Low-pass filter and communication equipment

Abstract

The embodiment of the utility model relates to the technical field of filters, in particular to a low-pass filter and communication equipment, the low-pass filter comprises a substrate and a topological structure, and the topological structure is arranged on the substrate. The topological structure comprises an input end, an output end, a variable capacitance diode, a first micro-strip line, a second micro-strip line, a third micro-strip line, a fourth micro-strip line, a fifth micro-strip line, a sixth micro-strip line, a seventh micro-strip line, a first open-circuit branch knot, a second open-circuit branch knot, a third open-circuit branch knot and a fourth open-circuit branch knot. The first end of the first microstrip line and the first end of the second microstrip line are both connected to the input end, and the first end of the sixth microstrip line and the first end of the seventh microstrip line are both connected to the output end. The topological structure of the embodiment of the utility model comprises a variable capacitance diode, seven microstrip lines and four open-circuit branches, is used for designing a high-performance bandwidth reconfigurable low-pass filter through reasonable design and layout, and has the advantage of miniaturization.

Description

Technical Field

[0001] The embodiments of the utility model relate to the technical field of filters, and in particular to a low-pass filter and communication equipment. 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 reality is that the RF filters currently widely used in multifunctional communication systems are non-adjustable and have fixed functions. In order to realize the multifunctionality of the communication system, a large number of RF filters with different indicators need to be arranged inside the system, which will undoubtedly increase the complexity and volume of the communication system. If the RF filter has a reconfigurable feature so that the same filter can flexibly meet different indicators, it will help simplify the design of the wireless communication system and greatly reduce its volume and weight.

[0003] In order to make full use of spectrum resources and suppress out-of-band signals, microwave communication systems require low-pass filters with high selectivity and wide stopband. However, existing high-performance bandwidth-reconfigurable low-pass filters are mainly realized by loading multiple branches, multiple resonators, and multiple varactor diodes, which often have the defect of large size, which greatly limits their use in modern wireless communication systems. Utility Model Content

[0004] In view of the above problems, the embodiments of the present invention provide a low-pass filter and a communication device, which overcome the above problems or at least partially solve the above problems.

[0005] According to one aspect of an embodiment of the utility model, a low-pass filter is provided, comprising a substrate and a topological structure, wherein the topological structure is arranged on the substrate, and the topological structure comprises an input end, an output end, a varactor diode, a first microstrip line, a second microstrip line, a third microstrip line, a fourth microstrip line, a fifth microstrip line, a sixth microstrip line, a seventh microstrip line, a first open branch node, a second open branch node, a third open branch node, and a fourth open branch node; wherein the first end of the first microstrip line and the first end of the second microstrip line are both connected to the input end, one end of the first open branch node is connected to the second end of the first microstrip line, and the sixth microstrip line is connected to the seventh microstrip line. The first end of the strip line and the first end of the seventh microstrip line are both connected to the output end, one end of the fourth open branch is connected to the second end of the seventh microstrip line, the second end of the second microstrip line and the second end of the sixth microstrip line are both connected to the first end of the third microstrip line, the second end of the third microstrip line is connected to the negative electrode of the varactor diode, the first end of the fourth microstrip line and the first end of the fifth microstrip line are both connected to the positive electrode of the varactor diode, one end of the second open branch is connected to the second end of the fourth microstrip line, and one end of the third open branch is connected to the second end of the fifth microstrip line.

[0006] In some embodiments, with the third microstrip line as the axis, the first microstrip line and the seventh microstrip line are symmetrically arranged, the second microstrip line and the sixth microstrip line are symmetrically arranged, the fourth microstrip line and the fifth microstrip line are symmetrically arranged, the first open branch and the fourth open branch are symmetrically arranged, and the second open branch and the third open branch are symmetrically arranged.

[0007] In some embodiments, the first microstrip line, the second microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line and the seventh microstrip line are parallel to each other, the first microstrip line, the second microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line and the seventh microstrip line are all perpendicular to the third microstrip line, and the third microstrip line, the first open branch, the second open branch, the third open branch and the fourth open branch are parallel to each other.

[0008] In some embodiments, the electrical length of the second microstrip line and the electrical length of the sixth microstrip line are both one-quarter wavelength corresponding to the stop band center frequency of the low-pass filter.

