Low-pass filter topological structure with reconfigurable bandwidth and filter

Abstract

The utility model discloses a bandwidth reconfigurable low-pass filter topological structure, which comprises a second microstrip line and a seventh microstrip line which are sequentially connected along a first direction, the second microstrip line is connected with an input end, and the seventh microstrip line is connected with an output end; a first microstrip line is connected between the input end and the second microstrip line, and the first microstrip line is connected with a first open circuit branch; an eighth microstrip line is connected between the seventh microstrip line and the output end, and the eighth microstrip line is connected with a sixth open-circuit branch knot; a third microstrip line and a fifth microstrip line are symmetrically connected between the second microstrip line and the seventh microstrip line; the third microstrip line is sequentially connected with a first variable capacitance diode and a fourth microstrip line, and the fourth microstrip line is symmetrically connected with a second open-circuit branch knot and a third open-circuit branch knot; the fifth microstrip line is sequentially connected with a second variable capacitance diode and a sixth microstrip line, and the sixth microstrip line is symmetrically connected with a fourth open-circuit branch knot and a fifth open-circuit branch knot; according to the utility model, the problem that the filter cannot satisfy high selectivity and wide stop band at the same time is solved.

Description

Technical Field

[0001] The utility model relates to the technical field of filters, in particular to a bandwidth-reconfigurable low-pass filter topology and a filter. Background Art

[0002] With the rapid development of modern 5G technology, the miniaturization of multifunctional communication systems has become an inevitable development trend. However, the radio frequency filters widely used in multifunctional communication systems at present have the characteristics of being non-adjustable and having fixed functions. To achieve the multifunction of a communication system, a large number of radio frequency filters with different indexes need to be arranged inside the system, which will undoubtedly increase the complexity and volume of the communication system. If the radio frequency filter has a reconfigurable feature, enabling the same filter to flexibly meet different indexes, it will help simplify the design of the wireless communication system and greatly reduce its volume and weight.

[0003] To make full use of spectrum resources and suppress out-of-band signals, low-pass filters with high selectivity and wide stopbands are required in each microwave communication system. However, the existing bandwidth-reconfigurable low-pass filters often cannot meet both high selectivity and wide stopbands at the same time, which greatly limits their use in modern wireless communication systems. Summary of the Utility Model

[0004] The main purpose of the utility model is to provide a bandwidth-reconfigurable low-pass filter topology and a filter, aiming to solve the problem that the existing bandwidth-reconfigurable low-pass filters cannot meet both high selectivity and wide stopbands at the same time.

[0005] To achieve the above purpose, the utility model proposes a bandwidth-reconfigurable low-pass filter topology, including a second microstrip line and a seventh microstrip line connected in sequence along a first direction. The other end of the second microstrip line is connected with an input end, and the other end of the seventh microstrip line is connected with an output end;

[0006] A first microstrip line is connected between the input end and the second microstrip line, and the other end of the first microstrip line is connected with a first open stub; an eighth microstrip line is connected between the seventh microstrip line and the output end, and the other end of the eighth microstrip line is connected with a sixth open stub;

[0007] A third microstrip line and a fifth microstrip line are symmetrically connected between the second microstrip line and the seventh microstrip line; the other end of the third microstrip line is sequentially connected with a first varactor diode and a fourth microstrip line, and the other end of the fourth microstrip line is symmetrically connected with a second open stub and a third open stub; the other end of the fifth microstrip line is sequentially connected with a second varactor diode and a sixth microstrip line, and the other end of the sixth microstrip line is symmetrically connected with a fourth open stub and a fifth open stub;

[0008] The first microstrip line, the eighth microstrip line, the second open stub, the third open stub, the fourth open stub, and the fifth open stub are all arranged along a first direction. The third microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line, the first open stub, and the sixth open stub are all arranged along a second direction, and the second direction is perpendicular to the first direction.

