A compact dual / tri-band switchable bandpass filter

Abstract

The application belongs to the technical field of microwave communication, and particularly relates to a compact dual-band / three-band switchable band-pass filter. The compact dual-band / three-band switchable band-pass filter is constructed, the multi-band filter response is realized by using the coupled microstrip line structure with different structures at two ends, the resonant frequency of the circuit is changed by loading variable capacitors, the switching of the dual-band / three-band band-pass filter response is flexibly realized without loading additional control switches and elements, the overall circuit has the advantages of simple and compact structure, excellent filter performance, good out-of-band suppression, flexible switching and low loss, etc. The open-circuit microstrip line stub and the short-circuit microstrip line stub are connected at two ends of the input and output feed line structure, respectively generate a transmission zero point at the right side of the first passband and the second passband of the filter, and enhance the frequency selectivity of the band-pass filter; the one-end short-circuit coupled microstrip line is used to realize the impedance matching between the feed port and the filter resonator.

Description

Technical Field

[0001] This invention belongs to the field of microwave communication technology, specifically relating to a compact dual-frequency / triple-frequency switchable bandpass filter. Background Technology

[0002] Switchable filters have wide applications in modern communication systems and signal processing, and their importance and demand are constantly growing. With the rapid development of wireless communication technology, communication equipment needs to simultaneously support multi-band, multi-mode signal processing to adapt to constantly changing communication standards and spectrum resource allocation. Traditional fixed filters, due to their limited frequency response, cannot meet this flexibility requirement. Therefore, switchable filters have emerged as a new technology. Switchable filters have the ability to quickly switch between multiple frequency bands and can dynamically adjust their frequency response according to the needs of different application scenarios, thereby achieving efficient multi-band signal processing. This flexibility significantly improves the system's spectrum utilization and communication quality, effectively coping with complex and ever-changing communication environments. Furthermore, switchable filters play a crucial role in the miniaturization and low-power design of wireless communication equipment, meeting the needs of portable and mobile communication devices for compact design and energy optimization. In addition, switchable filters also have significant application value in the defense and aerospace fields. For example, multi-band, multi-mode radar and communication systems require reliable data transmission and signal detection in various environments. The introduction of switchable filters facilitates seamless switching between different operating frequency bands in these systems, improving system flexibility and reliability, and enhancing operational capabilities in complex electromagnetic environments. Therefore, the research and application of switchable filters not only drive advancements in communication technology but also provide crucial support for achieving more efficient and flexible signal processing, and their role in future communication systems will become increasingly important.

[0003] Currently, the main technologies for realizing multi-band switchable filters include MEMS switching technology, PIN diode technology, and varactor diode technology. Among these, MEMS switching technology for reconfigurable filters primarily utilizes micromechanical switches to change the circuit configuration. MEMS switches achieve opening and closing through mechanical movement, thereby switching between different frequency bands. While this type of filter offers high Q values ​​and precise control, its structure is relatively complex, reliability is lower, and cost is relatively high. PIN diode-based switchable filters utilize the different resistance characteristics of PIN diodes under forward and reverse bias states to achieve circuit switching and frequency band switching. Under forward bias, the PIN diode exhibits low impedance, equivalent to a closed switch; under reverse bias, it exhibits high impedance, equivalent to an open switch. By controlling the diode's bias voltage, the filter's circuit structure can be switched to achieve frequency band switching. Although this circuit improves the integration of the RF system, the introduction of PIN diodes increases the circuit's insertion loss, and the diode's performance is significantly affected by temperature, requiring additional temperature compensation design. Compared to the previous two types of switchable filters, switchable filters based on varactor diode technology offer advantages such as ingenious design, low cost, and low power consumption. The junction capacitance of a varactor diode changes under different bias voltages. By adjusting the bias voltage, the resonant frequency of the filter can be changed, enabling free switching between single-frequency, dual-frequency, or tri-frequency operation. Although a small number of switchable filters based on varactor diodes have emerged, they typically require cascading multiple frequency bands of RF circuitry, resulting in complex circuit structures, significant design challenges, poor filtering performance, high losses, and the need for additional switching and control components. These drawbacks hinder the development of RF communication systems towards higher integration, miniaturization, and lower cost.

