Five-mode reconfigurable microstrip line filter

Abstract

The invention discloses a five-mode reconfigurable microstrip line filter which adopts a single symmetrical microstrip line unified topology and is composed of four sections of quarter-wavelength parallel coupling lines, seven sections of quarter-wavelength microstrip lines and seven PIN switches. On the premise of not changing a main body circuit frame, flexible reconstruction of five band-pass filtering modes, namely, a multi-transmission-zero-point high-isolation dual-frequency band-pass mode, a three-frequency-band band-pass mode, a multi-zero-point wide-frequency band-pass mode, a double-side high-suppression dual-frequency band-pass mode and a multi-zero-point high-isolation dual-frequency band-pass mode, is realized on the same hardware platform by controlling the on and off states of the PIN switches.

Description

Technical Field

[0001] This invention relates to the field of radio frequency communication electronics technology, and in particular to a multimode reconfigurable microstrip line filter circuit. Background Technology

[0002] With the rapid development of wireless communication, satellite communication, and radar detection technologies, as well as the iterative evolution of next-generation multi-frequency, multi-standard RF front-ends, the performance requirements of filters, as core components of RF front-ends, have gradually upgraded from traditional single fixed frequency response to multi-band adaptation, multi-functional integration, miniaturization, and high integration. In practical applications with limited circuit area, how to achieve flexible switching of multiple operating modes of filters while taking into account low insertion loss, high return loss, excellent out-of-band rejection, and steep transition band roll-off characteristics has become a core research direction and key technical challenge in the field of reconfigurable filters.

[0003] Currently, existing technologies mostly achieve reconfiguration switching between bandpass / bandstop and broadband / dual-band response modes of filters by introducing tunable components such as PIN diodes, MEMS switches, or varactor elements. While this approach improves the system's spectral adaptability to some extent, it still faces several technical bottlenecks and shortcomings in practical engineering applications, specifically in the following three aspects: First, high circuit redundancy limits integration. Existing multimode reconfigurable filters typically employ multiple relatively independent resonant units or combinations of multiple filter sub-circuits to achieve switching between different modes, resulting in significant circuit redundancy and low resource reuse, making it difficult to further achieve miniaturization and high integration of filters, and failing to meet the development requirements of high integration in RF front-ends. Second, significant performance differences exist between modes, making it difficult to balance overall performance indicators. Different operating modes often correspond to independent coupling paths and resonant mechanisms, which can easily lead to significant differences in filtering performance among modes. Specifically, some modes exhibit problems such as high insertion loss, insufficient return loss, weak out-of-band rejection, or insufficiently steep roll-off in the transition band, making it difficult to achieve a comprehensive balance of low passband loss, high return loss, strong out-of-band rejection, and steep roll-off characteristics. Thirdly, the number of switchable modes is limited, and achieving a unified topology is difficult. Most existing solutions can only achieve single or a few specific responses such as dual-band pass, wideband pass, or multi-band pass, with limited mode switching capabilities. Furthermore, flexibly switching between multiple high-performance bandpass modes within the same topology remains technically challenging. The insufficient ability to construct multi-zero responses also makes it difficult to simultaneously achieve excellent frequency selectivity and deep stopband rejection in multiple modes. Especially in high-frequency applications, traditional design methods fail to fully coordinate the working mechanisms of coupling paths and resonant units, further limiting the feasibility of flexibly achieving multiple response requirements such as dual-band pass, triple-band pass, wideband pass, and high-selectivity pass within a unified topology.

[0004] In summary, existing technologies struggle to simultaneously achieve optimal filter integration, mode switching flexibility, and balanced multi-mode performance. Therefore, there is an urgent need to propose a reconfigurable filter scheme based on a unified microstrip topology. This scheme should enable flexible switching between various filtering modes, including dual-band, tri-band, wideband, flat response, and high selectivity, without significantly increasing circuit size. Simultaneously, it should possess high structural integration and superior out-of-band rejection capabilities to address the numerous shortcomings of existing technologies and meet the application requirements of next-generation RF front-ends. Summary of the Invention

[0005] Purpose of the invention: In view of the above-mentioned prior art, a highly integrated five-mode reconfigurable microstrip line filter is proposed. Through the collaborative design of parallel coupling lines, microstrip lines and switching units, five different bandpass modes are integrated and flexibly switched.