[0009] In some embodiments, the sum of the electrical length of the first microstrip line and the electrical length of the first open branch is one-quarter wavelength corresponding to the center frequency of the stopband of the low-pass filter, and the sum of the electrical length of the seventh microstrip line and the electrical length of the fourth open branch is one-quarter wavelength corresponding to the center frequency of the stopband of the low-pass filter.

[0010] In some embodiments, the electrical length of the third microstrip line is one eighth of the wavelength corresponding to the stop band center frequency of the low-pass filter.

[0011] In some embodiments, the sum of the electrical length of the fourth microstrip line and the electrical length of the second open-circuit branch is three-eighths of a wavelength corresponding to the center frequency of the stopband of the low-pass filter, and the sum of the electrical length of the fifth microstrip line and the electrical length of the third open-circuit branch is three-eighths of a wavelength corresponding to the center frequency of the stopband of the low-pass filter.

[0012] In some embodiments, the characteristic impedance of the first microstrip line, the characteristic impedance of the seventh microstrip line, the characteristic impedance of the first open-circuit branch and the characteristic impedance of the fourth open-circuit branch are equal, and the characteristic impedance of the second microstrip line and the characteristic impedance of the sixth microstrip line are equal.

[0013] In some embodiments, the characteristic impedance of the fourth microstrip line, the characteristic impedance of the fifth microstrip line, the characteristic impedance of the second open-circuit stub, and the characteristic impedance of the third open-circuit stub are equal and are twice the characteristic impedance of the third microstrip line.

[0014] According to one aspect of the embodiments of the present utility model, a communication device is provided, comprising the above-mentioned low-pass filter.

[0015] The beneficial effects of the embodiments of the utility model are as follows: Different from the prior art, the embodiments of the utility model provide a low-pass filter, including a substrate and a topological structure, wherein the topological structure is arranged on the substrate, and the topological structure includes an input end, an output end, a varactor diode, a first microstrip line, a second microstrip line, a third microstrip line, a fourth microstrip line, a fifth microstrip line, a sixth microstrip line, a seventh microstrip line, a first open branch, a second open branch, a third open branch, and a fourth open branch; wherein the first end of the first microstrip line and the first end of the second microstrip line are both connected to the input end, and one end of the first open branch is connected to the first microstrip line. The second end, the first end of the sixth microstrip line and the first end of the seventh microstrip line are both connected to the output end, one end of the fourth open-circuit branch is connected to the second end of the seventh microstrip line, the second end of the second microstrip line and the second end of the sixth microstrip line are both connected to the first end of the third microstrip line, the second end of the third microstrip line is connected to the negative electrode of the varactor diode, the first end of the fourth microstrip line and the first end of the fifth microstrip line are both connected to the positive electrode of the varactor diode, one end of the second open-circuit branch is connected to the second end of the fourth microstrip line, and one end of the third open-circuit branch is connected to the second end of the fifth microstrip line. The topological structure of the embodiment of the utility model includes a varactor diode, seven microstrip lines and four open-circuit branches. Compared with the existing design technologies of various reconfigurable microstrip low-pass filters, the number of loaded branches, resonators or varactor diodes is reduced. Through reasonable design and layout, it is used to design a bandwidth reconfigurable low-pass filter with high performance, which has the advantage of miniaturization. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the embodiments of the utility model, the drawings required for use in the embodiments of the utility model will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the utility model. For ordinary technicians in this field, other drawings can be obtained based on the drawings without creative work.

[0017] Figure 1 It is a schematic diagram of the topological structure provided by the embodiment of the utility model;

[0018] Figure 2 It is a layout diagram of a low-pass filter provided in an embodiment of the utility model;

[0019] Figure 3 It is a schematic diagram of an odd-mode form of an equivalent topological structure provided by an embodiment of the utility model;

[0020] Figure 4 It is a schematic diagram of an even-mode form of an equivalent topological structure provided by an embodiment of the utility model;

[0021] Figure 5 This is a diagram of S parameter simulation results of the low-pass filter provided by the embodiment of the utility model in the first state;

[0022] Figure 6 This is a diagram of S parameter simulation results of the low-pass filter provided by the embodiment of the utility model in the second state;

[0023] Figure 7 is a diagram of S parameter simulation results of a low-pass filter provided by an embodiment of the utility model in a third state;

[0024] Figure 8 is the S of the low-pass filter provided by the embodiment of the utility model in different working states 21 Simulation result diagram.