[0009] Optionally, the second microstrip line and the seventh microstrip line, the first microstrip line and the eighth microstrip line, the first open stub and the sixth open stub, the second open stub and the third open stub, and the fourth open stub and the fifth open stub are all symmetric about the second direction;

[0010] The third microstrip line and the fifth microstrip line, the fourth microstrip line and the sixth microstrip line, the second open stub and the fourth open stub, and the third open stub and the fifth open stub are all symmetric about the first direction.

[0011] Optionally, the sum of the electrical length of the first microstrip line and the electrical length of the first open stub, and the sum of the electrical length of the eighth microstrip line and the electrical length of the sixth open stub are both one-quarter wavelength corresponding to the center frequency of the stopband of the filter.

[0012] Optionally, the electrical length of the second microstrip line, the electrical length of the seventh microstrip line, the electrical length of the second open stub, the electrical length of the third open stub, the electrical length of the fourth open stub, and the electrical length of the fifth open stub are all one-quarter wavelength corresponding to the center frequency of the stopband of the filter.

[0013] Optionally, the electrical length of the third microstrip line, the electrical length of the fourth microstrip line, the electrical length of the fifth microstrip line, and the electrical length of the sixth microstrip line are all one-eighth wavelength corresponding to the center frequency of the stopband of the filter.

[0014] Optionally, the characteristic impedance of the second microstrip line is equal to the characteristic impedance of the seventh microstrip line.

[0015] Optionally, the characteristic impedance of the first microstrip line, the characteristic impedance of the eighth microstrip line, the characteristic impedance of the first open stub, and the characteristic impedance of the sixth open stub are equal.

[0016] Optionally, the characteristic impedance of the third microstrip line, the characteristic impedance of the fourth microstrip line, the characteristic impedance of the fifth microstrip line, and the characteristic impedance of the sixth microstrip line are equal;

[0017] The characteristic impedance of the second open stub, the characteristic impedance of the third open stub, the characteristic impedance of the fourth open stub, and the characteristic impedance of the fifth open stub are equal.

[0018] To achieve the above object, the present invention also provides a filter, including any one of the above topological structures.

[0019] Optionally, the filter further includes a circuit board with a dielectric constant of 3.38, a dielectric loss of 0.0022, a thickness of 0.813 mm, and dimensions of 21.6 mm * 21.2 mm;

[0020] The line lengths of the first microstrip line and the eighth microstrip line are both set to l 1H = 6.5 mm, and the line lengths of the first open stub and the sixth open stub are both set to l 1V = 2.05 mm. The line widths between the first microstrip line, the eighth microstrip line, the first open stub, and the sixth open stub are all set to w 1 = 1.2 mm;

[0021] The line lengths of the second microstrip line and the seventh microstrip line are both set to l 2 = 8.7 mm, and the line widths are both set to w 2 = 0.1 mm. The line spacings between the first microstrip line and the second microstrip line, and between the eighth microstrip line and the seventh microstrip line are both set to s 1 = 0.4 mm;

[0022] The line lengths of the third microstrip line and the fifth microstrip line are both set to l 3 = 4.75 mm, and the line widths are both set to w 3 = 0.6 mm;

[0023] The line lengths of the fourth microstrip line and the sixth microstrip line are both set to l 4 = 4.95 mm, and the line widths are both set to w 4 = 0.6 mm;

[0024] The line lengths of the second open stub, the third open stub, the fourth open stub, and the fifth open stub are all set to l 5 = 9 mm, and the line widths are all set to w 5 = 0.6 mm.

[0025] The beneficial effects of the present utility model are as follows: It improves the topological structure of the existing low-pass filter. The topological structure includes a second microstrip line and a seventh microstrip line connected in sequence along a first direction. The second microstrip line is connected to an input end, and the seventh microstrip line is connected to an output end; a first microstrip line is connected between the input end and the second microstrip line, and the first microstrip line is connected to a first open stub; an eighth microstrip line is connected between the seventh microstrip line and the output end, and the eighth microstrip line is connected to a sixth open stub; a third microstrip line and a fifth microstrip line are symmetrically connected between the second microstrip line and the seventh microstrip line; the third microstrip line is sequentially connected to a first varactor diode and a fourth microstrip line, and the fourth microstrip line is symmetrically connected to a second open stub and a third open stub; the fifth microstrip line is sequentially connected to a second varactor diode and a sixth microstrip line, and the sixth microstrip line is symmetrically connected to a fourth open stub and a fifth open stub;