[0006] Technical solution: A five-mode reconfigurable microstrip line filter, including input port 1, output port 2, four quarter-wavelength parallel coupling lines CL1~CL4, seven quarter-wavelength microstrip lines ML1~ML7, and seven PIN switches P1~P7;

[0007] The parallel coupling line CL1, microstrip line ML2, microstrip line ML3, parallel coupling line CL2, switch P2, microstrip line ML6, parallel coupling line CL4, switch P5, parallel coupling line CL3, microstrip line ML5, and switch P1 are connected in series to form a loop.

[0008] Input port 1 is connected to the connection node of parallel coupling line CL1 and switch P1 via microstrip line ML1; output port 2 is connected to the connection node of parallel coupling line CL2 and switch P2 via microstrip line ML4; microstrip line ML7, with one end open, is connected to the connection node of microstrip line ML2 and microstrip line ML3 via switch P3; the connection node of switch P5 and parallel coupling line CL3 is also grounded via switch P6; the parallel coupling line CL3 is also grounded via switch P4, and the parallel coupling line CL4 is also grounded via switch P7.

[0009] By controlling the on and off states of PIN switches P1~P7, the reconstructed switching of five bandpass filter modes can be achieved under the same circuit topology.

[0010] Furthermore, the lower left end of the parallel coupling line CL1 is connected to the switch P1, the upper right end is connected to the microstrip line ML2, and the upper left end and the lower right end are open circuits; the upper left end of the parallel coupling line CL2 is connected to the microstrip line ML3, the lower right end is connected to the switch P2, and the lower left end and the upper right end are open circuits; the upper left end of the parallel coupling line CL3 is connected to the microstrip line ML5, the lower right end is connected to the switch P5, and the upper right end is open circuits; the upper right end of the parallel coupling line CL4 is connected to the microstrip line ML6, the lower left end is connected to the switch P5, and the upper left end is open circuits.

[0011] Furthermore, the lower left end of the parallel coupling line CL3 is grounded through switch P4, and the lower right end of the parallel coupling line CL4 is grounded through switch P7.

[0012] Furthermore, the five bandpass filtering modes include: multi-transmission zero dual-frequency bandpass mode, three-band bandpass mode, wideband multi-pole bandpass mode, dual-sided high-suppression dual-frequency bandpass mode, and passband isolation enhanced dual-frequency bandpass mode.

[0013] Furthermore, in the multi-transmission zero-point dual-frequency bandpass mode, switches P1, P2, and P5 are turned on, while switches P3, P4, P6, and P7 are turned off.

[0014] Furthermore, in the three-band bandpass mode, switches P1, P2, P5, and P6 are turned on, while switches P3, P4, and P7 are turned off.

[0015] Furthermore, in the broadband multi-pole bandpass filtering mode, switches P1, P2, and P3 are open, and switches P4 to P7 are in a non-conducting state.

[0016] Furthermore, in the dual-side high-suppression dual-bandpass mode, switches P1, P2, P4, and P7 are turned on, while switches P3, P5, and P6 are turned off.

[0017] Furthermore, in the passband isolation enhanced dual-bandpass filter mode, switches P1, P2, P3, and P5 are turned on, while switches P4, P6, and P7 are turned off.

[0018] Furthermore, the filter adopts a single symmetrical microstrip line topology, with the upper branch forming the basic resonant channel, and the lower branch and open stub ML7 participating in the coupling resonance or grounding loading through PIN switch control. The cross-coupling of the upper and lower branches generates multiple transmission zeros.

[0019] Beneficial Effects: This invention addresses the technical shortcomings of existing reconfigurable microstrip filters, such as large structural redundancy, low resource reuse rate, uneven performance across multiple modes, and a limited number of switchable modes. Through unified microstrip topology, PIN switch collaborative control, optimized main and auxiliary branch coupling, and precise multi-zero placement, it achieves efficient reconfiguration of five-mode bandpass filtering, offering the following significant advantages compared to existing technologies:

[0020] 1. This invention adopts a single symmetrical microstrip line topology, which can realize the reconstruction of five filtering modes by switching seven PIN switches. There is no need to set up multiple independent filter sub-circuits or redundant resonant units. All core components such as microstrip lines, parallel coupled lines, and open-circuit stubs are efficiently reused in different modes. This fundamentally solves the problems of structural redundancy and large circuit area in traditional solutions. Without increasing hardware costs and circuit scale, it realizes the miniaturization and high integration design of the filter.