[0025] The reference numerals in the specific implementation manner are as follows:

[0026] 1000, low-pass filter; 100, topology;

[0027] 10. Input terminal; 20. First microstrip line; 21. Second microstrip line; 22. Third microstrip line; 23. Fourth microstrip line; 24. Fifth microstrip line; 25. Sixth microstrip line; 26. Seventh microstrip line;

[0028] 31, the first open branch; 32, the second open branch; 33, the third open branch; 34, the fourth open branch;

[0029] 40. first solder pad; 50. second solder pad; 60. DC grounding hole; 70. output terminal;

[0030] 80. varactor diode; 81. resistor; 82. first inductor; 83. second inductor. DETAILED DESCRIPTION

[0031] In order to facilitate the understanding of the utility model, the utility model is described in more detail below in conjunction with the accompanying drawings and specific embodiments. It should be noted that when an element is described as "fixed to" another element, it can be directly on the other element, or there can be one or more centered elements therebetween. When an element is described as "connected to" another element, it can be directly connected to the other element, or there can be one or more centered elements therebetween. The terms "vertical", "horizontal", "left", "right" and similar expressions used in this specification are for illustrative purposes only.

[0032] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as those commonly understood by technicians in the technical field of the present invention. The terms used in this specification are only for the purpose of describing specific embodiments and are not used to limit the present invention. The term "and / or" used in this specification includes any and all combinations of one or more related listed items.

[0033] In order to make full use of spectrum resources and suppress out-of-band signals, microwave communication systems require low-pass filters with high selectivity and wide stopband. However, existing high-performance bandwidth reconfigurable low-pass filters often have the defect of large size, which greatly limits their use in modern wireless communication systems. For example, there are many existing design technologies for reconfigurable microstrip low-pass filters, which mainly adopt low-impedance branches loaded with step impedance resonators, cascaded hairpin-type step impedance resonators, fan-shaped branches and open branches loaded with coupling lines, multiple open branches loaded with hexagonal resonators, step impedance branches and fan-shaped branches loaded with coupling lines and other design methods to achieve reconfigurable low-pass filter response. However, the resonator size of the reconfigurable low-pass filter designed by the above traditional methods is large, and more branches are loaded, which makes the reconfigurable low-pass filter generally have the defect of large size.

[0034] The embodiment of the utility model adopts a newly designed topological structure, including a varactor diode, seven microstrip lines and four open branches. Compared with the existing design technologies of various reconfigurable microstrip low-pass filters, the number of loaded branches, resonators or varactor diodes is reduced. Through reasonable design layout, it is used to design a high-performance bandwidth reconfigurable low-pass filter with the advantage of miniaturization.

[0035] In order to facilitate readers to understand the inventive concept of the utility model, the specific structure of the low-pass filter is described below:

[0036] See also Figure 1-2The low-pass filter 1000 includes a substrate (not shown) and a topological structure 100. The topological structure 100 is arranged on the substrate. The substrate is used as a carrier and grounding for the topological structure 100. It can be understood that the material of the substrate can be selected according to needs. For example, the material of the substrate is a polymer material with a metal layer. The topological structure 100 includes an input terminal 10, an output terminal 70, a varactor diode 80, a first microstrip line 20, a second microstrip line 21, a third microstrip line 22, a fourth microstrip line 23, a fifth microstrip line 24, a sixth microstrip line 25, a seventh microstrip line 26, a first open branch 31, a second open branch 32, a third open branch 33 and a fourth open branch 34; wherein the first end of the first microstrip line 20 and the first end of the second microstrip line 21 are both connected to the input terminal 10, one end of the first open branch 31 is connected to the second end of the first microstrip line 20, the first end of the sixth microstrip line 25 and the first end of the seventh microstrip line 26 are both connected to the output terminal 70, and one end of the fourth open branch 34 is connected to the The second end of the seventh microstrip line 26, the second end of the second microstrip line 21 and the second end of the sixth microstrip line 25 are all connected to the first end of the third microstrip line 22, the second end of the third microstrip line 22 is connected to the cathode of the varactor 80, the first end of the fourth microstrip line 23 and the first end of the fifth microstrip line 24 are both connected to the anode of the varactor 80, one end of the second open branch 32 is connected to the second end of the fourth microstrip line 23, and one end of the third open branch 33 is connected to the second end of the fifth microstrip line 24. In this way, the external electrical signal can be input into the topological structure 100 from the input end 10, transmitted to various parts of the topological structure 100, and output from the output end 70.