[0026] The filter based on this topological structure has two odd-mode transmission poles, one even-mode transmission pole, and three transmission zeros, and no matter how the characteristic impedance values of its respective stubs are changed, the relative positions of the transmission poles and zeros will not change. Therefore, a low-pass filter can be designed based on this topological structure, and there are two transmission poles in the passband to ensure flatness, and three transmission zeros in the stopband to ensure wide and high selectivity, wide stopband, and high isolation. Description of the Drawings

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present utility model or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present utility model. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0028] Figure 1 It is a schematic diagram of the topological structure of the filter of the present utility model;

[0029] Figure 2 It is the odd-mode form diagram of the equivalent topological structure of the present utility model;

[0030] Figure 3 It is the even-mode form diagram of the equivalent topological structure of the present utility model;

[0031] Figure 4 It is the layout layout of the filter of the present utility model;

[0032] Figure 5 It is the S-parameter simulation result of the filter of the present utility model in State 1;

[0033] Figure 6 It is the S-parameter simulation result of the filter of the present utility model in State 2;

[0034] Figure 7 This is the S-parameter simulation result of the filter of the present utility model in State 3;

[0035] Figure 8 This is the S 21 simulation result of the filter of the present utility model under different working states;

[0036] The realization of the purpose, functional features and advantages of the present utility model will be further described with reference to the embodiments and the accompanying drawings. Detailed implementation manners

[0037] Next, the technical solutions in the embodiments of the present utility model will be clearly and completely described with reference to the accompanying drawings in the embodiments of the present utility model. Obviously, the described embodiments are only a part of the embodiments of the present utility model, rather than all the embodiments. Based on the embodiments of the present utility model, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present utility model.

[0038] It should be noted that if there are directional indications (such as up, down, left, right, front, back,...) involved in the embodiments of the present utility model, the directional indications are only used to explain the relative positional relationship and movement conditions between components in a specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indications will also change accordingly.

[0039] In addition, if there are descriptions such as "first" and "second" involved in the embodiments of the present utility model, the descriptions of "first" and "second" are only for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In addition, the meaning of "and / or" appearing throughout the text is that it includes three parallel solutions. Taking "A and / or B" as an example, it includes solution A, or solution B, or a solution where A and B are satisfied simultaneously. In addition, the technical solutions between the embodiments can be combined with each other, but it must be based on the fact that those of ordinary skill in the art can implement them. When the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present utility model.

[0040] An embodiment of the present utility model provides a bandwidth-reconfigurable low-pass filter topology structure. Refer to Figure 1 , which includes a second microstrip line and a seventh microstrip line connected in sequence along a first direction. The other end of the second microstrip line is connected to an input end, and the other end of the seventh microstrip line is connected to an output end;

[0041] A first microstrip line is connected between the input end and the second microstrip line, and the other end of the first microstrip line is connected to a first open stub; an eighth microstrip line is connected between the seventh microstrip line and the output end, and the other end of the eighth microstrip line is connected to a sixth open stub;

[0042] A third microstrip line and a fifth microstrip line are symmetrically connected between the second microstrip line and the seventh microstrip line; the other end of the third microstrip line is sequentially connected to a first varactor diode and a fourth microstrip line, and the other end of the fourth microstrip line is symmetrically connected to a second open stub and a third open stub; the other end of the fifth microstrip line is sequentially connected to a second varactor diode and a sixth microstrip line, and the other end of the sixth microstrip line is symmetrically connected to a fourth open stub and a fifth open stub;

[0043] The first microstrip line, the eighth microstrip line, the second open stub, the third open stub, the fourth open stub and the fifth open stub are all arranged along a first direction, the third microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line, the first open stub and the sixth open stub are all arranged along a second direction, and the second direction is perpendicular to the first direction.

