third coupling microstrip line 633, a fourth coupling line 634, a fifth coupling microstrip line 635, a first microstrip resonator 638, a second microstrip resonator 639, a first open-circuit branch segment 636, a second open-circuit branch segment 637 and a first capacitive plate 601 are arranged on the fifth conductive layer 6, a ground plate 74 is arranged on the sixth conductive layer 7, a ground plate 741 connected with the sixth conductive layer 7, and four ground ports 741, four ground ports are arranged at intervals.

A first conductive column L1 is penetratingly disposed between the first conductive layer 2 and the third conductive layer 4, a second conductive column L2 is penetratingly disposed between the first conductive layer 2 and the fifth conductive layer 6, a third conductive column L3 is penetratingly disposed between the second conductive layer 3 and the fourth conductive layer 5, a fourth conductive column L4 is penetratingly disposed between the second conductive layer 3 and the fifth conductive layer 6, a fifth conductive column L5 is connected between the third conductive layer 4 and the fifth conductive layer 6, the third capacitive piece 211 is connected with the first inductive element 401 through the first conductive column L1, the third capacitive piece 211 is connected with the fifth coupling microstrip line 635 through the second conductive column L2, the second capacitive piece 321 is connected with the second inductive element 501 through the third conductive column L3, the second capacitive piece 321 is connected with the fourth coupling microstrip line 634 through the fourth conductive column L4, and the first capacitive piece 601, the first inductive element 401, and the second inductive element 501 are connected through the fifth conductive column L5.

In this embodiment, the first conductive pillar L1, the second conductive pillar L2, the third conductive pillar L3, the fourth conductive pillar L4, and the fifth conductive pillar L5 are all via pillars.

In the first conductor layer 2, a segment of strip line is connected between the right edge of the third capacitor plate 211 and the first port through the first conductive pillar L1, and a segment of strip line is connected between the left edge of the third capacitor plate 211 and the second conductive pillar L2, so as to form a circuit path.

In the second conductor layer 3, a segment of strip line is connected between the right edge of the second capacitor plate 321 and the second port through the third conductive pillar L3, and a segment of strip line is connected between the left edge of the second capacitor plate 321 and the fourth conductive pillar L4, so as to form a circuit path.

In the third conductor layer 4, one end of the U-shaped stripline of the first inductance element 401 is connected to the first conductive pillar L1, and the other end is connected to the fifth conductive pillar L5; in the fourth conductor layer 5, one end of the U-shaped strip line of the second inductance element 501 is connected to the third conductive pillar L3, and the other end is connected to the fifth conductive pillar L5.

Preferably, the dielectric substrate 1 is formed by a low-temperature co-fired ceramic process.

The low temperature co-fired ceramic technology (LTCC technology) is that low temperature sintered ceramic powder is made into a green ceramic tape with accurate thickness and compactness, required circuit patterns are made on the green ceramic tape by utilizing the technologies of laser drilling, micropore grouting, precise conductor paste printing and the like, and then the green ceramic tape is laminated together, and the inner electrode and the outer electrode can be sintered at 900 ℃ respectively by using metals such as silver, copper, gold and the like to make a high-density circuit with three-dimensional space which is not interfered mutually. The dielectric substrate 1 is printed and formed by adopting a low-temperature co-fired ceramic process, and the filter has the advantages of high precision, high reliability and small volume.

Preferably, the defected ground structure 701 is a dumbbell-type grid.

In the embodiment, the self-feedback circuit, the filter circuit and the balun circuit are connected in series to form the balun filter, so that the balun filter is suitable for requirements of the balun filter in a frequency band of 5.1GHz-5.9GHz, out-of-band rejection of an isolation frequency band of 4.6GHz reaches 40dB, out-of-band rejection of an isolation frequency band of 6.7GHz reaches 45dB, loss of a passing frequency band is lower than 2.2dB, VSWR is smaller than 1.85, amplitude difference of two output ports is within 0.2dB, and phase difference of the two output ports is within 180 +/-3 degrees. Therefore, the balun filter has good filtering effect and can convert a balanced port into an unbalanced port.

To sum up, the embodiment of the present invention provides a balun filter based on a stub-loaded resonator, which is characterized in that on the basis that a first coupling microstrip line, a second coupling microstrip line, a third coupling microstrip line and a ground plane form a balun, a fourth coupling microstrip line is connected to a first open-circuit stub, a fifth coupling microstrip line is connected to a second open-circuit stub, and when the electrical length of the open-circuit stub is equal to 1/4 wavelength of an out-of-band frequency rejection point, due to the quarter-wavelength microstrip line principle, a signal is guided into the ground plane, so as to increase the selectivity of the edge of a pass band of the filter; in addition, the fourth coupling microstrip line is coupled with the first microstrip resonator in parallel, the fifth coupling microstrip line is coupled with the second microstrip resonator in parallel, the microstrip resonators generate multimode resonance with the circuit, a plurality of transmission zeros are generated, and the selectivity of the edge of the pass band of the filter is increased; the defected ground structure can change the distributed inductance and the distributed capacitance of the transmission line and cause multimode resonance, so that slow wave characteristics are obtained, the length of the needed microstrip line is reduced, the size of the filter is reduced, and the use frequency can be widened; in different use environments, the out-of-band rejection frequency of the open-circuit stub and the microstrip resonator can be adjusted by adjusting the lengths and the widths of the first microstrip resonator, the second microstrip resonator, the fourth coupled microstrip line, the fifth coupled microstrip line and the open-circuit stub, so that the bandwidth of the filter can be adjusted, and the filter can be used in different application scenes.