[0004] Current research on switchable bandpass filters mainly focuses on the cascading of multiple mode circuits, using switches to combine different circuit structures. Existing multi-band bandpass filters, whether by adding resonator units or changing the resonator's structure to generate more passbands, often cannot achieve flexible switching between multiple frequency bands. Furthermore, they suffer from drawbacks such as complex circuit structures, large size, high design difficulty, and the need for additional switches or control components.

[0005] Therefore, it is necessary to develop a new type of high-performance switchable filter that can flexibly switch between multiple frequency bands while maintaining a simple and compact structure. It can achieve flexible switching between the filtering performance of multiple frequency bands without the need for additional switches or control components, providing a reliable solution for improving the future RF front-end communication performance and achieving more efficient and flexible signal processing capabilities. Summary of the Invention

[0006] To address the technical problems in the prior art, this invention proposes a compact dual-frequency / triple-frequency switchable bandpass filter. This invention has a simple structure, small size, and can achieve flexible switching between multiple frequency bands without additional control circuits and components. It also has good filtering performance, low loss, and is easy to process and manufacture.

[0007] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0008] A compact dual-frequency / triple-frequency switchable bandpass filter includes an upper microstrip structure, a middle dielectric substrate, and a lower metal ground structure stacked from top to bottom. The upper microstrip structure includes a first capacitor C1, a second capacitor C2, parallel-coupled microstrip lines, a metallized via-pad ground structure, coupled microstrip lines, short-circuited microstrip line stubs, open-circuited microstrip line stubs, a microstrip line structure, an input feed structure, and an output feed structure. The middle dielectric substrate has several metal vias connecting the upper microstrip structure and the lower metal ground structure. The first capacitor C1, parallel-coupled microstrip lines, metallized via-pad ground structure, coupled microstrip lines, short-circuited microstrip line stubs, open-circuited microstrip line stubs, and microstrip line structure are mirror-replicated about the Y-axis to form a completely symmetrical circuit structure. The input feed structure and the output feed structure are symmetrical about the Y-axis. The parallel-coupled microstrip lines... One end of the parallel-coupled microstrip line is connected to one end of the microstrip line structure and to one end of the first capacitor C1; the other end of the first capacitor C1 is connected to the grounding structure of the metallized via pad; one end of the parallel-coupled microstrip line is connected to the second capacitor C2; the other end of the microstrip line structure is connected to one end of the coupled microstrip line; the other end of the coupled microstrip line is connected to one end of the input feed line structure; the connection between the other end of the coupled microstrip line and one end of the input feed line structure is respectively connected to one end of the short-circuited microstrip line stub and one end of the open-circuited microstrip line stub; the other end of the short-circuited microstrip line stub is grounded through a metal via; the other end of the open-circuited microstrip line stub is open-circuited; the open-circuited microstrip line stub is arranged parallel to the parallel-coupled microstrip line; the other end of the input feed line structure is connected to the signal input terminal; the other end of the output feed line structure, which is symmetrical about the Y-axis centerline, is connected to the signal output terminal.

[0009] As a further preferred embodiment of the present invention, the parallel coupled microstrip line includes a first microstrip line and a second microstrip line that are parallel to each other; one end of the first microstrip line is grounded through a via, and the other end is connected to one end of the microstrip line structure; one end of the second microstrip line is grounded through a via, and the other end is connected to one end of the microstrip line structure; a second capacitor C2 is connected between one end of the first microstrip line and one end of the second microstrip line.

[0010] As a further preferred embodiment of the present invention, the coupled microstrip line includes a parallel third microstrip line and a fourth microstrip line; one end of the third microstrip line is connected to one end of the fourth microstrip line; the other end of the fourth microstrip line is connected to the other end of the microstrip line structure; the other end of the third microstrip line is connected to one end of the input feed line structure; the connection point between the other end of the third microstrip line and one end of the input feed line structure is respectively connected to one end of a short-circuited microstrip line stub and one end of an open-circuited microstrip line stub.