[0021] 2. Under the same hardware platform and main circuit framework, this invention can flexibly switch between five bandpass filtering modes: multi-zero dual-band bandpass, tri-band bandpass, wideband multi-pole bandpass, dual-sided high-suppression dual-band bandpass, and passband isolation enhanced dual-band bandpass. It covers various response requirements such as dual-band, tri-band, and wideband, and breaks through the technical bottleneck of existing technologies that have a small number of switchable modes and difficulty in achieving high-performance multi-mode response with a unified topology. It can be adapted to diverse application scenarios of new-generation RF front-ends such as multi-frequency and multi-standard wireless communication, satellite communication, and radar detection.

[0022] 3. This invention achieves low insertion loss, high return loss, and good impedance matching across the entire frequency band in all five operating modes through the synergistic effect of main branch foundation resonance and auxiliary branch coupling, open-circuit stub adjustment, and grounding loading. This effectively solves the problems of significant performance differences, high insertion loss, and insufficient return loss in traditional schemes under different modes. The passband of each mode is uniform, the ripple is small, the performance consistency is high after mode switching, and the overall filtering index is balanced and stable.

[0023] 4. This invention can precisely control the coupling path and branch boundary conditions through the switching state, and introduce multiple transmission zeros as needed in dual-frequency, tri-frequency, and broadband modes: in dual-frequency mode, it significantly improves the isolation between passbands and the suppression capability of both sides of the passband stopband; in tri-frequency mode, it optimizes the transition band roll-off characteristics; and in broadband mode, it makes up for the lack of selectivity of traditional broadband filters. In the end, it achieves excellent characteristics of high isolation in the passband, deep suppression outside the band, and steep transition band, which greatly improves the frequency selectivity and anti-interference capability of the filter.

[0024] 5. This invention effectively reduces reflection loss during mode switching by optimizing the switching network and impedance matching design, ensuring stable system operation in all modes; the overall structure is compact and free of complex redundant design, making it easy to process and debug, and can be directly applied to high-performance scenarios such as 5G / 6G RF front-ends and multi-frequency communication terminals. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the circuit structure of the five-mode reconfigurable microstrip line filter of the present invention;

[0026] Figure 2The circuit diagrams for the five switching modes are shown below, where (a) is a dual-band bandpass filter with multiple transmission zeros, (b) is a three-band bandpass filter, (c) is a wideband multi-pole bandpass filter, (d) is a dual-side high-suppression dual-band bandpass filter, and (e) is a dual-band bandpass filter with enhanced inter-band isolation.

[0027] Figure 3 This is a schematic diagram of the parameters of the modulus reconfigurable filter in Example 5;

[0028] Figure 4 The following are the S-parameter response curves for the five switching modes in the embodiment: (a) represents the S-parameters of the high dual-band bandpass filter with multiple transmission zeros, (b) represents the S-parameters of the three-band bandpass filter, (c) represents the S-parameters of the wideband multi-pole bandpass filter, (d) represents the S-parameters of the dual-side high-suppression dual-band bandpass filter, and (e) represents the S-parameters of the high-isolation dual-band bandpass filter. Detailed Implementation

[0029] The invention will be further explained below with reference to the accompanying drawings. Five-mode reconfigurable microstrip line filter circuit.

[0030] A five-mode reconfigurable microstrip line filter, such as Figure 1 As shown, its circuit includes: port 1 and port 2, four quarter-wavelength parallel coupling lines CL1, CL2, CL3 and CL4, seven quarter-wavelength microstrip lines ML1~ML7, and switches P1~P7.

[0031] Parallel coupling line CL1, microstrip line ML2, microstrip line ML3, parallel coupling line CL2, switch P2, microstrip line ML6, parallel coupling line CL4, switch P5, parallel coupling line CL3, microstrip line ML5, and switch P1 are connected in series to form a loop.