The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and substitutions can be made without departing from the technical principle of the present invention, and these modifications and substitutions should also be regarded as the protection scope of the present invention.

Claims (10)

1. A balun filter based on a stub loaded resonator is characterized by comprising an input port, a first output port, a second output port, a first coupling microstrip line, a second coupling microstrip line, a third coupling microstrip line, a fourth coupling microstrip line, a fifth coupling microstrip line, a first microstrip resonator, a second microstrip resonator, a first open-circuit stub, a second open-circuit stub, a first capacitor piece and a ground plate;

the first coupling microstrip line is connected with the input port, a half-wavelength front section of the first coupling microstrip line and a quarter-wavelength section of the second coupling microstrip line are coupled in parallel to form a same-phase coupling circuit, a half-wavelength rear section of the first coupling microstrip line and a quarter-wavelength section of the third coupling microstrip line are coupled in parallel to form a differential phase coupling circuit, and the same-phase coupling circuit, the differential phase coupling circuit and the ground plate form a balun;

the second coupling microstrip line is connected with the fourth coupling microstrip line, the fourth coupling microstrip line is coupled with the first microstrip resonator in parallel, the first open-circuit branch is connected with the fourth coupling microstrip line, and the fourth coupling microstrip line is communicated with the second output port;

the third coupling microstrip line is connected with the fifth coupling microstrip line, the fifth coupling microstrip line is coupled with the second microstrip resonator in parallel, the second open-circuit branch is connected with the fifth coupling microstrip line, and the fifth coupling microstrip line is communicated with the first output port;

the grounding plate is provided with a defected ground structure, the first capacitor plate and the grounding plate form a first capacitor element, and the fourth coupling microstrip line and the fifth coupling microstrip line are both connected with the first capacitor plate.

2. The balun filter based on the stub-loaded resonator of claim 1, wherein the first microstrip resonator and the second microstrip resonator are both of an L-shaped structure, and the first microstrip resonator and the second microstrip resonator are symmetrically arranged.

3. The balun filter based on the stub-loaded resonator of claim 1, wherein the first open stub is vertically connected to the fourth coupled microstrip line, the second open stub is vertically connected to the fifth coupled microstrip line, and the first open stub and the second open stub are arranged in a central symmetry.

4. The balun filter based on the stub-loaded resonator according to any one of claims 1 to 3, further comprising a first inductive element and a second inductive element, wherein the first inductive element is connected between the first capacitive patch and the fifth coupled microstrip line, the second inductive element is connected between the first capacitive patch and the fourth coupled microstrip line, and the first inductive element, the second inductive element and the first capacitive element form a filter circuit.

5. The balun filter based on stub-loaded resonators according to claim 4, wherein the first inductive element and the second inductive element are both U-shaped strip lines, and the first inductive element and the second inductive element are symmetrically arranged.

6. The balun filter based on the stub loaded resonator of claim 4, further comprising a second capacitor plate and a third capacitor plate, wherein the second capacitor plate is connected between the fourth coupling microstrip line and the second output port, and the second inductor element is connected between the fourth coupling microstrip line and the fourth coupling microstrip line, the third capacitor plate is connected between the fifth coupling microstrip line and the first output port, and the fifth coupling microstrip line and the first inductor element, and the second capacitor plate and the third capacitor plate form a second capacitor element and form a self-feedback circuit.

7. The balun filter based on stub-loaded resonators according to claim 6, wherein the second capacitor piece and the third capacitor piece are both square capacitor pieces.

8. The balun filter based on the stub loaded resonator of claim 6, comprising a plurality of dielectric substrates arranged in a stacked manner, wherein a conductive layer is disposed between two adjacent dielectric substrates, the conductive layer is defined as a first conductive layer, a second conductive layer, a third conductive layer, a fourth conductive layer, a fifth conductive layer and a sixth conductive layer from top to bottom, the third capacitive plate and the first output port are disposed on the first conductive layer, the second capacitive plate and the second output port are disposed on the second conductive layer, the first inductive element is disposed on the third conductive layer, the second inductive element is disposed on the fourth conductive layer, the input port, the first coupling microstrip line, the second coupling microstrip line, the third coupling microstrip line, the fourth coupling microstrip line, the fifth coupling microstrip line, the first microstrip resonator, the second microstrip resonator, the first open-circuit stub, the second capacitive plate are disposed on the fifth conductive layer, the ground plate is disposed on the sixth conductive layer, the ground plate is further connected to the ground plate, and four ground ports are disposed along an edge, and four ground ports are spaced apart;

a first conductive column is arranged between the first conductor layer and the third conductor layer in a penetrating manner, a second conductive column is arranged between the first conductor layer and the fifth conductor layer in a penetrating manner, a third conductive column is arranged between the second conductor layer and the fourth conductor layer in a penetrating manner, a fourth conductive column is arranged between the second conductor layer and the fifth conductor layer in a penetrating manner, a fifth conductive column is connected between the third conductor layer and the fifth conductor layer, the third capacitive piece is connected with the first inductive element through the first conductive column, the third capacitive piece is connected with the fifth coupling microstrip line through the second conductive column, the second capacitive piece is connected with the second inductive element through the third conductive column, the second capacitive piece is connected with the fourth coupling microstrip line through the fourth conductive column, and the first capacitive piece, the first inductive element and the second inductive element are connected through the fifth conductive column.

9. The balun filter based on the stub loaded resonator of claim 8, wherein the dielectric substrate is formed by a low temperature co-fired ceramic process.

10. The balun filter based on stub-loaded resonators according to any one of claims 1-3, wherein the defected ground structure is a dumbbell-type grid.

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