[0011] As a further preferred embodiment of the present invention, the microwave signal is fed in through the signal input terminal, transmitted to the coupled microstrip line through the input feed line structure, and then transmitted to the parallel coupled microstrip line through the microstrip line structure. The microwave RF signal generates a filtering response in the two resonant structures of the coupled microstrip line and the parallel coupled microstrip line, respectively. The resonant structures of the coupled microstrip line and the parallel coupled microstrip line are different, resulting in two filtering responses. A second capacitor C2 is loaded between one end of the first microstrip line and one end of the second microstrip line to enhance the coupling strength of the parallel coupled microstrip line. By adjusting the size of the first variable capacitor C1, the resonant frequency of the filtering structure can be tuned, realizing free switching between dual-band and tri-band operating frequencies. At the same time, a short-circuited microstrip line stub connected in parallel at the connection between the input feed line structure and the coupled microstrip line generates a transmission zero on the right side of the second passband. Conversely, an open-circuited microstrip line stub connected in parallel at the connection between the input feed line structure and the coupled microstrip line forms a transmission zero on the right side of the first passband. The microwave RF signal with multi-band filtering response is transmitted to the signal output terminal through the output feed line structure.

[0012] The compact dual-frequency / triple-frequency switchable bandpass filter of this invention, compared with the prior art, has the following technical advantages:

[0013] (1) The compact dual-band / tri-band switchable bandpass filter constructed in this invention realizes multi-band filtering response by utilizing a cascaded coupled microstrip line structure with different structures at both ends. By loading a variable capacitor to change the resonant frequency of the circuit, the switching of dual-band / tri-band bandpass filtering response can be flexibly realized without loading additional control switches and components. The overall circuit has the advantages of simple and compact structure, excellent filtering performance, good out-of-band rejection, flexible switching, and low loss.

[0014] (2) The open and short-circuit microstrip line stubs of the present invention are connected to both ends of the input and output feed line structures, respectively generating a transmission zero on the right side of the first passband and the second passband of the filter, thereby enhancing the frequency selectivity of the bandpass filter; the coupled microstrip line with one end shorted is used to achieve impedance matching between the feed port and the filter resonator. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the structure of an embodiment of the present invention;

[0016] Figure 2 This is a side view of an embodiment of the present invention;

[0017] Figure 3 This is a schematic diagram of the upper microstrip structure dimensions according to an embodiment of the present invention;

[0018] Figure 4 This is the S of the dual-band bandpass filter in this embodiment of the invention. 11 and S 21 The frequency response curve;

[0019] Figure 5 This is the S of the three-band bandpass filter in this embodiment of the invention. 11 and S 21 The frequency response curve;

[0020] In the attached figures: 10, upper microstrip structure; 20, middle dielectric substrate; 30, lower metal ground structure; 100, parallel coupled microstrip line; 100-a, first microstrip line; 100-b, second microstrip line; 101, metallized via pad grounding structure; 102, coupled microstrip line; 102-a, third microstrip line; 102-b, fourth microstrip line; 103, short-circuited microstrip line stub; 104, open-circuited microstrip line stub; 105, microstrip line structure; 106, input feeder structure; 107, output feeder structure. Detailed Implementation

[0021] The present invention will be further explained in detail below with reference to the accompanying drawings, so that those skilled in the art can better understand and implement the present invention. However, the following examples are only used to explain the present invention and are not intended to limit the present invention.