[0032] In this configuration, the lower left end of parallel coupling line CL1 is connected to switch P1, and the upper right end of parallel coupling line CL1 is connected to microstrip line ML2. The upper left and lower right ends of parallel coupling line CL1 are open circuits. The upper left end of parallel coupling line CL2 is connected to microstrip line ML3, and the lower right end of parallel coupling line CL2 is connected to switch P2. The lower left and upper right ends of parallel coupling line CL2 are open circuits. The upper left end of parallel coupling line CL3 is connected to microstrip line ML5, and the lower right end of parallel coupling line CL3 is connected to switch P5. The upper right end of parallel coupling line CL4 is connected to microstrip line ML6, and the lower left end of parallel coupling line CL4 is connected to switch P5. The upper left end of parallel coupling line CL4 is open circuits.

[0033] In addition, port 1 is connected to the connection point of parallel coupling line CL1 and switch P1 via microstrip line ML1, and port 2 is connected to the connection point of parallel coupling line CL2 and switch P2 via microstrip line ML4; microstrip line ML7, which is open at one end, is connected to the connection point of microstrip line ML2 and microstrip line ML3 via switch P3; the connection point of switch P5 and parallel coupling line CL3 is also grounded via switch P6; the lower left end of parallel coupling line CL3 is grounded via switch P4, and the lower right end of parallel coupling line CL4 is grounded via switch P7.

[0034] Switches P1 to P7 are all PIN diodes; port 1 is used as the input port and port 2 is used as the output port.

[0035] In the circuit described above, the switching states of switches P1 to P7 can be controlled by adjusting the power supply voltage of the RF switches, enabling switching between five modes. Specifically:

[0036] Mode 1: Multi-transmission zero-point dual-bandpass filter. Switches P1, P2, and P5 are turned on, while switches P3, P4, P6, and P7 are turned off. The circuit diagram of this multi-transmission zero-point high-isolation dual-bandpass filter is shown below. Figure 2 As shown in (a), the input signal, injected through port 1, excites the parallel coupling lines CL1 and CL2 in the upper branch via microstrip line ML1. Since the two quarter-wavelength parallel coupling lines and the connecting microstrip lines ML2 and ML3 together form a composite resonant unit, the upper branch can establish the main transmission zero of the dual-bandpass response. Simultaneously, the conduction of switches P1 and P5 connects the microstrip line ML5, parallel coupling line CL3, and parallel coupling line CL4 in the lower branch to the main circuit, forming an additional coupling path. This additional coupling path changes the equivalent coupling matrix of the main transmission network and introduces additional transmission zeros on both sides of the passband and in the middle stopband region, thereby enhancing the isolation characteristics between the two passbands and improving the out-of-band rejection level.

[0037] Mode 2: Three-band bandpass filter. Switches P1, P2, P5, and P6 are turned on, while switches P3, P4, and P7 are turned off. The circuit diagram of this three-band bandpass filter is shown below. Figure 2 As shown in (b). The key difference between this mode and mode 1 is that the lower branch no longer serves only as an auxiliary zero-point loading network, but forms an additional resonant channel with a clear ground boundary condition after switch P6 is turned on, thus forming a three-resonant mode response together with the upper branch.

[0038] Specifically, after the input signal enters from port 1, it first excites the two-stage coupled resonant structure in the upper branch, consisting of parallel coupled lines CL1 and CL2, resulting in two main passbands in the upper branch. Simultaneously, the conduction of switches P1, P2, P5, and P6 causes the parallel coupled lines CL3 and CL4 in the lower branch and their grounding ports to form a new resonant circuit. Because the grounding condition alters the equivalent electrical length and boundary impedance of the lower branch, the branch that originally only participated in auxiliary coupling is transformed into a third bandpass channel that can independently contribute to the resonant poles. Therefore, the entire circuit forms three relatively independent but mutually coupled resonant paths under a unified topology, corresponding to bandpass responses in three discrete frequency bands.

[0039] From a mechanistic perspective, the upper branch is responsible for establishing the basic dual-frequency response, while the lower branch grounding coupling unit is responsible for introducing the third passband and further adjusting the out-of-band transmission characteristics. The cross-coupling between multiple resonant circuits can also create several transmission zeros outside the passband, thus enabling the three-band response to not only possess multi-passband characteristics but also a steep roll-off and a high out-of-band rejection level. Therefore, this mode is essentially a three-mode resonant coupling bandpass mechanism achieved through branch boundary reconstruction.