[0044] This embodiment improves the topological structure of the existing filter, which is composed of two varactor diodes, six open stubs and eight microstrip lines. Among them, the second microstrip line and the seventh microstrip line, the first microstrip line and the eighth microstrip line, the first open stub and the sixth open stub, the second open stub and the third open stub, the fourth open stub and the fifth open stub are all symmetric about the second direction;

[0045] The third microstrip line and the fifth microstrip line, the fourth microstrip line and the sixth microstrip line, the second open stub and the fourth open stub, the third open stub and the fifth open stub are all symmetric about the first direction.

[0046] Further, the sum of the electrical lengths of the first microstrip line and the first open stub, and the sum of the electrical lengths of the eighth microstrip line and the sixth open stub are both one-quarter of the wavelength corresponding to the center frequency of the stopband of the filter.

[0047] The electrical lengths of the second microstrip line, the seventh microstrip line, the second open stub, the third open stub, the fourth open stub and the fifth open stub are all one-quarter of the wavelength corresponding to the center frequency of the stopband of the filter.

[0048] The electrical lengths of the third microstrip line, the fourth microstrip line, the fifth microstrip line and the sixth microstrip line are all one-eighth of the wavelength corresponding to the center frequency of the stopband of the filter.

[0049] Further, the characteristic impedance of the second microstrip line is equal to that of the seventh microstrip line. In this embodiment, the characteristic impedance of the second microstrip line and that of the seventh microstrip line are denoted as Z 2 .

[0050] Further, the characteristic impedance of the first microstrip line, that of the eighth microstrip line, that of the first open stub, and that of the sixth open stub are equal. In this embodiment, the characteristic impedance of the first microstrip line, that of the eighth microstrip line, that of the first open stub, and that of the sixth open stub are denoted as Z 1 .

[0051] Further, the characteristic impedance of the third microstrip line, that of the fourth microstrip line, that of the fifth microstrip line, and that of the sixth microstrip line are equal; in this embodiment, the characteristic impedance of the third microstrip line, that of the fourth microstrip line, that of the fifth microstrip line, and that of the sixth microstrip line are denoted as Z 3 .

[0052] The characteristic impedance of the second open stub, that of the third open stub, that of the fourth open stub, and that of the fifth open stub are equal. In this embodiment, the characteristic impedance of the second open stub, that of the third open stub, that of the fourth open stub, and that of the fifth open stub are denoted as Z 4 .

[0053] For this topology, when the capacitance value of the varactor diode is infinite, the varactor diode has no influence on the performance of the filter designed based on this topology. At this time, the varactor diode can be ignored when analyzing the topology. In this case, this topology is a left-right symmetric structure, and its transmission zeros and poles can be analyzed by odd and even modes. Specifically, Figure 2 is the odd-mode form diagram of the equivalent topology:

[0054] When Y ino = ∞, it can be obtained that this topology has two odd-mode transmission poles, and their corresponding frequencies are respectively:

[0055] f op1 = 0

[0056] f op2 = f 0

[0057] Here, f 0 is the center frequency of the band-stop filter.

[0058] Figure 3 is the even-mode form diagram of the equivalent topology:

[0059] Here, the electrical lengths of the ninth microstrip line and the tenth microstrip line are both one-eighth of the wavelength corresponding to the center frequency of the stopband; the characteristic impedances of the ninth microstrip line and the tenth microstrip line are both twice the characteristic impedances of the third microstrip line and the fourth microstrip line, i.e., 2Z 3 . When Y ine = ∞, it can be obtained that this topological structure has an even-mode transmission pole, and the corresponding frequency is:

[0060]

[0061] For this topological structure, its transmission zeros can be calculated by the following method: Multiply the ABCD matrices of the cascaded resonators that make up the topological structure in sequence to obtain the ABCD matrix corresponding to this topological structure; convert the ABCD matrix of this topological structure into the corresponding S matrix. When |S 21 | = 0, it can be obtained that this topological structure has three transmission zeros, and the frequencies corresponding to the three transmission zeros are respectively:

[0062]

[0063] f z2 = f 0

[0064]

[0065] From the above analysis, it can be seen that this topological structure has two odd-mode transmission poles, one even-mode transmission pole, and three transmission zeros. And regardless of the values of the parameters Z 1 , Z 2 , Z 3 , Z 4 , the relative positions of these transmission zeros and poles, i.e., f op1 < f ep1 < f z1 < f op2 = f 0 = f z2 < f z3 , will not change. In addition, according to the characteristics of the RF filter, when the positions of the transmission zeros and poles coincide, only the characteristics of the transmission zeros are shown. Therefore, the RF filter designed based on this topological structure can only be a low-pass filter, and there are two transmission poles in the passband to ensure flatness, and three transmission zeros in the stopband to ensure wide and high selectivity, wide stopband, and high isolation.

[0066] In addition, further research on the equivalent topological structure shows that for the low-pass filter designed based on this topological structure, when the capacitance value of the varactor diode is reduced, the maximum operating frequency of the passband will move downward, but the maximum operating frequency of the stopband remains unchanged. Therefore, by controlling the capacitance value of the varactor diode with an independent DC power supply, the bandwidth of the low-pass filter designed based on this topological structure can be individually reconfigured.

[0067] To verify the above theory, the present utility model also provides a filter, including the topological structure described in any one of the above. Figure 4 For the layout layout of the filter of the present utility model, from Figure 4 it can be seen that the varactor diodes D V1 and D V2 require an independent power supply and DC ground vias to form the required DC voltage drop, so as to control their capacitance values. Among them, R1 is a resistor for protecting the varactor diodes D V1 and D V2 ; L1, L2, and L3 are blocking inductors, the purpose of which is to prevent the power supply circuit and the ground circuit from participating in the operation of the RF circuit; the pads of the protection resistor and the blocking inductor are both provided with pads; R1 is also connected to the pad of the independent power supply to introduce the voltage of the DC power supply; L2 and L3 are both connected to the DC ground vias to achieve the grounding function.

[0068] Furthermore, the filter further includes a circuit board, the dielectric constant of the circuit board is 3.38, the dielectric loss is 0.0022, the thickness is 0.813 mm, and the size is 21.6 mm * 21.2 mm;

[0069] Referring to Figure 4 , the line lengths of the first microstrip line and the eighth microstrip line are both set to l 1H = 6.5 mm, the line lengths of the first open stub and the sixth open stub are both set to l 1V = 2.05 mm, and the line widths between the first microstrip line, the eighth microstrip line, the first open stub, and the sixth open stub are all set to w 1 = 1.2 mm;

[0070] The line lengths of the second microstrip line and the seventh microstrip line are both set to l 2 = 8.7 mm, and the line widths are both set to w 2 = 0.1 mm; the line spacings between the first microstrip line and the second microstrip line, and between the eighth microstrip line and the seventh microstrip line are both set to s 1 = 0.4 mm;

[0071] The line lengths of the third microstrip line and the fifth microstrip line are both set to l 3 = 4.75 mm, and the line widths are both set to w 3 = 0.6 mm;

[0072] The line lengths of the fourth microstrip line and the sixth microstrip line are both set to l 4 = 4.95 mm, and the line widths are both set to w 4 = 0.6 mm;

[0073] The line lengths of the second open stub, the third open stub, the fourth open stub, and the fifth open stub are both set to l 5 = 9 mm, and the line widths are both set to w 5 = 0.6 mm.

[0074] Figure 5 This is the S-parameter simulation result of the filter of the present invention in State 1. The so-called State 1 means that when the varactor diode is loaded with an appropriate voltage, its capacitance value is 0.8 pF. At this time, the impedance bandwidth range where the reflection coefficient of the filter is less than -10 dB is from DC to 1.877 GHz, the maximum in-band loss is 0.48 dB; the stopband with isolation greater than 20 dB ranges from 2.871 GHz to 9 GHz; there are two transmission poles in the passband, located at 0 and 1.66 GHz respectively, to ensure in-band flatness; there are three transmission zeros in the stopband, located at 3.4 GHz, 5.515 GHz, and 8.8 GHz respectively, to ensure high selectivity, wide stopband, and high isolation.