[0022] like Figure 1-2As shown, a compact dual-band / triple-band switchable bandpass filter includes an upper microstrip structure 10, a middle dielectric substrate 20, and a lower metal ground structure 30 stacked from top to bottom. The upper microstrip structure 10 includes a first capacitor C1, a second capacitor C2, a parallel coupled microstrip line 100, a metallized via pad ground structure 101, a coupled microstrip line 102, a short-circuited microstrip line stub 103, an open-circuited microstrip line stub 104, a microstrip line structure 105, an input feed structure 106, and an output feed structure 107. The middle layer dielectric substrate 20 is provided with several through-holes connecting the upper microstrip structure 10 and the lower metal ground structure 30; the first capacitor C1, the parallel coupled microstrip line 100, the metallized through-hole pad ground structure 101, the coupled microstrip line 102, the short-circuited microstrip line stub 103, the open-circuited microstrip line stub 104, and the microstrip line structure 105 are mirrored about the Y-axis centerline to form a completely symmetrical circuit structure; the input feed structure 106 and the output feed structure 107 are symmetrical about the Y-axis centerline; the parallel coupled... One end of the microstrip line 100 is connected to one end of the microstrip line structure 105 and to one end of the first capacitor C1; the other end of the first capacitor C1 is connected to the metallized via pad grounding structure 101; one end of the parallel coupled microstrip line 100 is connected to the second capacitor C2; the other end of the microstrip line structure 105 is connected to one end of the coupled microstrip line 102; the other end of the coupled microstrip line 102 is connected to one end of the input feed structure 106; the other end of the coupled microstrip line 102 is connected to the input feed structure 106. One end of the short-circuited microstrip line stub 103 is connected to one end of the open-circuited microstrip line stub 104; the other end of the short-circuited microstrip line stub 103 is grounded through a metal via; the other end of the open-circuited microstrip line stub 104 is open-circuited; the open-circuited microstrip line stub 104 is arranged parallel to the parallel coupled microstrip line 100; the other end of the input feed structure 106 is connected to the signal input terminal; the other end of the output feed structure 107, which is symmetrical about the Y-axis centerline, is connected to the signal output terminal.

[0023] The relative permittivity of the middle dielectric substrate 20 is ε r The thickness is H, and the thickness of the upper microstrip structure 10 and the lower metal ground structure 30 is T.

[0024] The parallel-coupled microstrip line 100 includes a first microstrip line 100-a and a second microstrip line 100-b that are parallel to each other. One end of the first microstrip line 100-a is grounded through a via, and the other end is connected to one end of the microstrip line structure 105. One end of the second microstrip line 100-b is grounded through a via, and the other end is connected to one end of the microstrip line structure 105. A second capacitor C2 is connected between one end of the first microstrip line 100-a and one end of the second microstrip line 100-b.

[0025] The coupled microstrip line 102 includes a parallel third microstrip line 102-a and a fourth microstrip line 102-b; one end of the third microstrip line 102-a is connected to one end of the fourth microstrip line 102-b; the other end of the fourth microstrip line 102-b is connected to the other end of the microstrip line structure 105; the other end of the third microstrip line 102-a is connected to one end of the input feed line structure 106; the connection between the other end of the third microstrip line 102-a and one end of the input feed line structure 106 is respectively connected to one end of the short-circuited microstrip line stub 103 and one end of the open-circuited microstrip line stub 104.

[0026] Microwave signals are fed in through the signal input terminal, transmitted to the coupled microstrip line 102 via the input feed structure 106, and then transmitted to the parallel coupled microstrip line 100 via the microstrip line structure 105. The microwave RF signals generate filtering responses in the two resonant structures of the coupled microstrip line 102 and the parallel coupled microstrip line 100, respectively. The different resonant structures of the coupled microstrip line 102 and the parallel coupled microstrip line 100 result in two filtering responses. A second capacitor C2 is loaded between one end of the first microstrip line 100-a and one end of the second microstrip line 100-b, enhancing the coupling strength of the parallel coupled microstrip line 100. This helps achieve in-band matching of multi-frequency passbands. The first capacitor C1 can be adjusted... The size of the filter structure can be adjusted to tune its resonant frequency, enabling free switching between dual-band and tri-band operation. Simultaneously, a short-circuited microstrip line stub 103 connected in parallel at the connection between the input feed structure 106 and the coupled microstrip line 102 generates a transmission zero on the right side of the second passband. Conversely, an open-circuit microstrip line stub 104 connected in parallel at the connection between the input feed structure 106 and the coupled microstrip line 102 forms a transmission zero on the right side of the first passband. Therefore, there is a transmission zero on each side of the two passbands. These transmission zeros enhance the frequency selectivity of the filter and improve out-of-band rejection performance. The microwave RF signal with multi-band filtering response is transmitted to the signal output terminal via the output feed structure 107.