[0040] Mode 3: Wideband Multi-Pole Bandpass Filter. Switches P1-P3 are all open. The circuit diagram of this multi-zero wideband pass filter is shown below. Figure 2 As shown in (c), the lower branch is essentially decoupled from the main transmission path. The filter mainly consists of the parallel coupling lines CL1 and CL2 in the upper branch and the microstrip lines ML1 to ML4, forming a broadband bandpass transmission network. Since there is no additional switching branch to conduct in this mode, the main transmission path maintains a relatively continuous distributed parameter coupling state, thus the circuit exhibits strong broadband transmission capability.

[0041] From an electromagnetic mechanism perspective, both parallel coupled lines CL1 and CL2 are quarter-wavelength parallel coupled line structures. With appropriate even-mode / odd-mode impedance configurations, they can form multiple adjacent resonant poles. These resonant poles couple and broaden in the frequency domain, ultimately synthesizing into a continuous broadband bandpass response. Simultaneously, parasitic coupling and boundary resonance effects still exist between microstrip lines ML2 and ML3 and the central open-circuit stub ML7. This effect can introduce additional transmission zeros at the wide passband edge, thereby improving the selectivity deficiency problem commonly found in broadband filters. Therefore, this mode is not a simple single broadband bandpass structure, but rather utilizes the continuous resonance characteristics of the two-stage coupled line network and the edge zero-shaping effect of the open-circuit stub to achieve a bandpass response that balances "wide passband, multiple zeros, and high selectivity." Its core feature lies in achieving a unified broadband transmission and band-edge suppression through the superposition of the natural frequency responses of the distributed parameter coupled resonant units under the simplest topology excitation state.

[0042] Mode 4: Dual-sided high-suppression dual-bandpass filter. Switches P1, P2, P4, and P7 are turned on, while switches P3, P5, and P6 are turned off. The circuit diagram of this dual-sided high-suppression dual-bandpass filter is shown below. Figure 2 As shown in (d), under this switch configuration, both the upper and lower branches participate in signal transmission simultaneously, forming a composite resonant structure. The input signal enters through port 1 and is excited by the coupling structure in the upper branch via microstrip line ML1. The main resonant unit, composed of parallel coupling lines CL1 and CL2, generates two resonant poles, thus forming two passbands in the dual-bandpass response. Simultaneously, with switches P1 and P2 on, microstrip lines ML5 and ML6, as well as parallel coupling lines CL3 and CL4 in the lower branch, are connected to the main transmission path and, under the grounding load formed by switches P4 and P7 on, constitute an auxiliary resonant branch. This auxiliary coupling path forms a cross-coupling relationship with the main transmission path, generating transmission zeros on both sides of the passband, resulting in strong attenuation capabilities in both the low-frequency and high-frequency stopbands. Therefore, in this mode, the filter not only achieves a dual-bandpass response but also exhibits significantly enhanced stopband suppression characteristics on both sides of the passband, thus forming a dual-sided high-suppression dual-bandpass filter.

[0043] Mode 5: Enhanced Inter-bandband Isolation Dual-bandpass Filter. Switches P1, P2, P3, and P5 are turned on, while switches P4, P6, and P7 are turned off. The circuit diagram of this multi-zero high-isolation dual-bandpass filter is shown below. Figure 2 As shown in (e), this switch configuration creates a composite coupling network between the upper and lower branches, introducing multiple auxiliary resonant paths and thus generating multiple transmission zeros in the dual-bandpass response. The input signal enters through port 1 and excites the coupling structure in the upper branch via microstrip line ML1. The main resonant unit, composed of parallel coupling lines CL1 and CL2, generates two main passbands. Based on this, microstrip lines ML5 and ML6, and parallel coupling lines CL3 and CL4 in the conducting lower branch form cross-coupled connections with the main branch, and the open-circuit stub ML7, introduced through switch P3, participates in resonance modulation. This composite coupling structure creates multiple transmission zeros within and around the passbands. Zeros between and near the passbands improve isolation between the two passbands, while zeros outside the passband enhance stopband rejection. Therefore, this mode achieves high isolation, high selectivity, and good stopband rejection while maintaining dual-bandpass capability through precise design of multiple zeros. This mode is suitable for RF systems with high requirements for frequency selectivity, adjacent channel rejection, and passband isolation in dual-bandpass filters.