[0037] In some embodiments, with the third microstrip line 22 as the axis, the first microstrip line 20 and the seventh microstrip line 26 are symmetrically arranged, the second microstrip line 21 and the sixth microstrip line 25 are symmetrically arranged, the fourth microstrip line 23 and the fifth microstrip line 24 are symmetrically arranged, the first open branch 31 and the fourth open branch 34 are symmetrically arranged, and the second open branch 32 and the third open branch 33 are symmetrically arranged, that is, the topological structure 100 is left-right symmetrical about the third microstrip line 22, so as to simplify the design of the low-pass filter 1000.

[0038] In some embodiments, the first microstrip line 20, the second microstrip line 21, the fourth microstrip line 23, the fifth microstrip line 24, the sixth microstrip line 25 and the seventh microstrip line 26 are parallel to each other, the first microstrip line 20, the second microstrip line 21, the fourth microstrip line 23, the fifth microstrip line 24, the sixth microstrip line 25 and the seventh microstrip line 26 are all perpendicular to the third microstrip line 22, and the third microstrip line 22, the first open branch 31, the second open branch 32, the third open branch 33 and the fourth open branch 34 are parallel to each other, so as to simplify the design of the low-pass filter 1000.

[0039] In some embodiments, the electrical length of the second microstrip line 21 and the electrical length of the sixth microstrip line 25 are both one-quarter wavelength corresponding to the stopband center frequency of the low-pass filter 1000, the sum of the electrical length of the first microstrip line 20 and the electrical length of the first open branch 31 is one-quarter wavelength corresponding to the stopband center frequency of the low-pass filter 1000, and the sum of the electrical length of the seventh microstrip line 26 and the electrical length of the fourth open branch 34 is one-quarter wavelength corresponding to the stopband center frequency of the low-pass filter 1000. The electrical length of the third microstrip line 22 is one eighth of the wavelength corresponding to the stopband center frequency of the low-pass filter 1000, the sum of the electrical length of the fourth microstrip line 23 and the electrical length of the second open branch 32 is three eighths of the wavelength corresponding to the stopband center frequency of the low-pass filter 1000, and the sum of the electrical length of the fifth microstrip line 24 and the electrical length of the third open branch 33 is three eighths of the wavelength corresponding to the stopband center frequency of the low-pass filter 1000. In this way, the design of the low-pass filter 1000 is simplified.

[0040] In some embodiments, the characteristic impedance of the first microstrip line 20, the characteristic impedance of the seventh microstrip line 26, the characteristic impedance of the first open-circuit branch 31 and the characteristic impedance of the fourth open-circuit branch 34 are equal and are all Z1; the characteristic impedance of the second microstrip line 21 and the characteristic impedance of the sixth microstrip line 25 are equal and are all Z2. The characteristic impedance of the third microstrip line 22 is Z3, the characteristic impedance of the fourth microstrip line 23, the characteristic impedance of the fifth microstrip line 24, the characteristic impedance of the second open-circuit branch 32 and the characteristic impedance of the third open-circuit branch 33 are equal and are all 2Z3. In this way, the design of the low-pass filter 1000 is simplified.

[0041] Please also read Figure 3-4For the above topological structure 100, when the capacitance of the varactor diode 80 is infinite, the varactor diode 80 has no effect on the performance of the filter designed based on the topological structure 100. At this time, the varactor diode 80 can be ignored when analyzing the topological structure 100. In this case, the topological structure 100 is a bilaterally symmetrical structure, and its transmission zero poles can be analyzed using odd and even modes.

[0042] Figure 3 is the odd-mode form of the equivalent passive topology 100, when Y ino =∞, it can be concluded that the topology 100 has two odd-mode transmission poles, and their corresponding frequencies are:

[0043] f op1 =0

[0044] f op2 =f0

[0045] Here, f0 is the center frequency of the band-stop filter.