[0075] Figure 6 This is the S-parameter simulation result of the filter of the present invention in State 2. The so-called State 2 means that when the varactor diode is loaded with an appropriate voltage, its capacitance value is 2.3 pF. At this time, the impedance bandwidth range where the reflection coefficient of the filter is less than -10 dB is from DC to 1.439 GHz, the maximum in-band loss is 0.48 dB; the stopband with isolation greater than 20 dB ranges from 2.337 GHz to 8.988 GHz; there are two transmission poles in the passband, located at 0 and 1.18 GHz respectively, to ensure in-band flatness; there are three transmission zeros in the stopband, located at 2.68 GHz, 5.54 GHz, and 8.76 GHz respectively, to ensure high selectivity, wide stopband, and high isolation.

[0076] Figure 7This is the S-parameter simulation result of the filter of the present utility model in State 3. The so-called State 3 means that when the varactor diode is under an appropriate applied voltage, its capacitance value is 50 pF. At this time, the impedance bandwidth range where the reflection coefficient of this filter is less than -10 dB is from DC to 1.101 GHz, and the maximum in-band loss is 0.48 dB; the stopband with an isolation greater than 20 dB ranges from 1.937 GHz to 8.966 GHz; there are two transmission poles in the passband, located at 0 and 0.76 GHz respectively, to ensure flatness in the passband; there are three transmission zeros in the stopband, located at 2.2 GHz, 5.54 GHz, and 8.74 GHz respectively, to ensure high selectivity, wide stopband, and high isolation.

[0077] From Figures 5-7 the simulation results, it can be seen that when the low-pass filter designed based on the topological structure proposed in this application is under the control of an independent DC power supply, its maximum operating frequency is shifted down from 1.877 GHz to 1.101 GHz.

[0078] From Figure 8 it can be seen that for the band-pass filter designed based on this topological structure, in different operating states, its bandwidth can be separately and continuously reconfigured, and it has high selectivity, wide stopband, and high isolation.

[0079] Therefore, the topological structure of this embodiment can be used to design a low-pass filter with reconfigurable bandwidth, having the advantages of high selectivity, wide stopband, and high isolation.

[0080] The above are only the optional embodiments of the present utility model, and do not limit the patent scope of the present utility model accordingly. Any equivalent structural transformation made under the inventive concept of the present utility model by using the content of the specification and drawings of the present utility model, or any direct / indirect application in other related technical fields is included in the patent protection scope of the present utility model.

Claims

1. A bandwidth reconfigurable low-pass filter topology structure, characterized in that: It includes a second microstrip line and a seventh microstrip line connected in sequence along a first direction, the other end of the second microstrip line is connected to an input end, and the other end of the seventh microstrip line is connected to an output end; A first microstrip line is connected between the input end and the second microstrip line, and a first open-circuit branch is connected to the other end of the first microstrip line; An eighth microstrip line is connected between the seventh microstrip line and the output end, and the other end of the eighth microstrip line is connected to a sixth open-circuit branch; The third microstrip line and the fifth microstrip line are symmetrically connected between the second microstrip line and the seventh microstrip line; the other end of the third microstrip line is sequentially connected to the first varactor diode and the fourth microstrip line, and the other end of the fourth microstrip line is symmetrically connected to the second open branch node and the third open branch node; the other end of the fifth microstrip line is sequentially connected to the second varactor diode and the sixth microstrip line, and the other end of the sixth microstrip line is symmetrically connected to the fourth open branch node and the fifth open branch node; The first microstrip line, the eighth microstrip line, the second open branch, the third open branch, the fourth open branch and the fifth open branch are all arranged along the first direction, and the third microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line, the first open branch and the sixth open branch are all arranged along the second direction, and the second direction is perpendicular to the first direction.