[0044] To better illustrate the technical effects of the present invention, this embodiment designs a five-mode reconfigurable filter circuit with a center frequency of 2 GHz, and models and verifies it using simulation software. Figure 3 This is a schematic diagram of the circuit parameters for this embodiment. The characteristic impedances of each microstrip transmission line and coupling structure are set as follows: Ze1 =175.916 Ω, Z o1 =41.31993Ω, Z e2 =181.963 Ω, Z o2 =77.5344Ω, Z1=55.5659Ω, Z2=67.977 Ω, Z3=44.904 Ω, Z4=105.233 Ω, Z5=175.568 Ω, Z6=124.128 Ω; the electrical length of each microstrip segment is designed to be θ=90°.

[0045] Figure 4 Simulated frequency response curves for this embodiment under five different switching states are presented to characterize the transmission and reflection characteristics of the filter in different modes.

[0046] In mode 1, it is a dual-bandpass filter with multiple transmission zeros, and its S-parameter response curve is as follows: Figure 4 As shown in (a), the center passbands are located at approximately 1.5 GHz and 2.5 GHz, respectively. 21 It approaches 0 dB within the passband, exhibiting low insertion loss characteristics; three distinct transmission zeros are introduced between the passbands and at both ends of the passband, and S 21 Below -15.5 dB, effectively enhancing the isolation performance between the two bands; simultaneously, the in-band reflection coefficient S of both passbands is reduced. 11 All frequencies are less than -15.2 dB, with good input matching; the passband and stopband rejection exceed -40 dB, effectively suppressing interference signals in non-operating frequency bands. This mode achieves a comprehensive performance of dual-band passability, high isolation, and good out-of-band rejection through upper and lower branch composite resonance and zero-point loading.

[0047] In mode 2, it is a three-band bandpass filter, and its S-parameter response curve is as follows: Figure 4 As shown in (b), Mode 2 achieves a three-band bandpass response, with center frequencies of approximately 1.24 GHz, 2.0 GHz, and 2.76 GHz for the three passbands, respectively. 21 The signal strength is close to 0 dB within the passband, exhibiting low insertion loss, and each of the three passbands has three reflection poles. Specifically, the -10 dB bandwidth of the first passband is 0.84–1.56 GHz; the -10 dB bandwidth of the second passband is 1.77–2.23 GHz; and the -10 dB bandwidth of the third passband is 2.44–3.16 GHz. 11 All values ​​are less than -15 dB, indicating excellent input matching. There are at least four transmission zeros outside the passband, with a depth of approximately -40 dB. The out-of-band rejection on both sides is above 30.2 dB, effectively isolating each passband and enhancing the out-of-band stopband.

[0048] In mode 3, it is a broadband multi-pole bandpass filter, and its S-parameter response curve is as follows: Figure 4 As shown in (c). In mode 3, the filter exhibits a wideband pass response, covering approximately 1.0–3.0 GHz, with a relative bandwidth of 100%, and has 5 reflection poles within the passband, demonstrating good passband range and impedance matching characteristics. Transmission coefficient S 21 Above -0.25 dB, it exhibits low insertion loss characteristics.

[0049] In mode 4, it is a dual-sided high-suppression dual-bandpass filter, and its S-parameter response curve is as follows: Figure 4 As shown in (d), the centers of the two passbands are located at approximately 1.15 GHz and 2.85 GHz, respectively. The -15 dB bandwidth of the first passband is 0.85-1.58 GHz, with a minimum insertion loss of 0.18 dB. The -15 dB bandwidth of the second passband is 2.44-3.14 GHz, with a minimum insertion loss of 0.19 dB, demonstrating good in-band matching. There is one transmission zero between the passbands, and zeros on both sides of the passband at 0.67 GHz and 3.33 GHz, respectively. The out-of-band rejection is greater than 30.2 dB, exhibiting good inter-passband isolation and high out-of-band rejection performance on both sides. This mode achieves a comprehensive performance of dual-band passability, high isolation, and good out-of-band rejection through upper and lower branch composite resonance and zero-point loading.

[0050] In mode 5, it is a passband isolation enhanced dual-band pass filter, and its S-parameter response curve is as follows: Figure 4 As shown in (e), the centers of the two passbands are located at approximately 1.28 GHz and 2.71 GHz, respectively. The -10 dB bandwidth of the first passband is 0.81–1.75 GHz, and the -10 dB bandwidth of the second passband is 2.24–3.18 GHz. Each passband has four reflection poles, and the insertion loss is close to 0 dB, demonstrating good in-band matching. There are three transmission zeros between the passbands, and the transmission coefficient S between the passbands is... 21 Less than -28.2dB, exhibiting enhanced passband isolation.