[0046] Figure 4 is the even-mode form of the equivalent topology 100, Figure 4 In the embodiment, the electrical length of the eighth microstrip line is equal to the electrical length of the third microstrip line 22; the characteristic impedance of the eighth microstrip line is twice the characteristic impedance of the third microstrip line 22, that is, 2Z3. ine =∞, it can be concluded that the topology 100 has an even-mode transmission pole, and its corresponding frequency is:

[0047]

[0048] For this topology 100, its transmission zero point can be calculated by the following method: multiply the ABCD matrices of the cascade resonators constituting the topology 100 in sequence to obtain the ABCD matrix corresponding to the topology 100; and convert the ABCD matrix of the topology 100 into the corresponding S matrix. 21 When |=0, it can be concluded that the topology 100 has three transmission zeros, and the frequencies corresponding to the three transmission zeros are:

[0049]

[0050] f z2 =f0

[0051]

[0052] From the above analysis, it can be seen that the topology 100 has two odd-mode transmission poles, one even-mode transmission pole, and three transmission zeros. Regardless of how the values ​​of the parameters Z1, Z2, and Z3 change, the relative positions of these transmission zero poles, i.e., f op1<f ep1 <f z1 <f op2 =f0=f z2 <f z3 , will not change. In addition, according to the characteristics of the RF filter, when the positions of the transmission zero and the transmission pole coincide, only the characteristics of the transmission zero are displayed. Therefore, the RF filter designed based on this topology 100 can only be a low-pass filter 1000, 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.

[0053] In addition, further research on the equivalent topological structure 100 shows that, for the low-pass filter 1000 designed based on the topological structure 100, when the capacitance of the varactor diode 80 is reduced, the maximum operating frequency of the passband will move upward, but the maximum operating frequency of the stopband will remain unchanged. Therefore, by controlling the capacitance of the varactor diode 80 through an independent DC power supply, the bandwidth of the low-pass filter 1000 designed based on the topological structure 100 can be independently reconfigured.

[0054] To verify the above theory, the present invention also provides a design example. Figure 2 The layout diagram of the bandwidth reconfigurable filter designed based on the topology 100 is shown in FIG. Figure 2 It can be seen that the varactor diode 80 requires an independent power supply and a DC grounding hole 60 to form the required DC voltage drop, so as to control its capacitance. Therefore, the low-pass filter 1000 is designed with a first pad 40, a second pad 50, a DC grounding hole 60, a resistor 81, a first inductor 82 and a second inductor 83; wherein the resistor 81 is used to protect the varactor diode 80; the first inductor 82 and the first inductor 82 are used as blocking inductors to prevent the power circuit and the grounding circuit from participating in the operation of the radio frequency circuit; the first pad 40 is used to protect the resistor 81 and the blocking inductor; the second pad 50 is used as a pad of an independent power supply to introduce the voltage of the DC power supply; the DC grounding hole 60 is used to realize the function of the ground.

[0055] In this design example, the low-pass filter 1000 is designed on a substrate with a dielectric loss of 0.0022 and a thickness of 0.813 mm, and the size of the substrate is 26.9 mm*11.3 mm. It can be understood that the dielectric loss, thickness and size of the substrate are not limited to the above values ​​and can be adjusted according to needs. Based on the above, it can be seen that the topological structure 100 is left-right symmetrical about the third microstrip line 22. Therefore, the size of the first microstrip line 20 is the same as the size of the seventh microstrip line 26, the size of the second microstrip line 21 is the same as the size of the sixth microstrip line 25, the size of the fourth microstrip line 23 is the same as the size of the fifth microstrip line 24, the size of the first open branch 31 is the same as the size of the fourth open branch 34, and the size of the second open branch 32 is the same as the size of the third open branch 33. Assumption: l 1H is the physical length of the first microstrip line 20 or the physical length of the seventh microstrip line 26, l 1V is the physical length of the first open branch 31 or the physical length of the fourth open branch 34, l2 is the physical length of the second microstrip line 21 or the physical length of the sixth microstrip line 25, l3 is the physical length of the third microstrip line 22, l4 is the physical length of the fourth microstrip line 23 or the physical length of the fifth microstrip line 24, l5 is the physical length of the second open branch 32 or the physical length of the third open branch 33, w1 is the physical width of the first microstrip line 20 or the physical width of the seventh microstrip line 26, w2 is the physical width of the second microstrip line 21 or the physical width of the sixth microstrip line 25, w3 is the physical width of the third microstrip line 22, w4 is the physical width of the fourth microstrip line 23 or the physical width of the fifth microstrip line 24, and w5 is the physical width of the second open branch 32 or the physical width of the third open branch 33.