[0027] The entire filter structure is symmetrical about the left and right centers of the Y-axis, ensuring balanced signal transmission in the two channels, reducing signal distortion and insertion loss. Furthermore, by simply adjusting the value of the first capacitor C1, the switching of dual-band / tri-band bandpass filter response can be flexibly achieved without the need for additional control switches and components. The overall structure is simple and compact, with excellent filtering performance.

[0028] Figure 3 This is a schematic diagram of the upper microstrip metal structure of a compact dual-band / tri-band switchable bandpass filter according to the present invention. Figure 3 As shown, the parallel coupled microstrip line 100 has a linewidth of W1, a length of L1, and a spacing of S1; the coupled microstrip line 102 has a linewidth of W2, and the shorting section has a linewidth of W... 21The length is L2 and the spacing is S2; the microstrip line structure 105 has a linewidth of W2 and a length of L5; the short-circuited microstrip line stub 103 has a linewidth of W3 and a length of L3; the open-circuit microstrip line stub 104 has a linewidth of W4 and a length of L4; the input feed structure 106 and the output feed structure 107 have linewidths of W... P The length is L P The metallized through-hole pad grounding structure 101 has a side length of L. pad The radius of the metal through hole is R via .

[0029] This invention utilizes two different coupled-line resonant structures to form a multi-band bandpass filter response. The added short-circuited microstrip line stub 103 and open-circuited microstrip line stub 104 provide two additional transmission zeros for the filter. This results in a total of four out-of-band transmission zeros, achieving excellent isolation between the two passbands and out-of-band suppression for both dual-band and tri-band applications. By cascading two different coupled-line resonant structures, a multi-band bandpass filter response is formed. A tunable capacitor C1 is loaded between the first microstrip line 100-a and the second microstrip line 100-b of the parallel coupled microstrip line 100 to adjust the circuit's resonant frequency. Switching between dual-band and tri-band bandpass filter responses can be flexibly achieved without additional control switches and components. Switching the circuit's operating state via capacitor C1 results in a simple and compact circuit, avoiding the power consumption associated with PIN diodes and MEMS switches, thus reducing power consumption. The circuit reduces signal distortion and insertion loss. A second capacitor C2 is added between the first microstrip line 100-a and the second microstrip line 100-b of the parallel coupled microstrip line 100, which enhances the coupling strength of the parallel coupled microstrip line 100 and achieves in-band matching of the multi-frequency passband. A short-circuited microstrip line stub 103 and an open-circuit microstrip line stub 104 are connected in parallel at the connection between the input feed structure 106 and the coupled microstrip line 102, forming a transmission zero on each side of the two passbands, which enhances the frequency selectivity and out-of-band rejection performance of the filter.

[0030] This invention generates a multi-band filtering response and utilizes a variable capacitor C1 to tune the resonant frequency of the filter structure, achieving free switching between dual-band and tri-band operation without the need for additional control switches or components. Furthermore, a short-circuited microstrip stub 103 and an open-circuit microstrip stub 104 are connected in parallel at the connection point between the input feed structure 106 and the coupling microstrip line 102, forming a transmission zero on each side of the two passbands, enhancing the filter's frequency selectivity and out-of-band rejection performance. The entire filter structure is symmetrical about the left and right centers of the Y-axis, ensuring balanced signal transmission in both channels, reducing signal distortion and insertion loss. Moreover, by simply adjusting the value of the first capacitor C1, switching between dual-band and tri-band bandpass filtering responses can be flexibly achieved without additional control switches or components. The overall structure is simple, compact, and exhibits excellent filtering performance.

[0031] This invention designs a compact dual-band / tri-band switchable bandpass filter with a center frequency of 2 GHz, with overall dimensions of 5.8 cm × 5.3 cm. Its specific dimensional parameters are as follows: Parallel coupled microstrip line 100: W1 = 2.615 mm, L1 = 24.9 mm, S1 = 5.44 mm; Coupled microstrip line 102: W2 = 1.3 mm, W... 21 =0.1mm, L2=10.2mm, S2=1.6mm; Microstrip line structure 105: L5=4.5mm; Short-circuited microstrip line stub 103: W3=0.628mm, L3=15.8mm; Open-circuited microstrip line stub 104: W4=0.583mm, L4=18.58mm; Input feed line structure 106 and output feed line structure 107: W P =3.38mm, L P =15mm; Metallized through-hole pad grounding structure 101:L pad =1.5mm, R via =0.2mm; Capacitor C1 = 1.42pF or 0.66pF, C2 = 1pF; Intermediate dielectric substrate MS uses RO4003C material, relative permittivity ε r The value is 3.55, and the thickness h is 1.524mm. Wide stopband absorption filter power dividers for other frequency bands can be obtained by scaling up in this embodiment.