2. The bandwidth reconfigurable low-pass filter topology structure according to claim 1, characterized in that: The second microstrip line and the seventh microstrip line, the first microstrip line and the eighth microstrip line, the first open branch and the sixth open branch, the second open branch and the third open branch, the fourth open branch and the fifth open branch are all symmetrical about the second direction; The third microstrip line and the fifth microstrip line, the fourth microstrip line and the sixth microstrip line, the second open branch and the fourth open branch, the third open branch and the fifth open branch are all symmetrical about the first direction.

3. The bandwidth reconfigurable low-pass filter topology structure according to claim 1, characterized in that: The sum of the electrical length of the first microstrip line and the electrical length of the first open branch, and the sum of the electrical length of the eighth microstrip line and the electrical length of the sixth open branch are all one-quarter wavelength corresponding to the center frequency of the filter stopband.

4. The bandwidth reconfigurable low-pass filter topology structure according to claim 3, characterized in that: The electrical length of the second microstrip line, the electrical length of the seventh microstrip line, the electrical length of the second open branch, the electrical length of the third open branch, the electrical length of the fourth open branch and the electrical length of the fifth open branch are all one-quarter wavelength corresponding to the center frequency of the filter stopband.

5. The bandwidth reconfigurable low-pass filter topology structure according to claim 4, characterized in that: The electrical lengths of the third microstrip line, the fourth microstrip line, the fifth microstrip line and the sixth microstrip line are all one eighth of the wavelength corresponding to the center frequency of the filter stopband.

6. The bandwidth reconfigurable low-pass filter topology structure according to claim 1, characterized in that: The characteristic impedance of the second microstrip line is equal to the characteristic impedance of the seventh microstrip line.

7. The bandwidth reconfigurable low-pass filter topology structure according to claim 6, characterized in that: The characteristic impedance of the first microstrip line, the characteristic impedance of the eighth microstrip line, the characteristic impedance of the first open-circuit branch, and the characteristic impedance of the sixth open-circuit branch are equal.

8. The bandwidth reconfigurable low-pass filter topology structure according to claim 7, characterized in that: The characteristic impedance of the third microstrip line, the characteristic impedance of the fourth microstrip line, the characteristic impedance of the fifth microstrip line and the characteristic impedance of the sixth microstrip line are equal; The characteristic impedance of the second open-circuit branch, the characteristic impedance of the third open-circuit branch, the characteristic impedance of the fourth open-circuit branch, and the characteristic impedance of the fifth open-circuit branch are equal.

9. A filter, characterized in that: It comprises the topological structure described in any one of claims 1 to 8.

10. The filter according to claim 9, characterized in that The filter further comprises a circuit board, the circuit board has a dielectric constant of 3.38, a dielectric loss of 0.0022, a thickness of 0.813 mm, and a size of 21.6 mm*21.2 mm; The lengths of the first microstrip line and the eighth microstrip line are both set to l 1H =6.5mm, the line lengths of the first open-circuit branch and the sixth open-circuit branch are both set to l 1V =2.05mm, the line widths of the first microstrip line, the eighth microstrip line, the first open branch and the sixth open are all set to w1=1.2mm; The line lengths of the second microstrip line and the seventh microstrip line are both set to l2=8.7 mm, and the line widths are both set to w2=0.1 mm; the line spacing between the first microstrip line and the second microstrip line, and the line spacing between the eighth microstrip line and the seventh microstrip line are both set to s1=0.4 mm; The line lengths of the third microstrip line and the fifth microstrip line are both set to l3=4.75 mm, and the line widths are both set to w3=0.6 mm; The line lengths of the fourth microstrip line and the sixth microstrip line are both set to l4=4.95 mm, and the line widths are both set to w4=0.6 mm; The line lengths of the second open branch, the third open branch, the fourth open branch and the fifth open branch are all set to l5=9 mm, and the line widths are all set to w5=0.6 mm.

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