[0056] One set of typical size parameters is: l 1H =8.2mm,l 1V =2.05mm, l2=10.3mm, l3=5.8mm, l4=11.25mm, l5=4.35mm, w1=1.2mm, w2=0.1mm, w3=1.7mm, w4=0.85mm, w5=0.85mm.

[0057] Please also read Figure 5 , Figure 5This is a diagram of S parameter simulation results of the low-pass filter 1000 designed with the above parameters in the first state. The so-called first state refers to the case where the capacitance of the varactor diode 80 is 2.5pF when an appropriate voltage is applied. At this time, the impedance bandwidth of the filter with a reflection coefficient less than -10dB ranges from DC to 1.578GHz, and the maximum loss within the band is 0.51dB; the stopband with an isolation greater than 20dB ranges from 2.359GHz to 7GHz; there are two transmission poles in the passband, located at 0 and 1.401GHz respectively, to ensure flatness within the band; there are three transmission zeros in the stopband, located at 2.78GHz, 4.62GHz, and 6.78GHz respectively, to ensure high selectivity, wide stopband and high isolation.

[0058] Please also read Figure 6 , Figure 6 This is the S parameter simulation result diagram of the low-pass filter 1000 designed by the above parameters in the second state. The so-called second state refers to the case where the capacitance of the varactor diode 80 is 5.6pF when the appropriate voltage is applied. At this time, the impedance bandwidth of the filter with a reflection coefficient less than -10dB ranges from DC to 1.362GHz, and the maximum loss in the band is 0.48dB; the stopband with an isolation greater than 20dB ranges from 2.102GHz to 7GHz; there are two transmission poles in the passband, located at 0 and 1.18GHz, respectively, to ensure flatness in the band; there are three transmission zeros in the stopband, located at 2.419GHz, 4.64GHz, and 6.74GHz, respectively, to ensure high selectivity, wide stopband and high isolation.

[0059] Please also read Figure 7 , Figure 7 This is the S parameter simulation result diagram of the low-pass filter 1000 designed by the above parameters in the third state. The so-called third state refers to the case where the capacitance of the varactor diode 80 is 50pF when the appropriate voltage is applied. At this time, the impedance bandwidth of the filter with a reflection coefficient less than -10dB ranges from DC to 1.162GHz, and the maximum loss in the band is 0.48dB; the stopband with an isolation greater than 20dB ranges from 1.863GHz to 6.962GHz; there are two transmission poles in the passband, located at 0 and 0.96GHz, respectively, to ensure flatness in the band; there are three transmission zeros in the stopband, located at 2.12GHz, 4.658GHz, and 6.72GHz, respectively, to ensure high selectivity, wide stopband and high isolation.

[0060] Depend on Figure 5-7 It can be seen from the simulation results that the maximum operating frequency of the low-pass filter 1000 designed based on the topological structure 100 proposed in the present invention can be independently shifted down from 1.578 GHz to 1.162 GHz under the control of an independent DC power supply.

[0061] Please also read Figure 8 , Figure 8 is the S parameter simulation result of the low-pass filter 1000 designed by the above parameters under different working conditions. Figure 8 It can be seen that the bandwidth of the low-pass filter 1000 designed based on the topological structure 100 can be independently and continuously reconfigured under different working conditions, and has high selectivity, wide stopband and high isolation. It can be understood that the above parameters l 1H , l 1V , l2, l3, l4, l5, w1, w2, w3, w4 and w5 are not limited to the above values ​​and can be adjusted according to needs.

[0062] In the embodiment of the utility model, the low-pass filter 1000 includes a substrate and a topological structure 100, the topological structure 100 is arranged on the substrate, and the topological structure 100 includes an input end 10, an output end 70, a varactor 80, a first microstrip line 20, a second microstrip line 21, a third microstrip line 22, a fourth microstrip line 23, a fifth microstrip line 24, a sixth microstrip line 25, a seventh microstrip line 26, a first open branch 31, a second open branch 32, a third open branch 33 and a fourth open branch 34. The topological structure 100 of the embodiment of the utility model includes a varactor 80, seven microstrip lines and four open branches, and is used to design a bandwidth reconfigurable low-pass filter 1000 with high performance through a reasonable design layout, which has the advantage of miniaturization.

[0063] The utility model further provides an embodiment of a communication device, wherein the communication device comprises the above-mentioned low-pass filter 1000. The function and structure of the low-pass filter 1000 may refer to the above-mentioned embodiment, which will not be described in detail here.