[0032] Figure 4 The reflection coefficient S of the dual-band bandpass filter obtained in this embodiment using CST MWS three-dimensional simulation software under the condition of variable capacitor C1 = 1.42pF is... 11 With transmission coefficient S 21 The frequency response curve. For example... Figure 4 As shown, when C1 = 1.42pF, the circuit exhibits a dual-band bandpass filter response. The center frequencies of the two passbands are 1.05GHz and 3.04GHz, respectively, with minimum insertion losses of 0.58dB and 0.63dB, respectively. The corresponding 3-dB relative bandwidths for the two passbands are 42% (0.89–1.33GHz) and 12.8% (2.84–3.23GHz), respectively. The first passband has two transmission poles located at 1.0GHz and 1.08GHz, respectively, and a transmission zero located at 0.31GHz on the left side of the passband, exhibiting out-of-band rejection better than -62.45dB. Between the two passbands are two transmission zeros located at 2.22GHz and 2.39GHz, respectively, with inter-passband rejection better than -53.76dB, demonstrating good frequency selectivity. Furthermore, the out-of-band rejection level in the 3.7–5GHz frequency range on the right side of the second passband reaches over 29dB.

[0033] Figure 5 The reflection coefficient S of the three-band bandpass filter obtained in this embodiment using CST MWS three-dimensional simulation software under the condition of variable capacitor C1 = 0.68pF is... 11 With transmission coefficient S 21 The frequency response curve is shown. When C1 = 0.68pF, the circuit exhibits a three-band bandpass filter response. The center frequencies of the three passbands are 1.28GHz, 3.1GHz, and 4.1GHz, respectively. The minimum insertion loss within the passbands is 0.53dB, 0.68dB, and 0.91dB, corresponding to 3-dB relative bandwidths of 56.3% (0.96~1.68GHz), 17.8% (2.85~3.40GHz), and 13.2% (3.84~4.38GHz), respectively. There are two transmission poles in the first passband, located at 1.07GHz and 1.39GHz, respectively. The left side of the first passband... There is a transmission zero at 0.31 GHz, with out-of-band rejection better than -62.23 dB; there are two transmission poles in the second passband, located at 3.06 GHz and 3.27 GHz respectively, and two transmission zeros in the stopband between the first and second passbands, located at 2.24 GHz and 2.36 GHz respectively, with out-of-band rejection better than -62.45 dB and better than 54 dB; there are also two transmission poles in the third passband, located at 3.92 GHz and 4.19 GHz respectively, and a transmission zero at 5.46 GHz to the right of the third passband, exhibiting good out-of-band rejection and frequency selectivity characteristics.

[0034] The specific implementation schemes described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific implementation schemes of the present invention and are not intended to limit the scope of the present invention. Any equivalent changes and modifications made by those skilled in the art without departing from the concept and principles of the present invention should fall within the scope of protection of the present invention.