[0051] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A five-mode reconfigurable microstrip line filter, characterized in that, It includes input port 1, output port 2, four quarter-wavelength parallel coupling lines CL1~CL4, seven quarter-wavelength microstrip lines ML1~ML7, and seven PIN switches P1~P7; The parallel coupling line CL1, microstrip line ML2, microstrip line ML3, parallel coupling line CL2, switch P2, microstrip line ML6, parallel coupling line CL4, switch P5, parallel coupling line CL3, microstrip line ML5, and switch P1 are connected in series to form a loop. Input port 1 is connected to the connection node of parallel coupling line CL1 and switch P1 via microstrip line ML1; output port 2 is connected to the connection node of parallel coupling line CL2 and switch P2 via microstrip line ML4; microstrip line ML7, with one end open, is connected to the connection node of microstrip line ML2 and microstrip line ML3 via switch P3; the connection node of switch P5 and parallel coupling line CL3 is also grounded via switch P6; the parallel coupling line CL3 is also grounded via switch P4, and the parallel coupling line CL4 is also grounded via switch P7. By controlling the on and off states of PIN switches P1~P7, the reconstructed switching of five bandpass filter modes can be achieved under the same circuit topology.

2. The five-mode reconfigurable microstrip line filter according to claim 1, characterized in that, The lower left end of the parallel coupling line CL1 is connected to switch P1, the upper right end is connected to microstrip line ML2, and the upper left and lower right ends are open circuits; the upper left end of the parallel coupling line CL2 is connected to microstrip line ML3, the lower right end is connected to switch P2, and the lower left and upper right ends are open circuits; the upper left end of the parallel coupling line CL3 is connected to microstrip line ML5, the lower right end is connected to switch P5, and the upper right end is open circuits; the upper right end of the parallel coupling line CL4 is connected to microstrip line ML6, the lower left end is connected to switch P5, and the upper left end is open circuits.

3. The five-mode reconfigurable microstrip line filter according to claim 2, characterized in that, The lower left end of the parallel coupling line CL3 is grounded through switch P4, and the lower right end of the parallel coupling line CL4 is grounded through switch P7.

4. The five-mode reconfigurable microstrip line filter according to any one of claims 1-3, characterized in that, The five bandpass filtering modes include: multi-transmission zero dual-frequency bandpass mode, three-band bandpass mode, wideband multi-pole bandpass mode, dual-sided high-suppression dual-frequency bandpass mode, and passband isolation enhanced dual-frequency bandpass mode.

5. The five-mode reconfigurable microstrip line filter according to claim 4, characterized in that, In the multi-transmission zero-point dual-frequency bandpass mode, switches P1, P2, and P5 are turned on, while switches P3, P4, P6, and P7 are turned off.

6. The five-mode reconfigurable microstrip line filter according to claim 4, characterized in that, In the three-band bandpass mode, switches P1, P2, P5, and P6 are turned on, while switches P3, P4, and P7 are turned off.

7. The five-mode reconfigurable microstrip line filter according to claim 4, characterized in that, In the broadband multi-pole bandpass filtering mode, switches P1, P2, and P3 are open, and switches P4 to P7 are in a non-conducting state.

8. The five-mode reconfigurable microstrip line filter according to claim 4, characterized in that, In the dual-side high-suppression dual-bandpass mode, switches P1, P2, P4, and P7 are turned on, while switches P3, P5, and P6 are turned off.

9. The five-mode reconfigurable microstrip line filter according to claim 4, characterized in that, In the passband isolation enhanced dual-bandpass filter mode, switches P1, P2, P3, and P5 are turned on, while switches P4, P6, and P7 are turned off.

10. The five-mode reconfigurable microstrip line filter according to any one of claims 1-3, characterized in that, The filter adopts a single symmetrical microstrip line topology. The upper branch forms the basic resonant channel, and the lower branch and open stub ML7 participate in the coupling resonance or ground loading through PIN switch control. The cross coupling of the upper and lower branches generates multiple transmission zeros.

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