[0064] It should be noted that the preferred embodiments of the utility model are given in the specification and drawings of the utility model, but the utility model can be implemented in many different forms and is not limited to the embodiments described in the specification. These embodiments are not used as additional restrictions on the content of the utility model. The purpose of providing these embodiments is to make the understanding of the disclosure of the utility model more thorough and comprehensive. In addition, the above-mentioned technical features continue to be combined with each other to form various embodiments not listed above, which are all regarded as the scope of the description of the utility model; further, for ordinary technicians in this field, they can be improved or transformed according to the above description, and all these improvements and transformations should belong to the scope of protection of the claims attached to the utility model.

Claims

1. A low-pass filter, characterized in that: include: substrate; A topological structure is arranged on the substrate, and the topological structure includes an input end, an output end, a varactor diode, a first microstrip line, a second microstrip line, a third microstrip line, a fourth microstrip line, a fifth microstrip line, a sixth microstrip line, a seventh microstrip line, a first open branch node, a second open branch node, a third open branch node, and a fourth open branch node; Among them, the first end of the first microstrip line and the first end of the second microstrip line are both connected to the input end, one end of the first open branch is connected to the second end of the first microstrip line, the first end of the sixth microstrip line and the first end of the seventh microstrip line are both connected to the output end, one end of the fourth open branch is connected to the second end of the seventh microstrip line, the second end of the second microstrip line and the second end of the sixth microstrip line are both connected to the first end of the third microstrip line, the second end of the third microstrip line is connected to the negative electrode of the varactor diode, the first end of the fourth microstrip line and the first end of the fifth microstrip line are both connected to the positive electrode of the varactor diode, one end of the second open branch is connected to the second end of the fourth microstrip line, and one end of the third open branch is connected to the second end of the fifth microstrip line.

2. The low-pass filter according to claim 1, characterized in that With the third microstrip line as the axis, the first microstrip line and the seventh microstrip line are symmetrically arranged, the second microstrip line and the sixth microstrip line are symmetrically arranged, the fourth microstrip line and the fifth microstrip line are symmetrically arranged, the first open branch and the fourth open branch are symmetrically arranged, and the second open branch and the third open branch are symmetrically arranged.

3. The low-pass filter according to claim 1, characterized in that The first microstrip line, the second microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line and the seventh microstrip line are parallel to each other, the first microstrip line, the second microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line and the seventh microstrip line are all perpendicular to the third microstrip line, and the third microstrip line, the first open branch, the second open branch, the third open branch and the fourth open branch are parallel to each other.

4. The low-pass filter according to claim 1, characterized in that The electrical length of the second microstrip line and the electrical length of the sixth microstrip line are both one-quarter wavelength corresponding to the stop band center frequency of the low-pass filter.

5. The low-pass filter 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 is a quarter wavelength corresponding to the center frequency of the stop band of the low-pass filter, and the sum of the electrical length of the seventh microstrip line and the electrical length of the fourth open branch is a quarter wavelength corresponding to the center frequency of the stop band of the low-pass filter.

6. The low-pass filter according to claim 1, characterized in that The electrical length of the third microstrip line is one eighth of the wavelength corresponding to the stop band center frequency of the low-pass filter.

7. The low-pass filter according to claim 1, characterized in that The sum of the electrical length of the fourth microstrip line and the electrical length of the second open branch is three-eighths of a wavelength corresponding to the center frequency of the stopband of the low-pass filter, and the sum of the electrical length of the fifth microstrip line and the electrical length of the third open branch is three-eighths of a wavelength corresponding to the center frequency of the stopband of the low-pass filter.

8. The low-pass filter according to any one of claims 1 to 7, characterized in that: The characteristic impedance of the first microstrip line, the characteristic impedance of the seventh microstrip line, the characteristic impedance of the first open-circuit branch, and the characteristic impedance of the fourth open-circuit branch are equal, and the characteristic impedance of the second microstrip line and the characteristic impedance of the sixth microstrip line are equal.

9. The low-pass filter according to any one of claims 1 to 7, characterized in that: The characteristic impedance of the fourth microstrip line, the characteristic impedance of the fifth microstrip line, the characteristic impedance of the second open-circuit branch, and the characteristic impedance of the third open-circuit branch are equal, and are all twice the characteristic impedance of the third microstrip line.

10. A communication device, characterized in that: Comprising a low-pass filter as claimed in any one of claims 1-9.

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