Claims

1. A compact dual-frequency / triple-frequency switchable bandpass filter, characterized in that, The structure includes an upper microstrip structure (10), a middle dielectric substrate (20), and a lower metal ground structure (30) stacked from top to bottom. The upper microstrip structure (10) includes a first capacitor C1, a second capacitor C2, a parallel coupled microstrip line (100), a metallized via pad ground structure (101), a coupled microstrip line (102), a short-circuited microstrip line stub (103), an open-circuit microstrip line stub (104), a microstrip line structure (105), an input feed structure (106), and an output feed structure (107). The middle dielectric substrate (20) is provided with several... A metal via is used to connect the upper microstrip structure (10) and the lower metal ground structure (30); the first capacitor C1, the parallel coupled microstrip line (100), the metallized via pad ground structure (101), the coupled microstrip line (102), the short-circuited microstrip line stub (103), the open-circuited microstrip line stub (104), and the microstrip line structure (105) are mirrored about the Y-axis to form a completely symmetrical circuit structure; the input feed structure (106) and the output feed structure (107) are symmetrical about the Y-axis; the parallel coupled microstrip line (100) One end of the parallel coupled microstrip line (100) is connected to one end of the microstrip line structure (105) and to one end of the first capacitor C1; the other end of the first capacitor C1 is connected to the metallized via pad grounding structure (101); one end of the parallel coupled microstrip line (100) is connected to the second capacitor C2; the other end of the microstrip line structure (105) is connected to one end of the coupled microstrip line (102); the other end of the coupled microstrip line (102) is connected to one end of the input feed structure (106); the other end of the coupled microstrip line (102) is connected to one end of the input feed structure (106). One end of the short-circuited microstrip line stub (103) is connected to one end of the open-circuited microstrip line stub (104); the other end of the short-circuited microstrip line stub (103) is grounded through a metal via; the other end of the open-circuited microstrip line stub (104) is open-circuited; the open-circuited microstrip line stub (104) is arranged parallel to the parallel coupled microstrip line (100); the other end of the input feed structure (106) is connected to the signal input terminal; the other end of the output feed structure (107), which is symmetrical about the Y-axis center line, is connected to the signal output terminal; the Y-axis and the X-axis are perpendicular to each other; the X-axis is the direction from the signal input terminal to the signal output terminal.

2. A compact dual-frequency / triple-frequency switchable bandpass filter according to claim 1, characterized in that, The parallel coupled microstrip line (100) includes a first microstrip line (100-a) and a second microstrip line (100-b) that are parallel to each other. One end of the first microstrip line (100-a) is grounded through a via, and the other end is connected to one end of the microstrip line structure (105). One end of the second microstrip line (100-b) is grounded through a via, and the other end is connected to one end of the microstrip line structure (105). A second capacitor C2 is connected between one end of the first microstrip line (100-a) and one end of the second microstrip line (100-b).

3. A compact dual-frequency / triple-frequency switchable bandpass filter according to claim 2, characterized in that, The coupled microstrip line (102) includes a parallel third microstrip line (102-a) and a fourth microstrip line (102-b); one end of the third microstrip line (102-a) is connected to one end of the fourth microstrip line (102-b); the other end of the fourth microstrip line (102-b) is connected to the other end of the microstrip line structure (105); the other end of the third microstrip line (102-a) is connected to one end of the input feed structure (106); the connection between the other end of the third microstrip line (102-a) and one end of the input feed structure (106) is respectively connected to one end of the short-circuited microstrip line stub (103) and one end of the open-circuit microstrip line stub (104).

4. A compact dual-frequency / triple-frequency switchable bandpass filter according to claim 3, characterized in that, The microwave signal is fed in through the signal input terminal, transmitted through the input feed line structure (106) to the coupled microstrip line (102), and then transmitted through the microstrip line structure (105) to the parallel coupled microstrip line (100). The microwave radio frequency signal generates a filter response in the two resonant structures of the coupled microstrip line (102) and the parallel coupled microstrip line (100). The coupled microstrip line (102) and the parallel coupled microstrip line (100) have different resonant structures, resulting in two filtering responses; A second capacitor C2 is loaded between one end of the first microstrip line (100-a) and one end of the second microstrip line (100-b), enhancing the coupling strength of the parallel coupled microstrip line (100). By adjusting the size of the first variable capacitor C1, the resonant frequency of the filter structure can be tuned, enabling free switching between dual-frequency and tri-frequency operating bands. Simultaneously, a short-circuited microstrip line stub (103) is connected in parallel at the connection between the input feed line structure (106) and the coupled microstrip line (102), generating a transmission zero on the right side of the second passband. Meanwhile, an open-circuit microstrip line stub (104) is connected in parallel at the connection between the input feed line structure (106) and the coupled microstrip line (102), forming a transmission zero on the right side of the first passband. The microwave radio frequency signal with multi-band filtering response is transmitted to the signal output terminal through the output feed line structure (107).

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