Electrically adjustable low-pass filter and communication equipment

Abstract

The embodiment of the invention relates to the technical field of filters, and particularly discloses an electrically adjustable low-pass filter and communication equipment. The electrically tunable low-pass filter includes a substrate and a topological structure. The topological structure comprises an input end, an output end, eight microstrip lines, six open circuit branches and a variable capacitance diode. Wherein the first microstrip line, the second microstrip line and the third microstrip line are all connected with the input end, the first open-circuit branch knot is connected with the first microstrip line, the second open-circuit branch knot is connected with the third microstrip line, the second microstrip line is connected with the seventh microstrip line and the fifth microstrip line, and the fifth microstrip line, the fourth microstrip line and the sixth microstrip line are all connected with the output end. The third open-circuit branch knot is connected with the fourth microstrip line, the fourth open-circuit branch knot is connected with the sixth microstrip line, the seventh microstrip line is electrically connected with the variable capacitance diode, the variable capacitance diode is connected with the eighth microstrip line, and the fifth open-circuit branch knot and the sixth open-circuit branch knot are both connected with the eighth microstrip line. Through the structural design, miniaturization of the electrically tunable filter is facilitated, and the use scene of the electrically tunable filter is enlarged.

Description

Technical Field

[0001] The embodiments of the present application relate to the field of filter technology, and in particular to an electrically adjustable low-pass filter and a communication device. 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 the feature of electrically adjustable bandwidth, so that the same filter can flexibly meet different indicators, it will help to simplify the design of the wireless communication system and greatly reduce its volume and weight.

[0003] In the process of realizing the present application, the inventors found that: currently, high-performance bandwidth electrically adjustable low-pass filters are mainly realized by loading multiple branches, multiple resonators and multiple varactor diodes, which often have the defect of large size, greatly limiting their use in modern wireless communication systems. Summary of the invention

[0004] The embodiments of the present application provide an electrically adjustable low-pass filter and a communication device, which can improve the current defect of the electrically adjustable low-pass filter being large in size.

[0005] In order to solve the above technical problems, a technical solution adopted by the present application is: to provide an electrically adjustable low-pass filter and a communication device. The electrically adjustable low-pass filter includes: a substrate and a topological structure. The topological structure includes an input end, an output end, 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, an eighth microstrip line, a first open branch, a second open branch, a third open branch, a fourth open branch, a fifth open branch, a sixth open branch and a varactor diode. Among them, the first end of the first microstrip line, the first end of the second microstrip line, and the first end of the third microstrip line are all connected to the input end, the first open branch is connected to the second end of the first microstrip line, the second open branch is connected to the second end of the third microstrip line, the second end of the second microstrip line is connected to the first end of the seventh microstrip line and the first end of the fifth microstrip line, the second end of the fifth microstrip line, the first end of the fourth microstrip line, and the first end of the sixth microstrip line are all connected to the output end, the third open branch is connected to the second end of the fourth microstrip line, the first end of the fourth open branch is connected to the second end of the sixth microstrip line, the second end of the seventh microstrip line is electrically connected to the first end of the varactor diode, the second end of the varactor diode is connected to the first end of the eighth microstrip line, and the fifth open branch and the sixth open branch are both connected to the second end of the eighth microstrip line.

[0006] In some embodiments, the first microstrip line, the second microstrip line, the third microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line, the fifth open branch and the sixth open branch are all arranged in parallel along the first direction. The seventh microstrip line, the first open branch, the second open branch, the third open branch and the fourth open branch are arranged in parallel along the second direction, and the seventh microstrip line, the varactor diode and the eighth microstrip line are collinear. The first direction is perpendicular to the second direction.

[0007] In some embodiments, the first microstrip line and the fourth microstrip line, the second microstrip line and the fifth microstrip line, the third microstrip line and the sixth microstrip line, the first open branch and the third open branch, the second open branch and the fourth open branch, the fifth open branch and the sixth open branch are all symmetrical about the central axis of the seventh microstrip line.

[0008] In some embodiments, the first open branch and the second open branch are symmetrical about the central axis of the second microstrip line, the first microstrip line and the third microstrip line are symmetrical about the central axis of the second microstrip line, the third open branch and the fourth open branch are symmetrical about the central axis of the fifth microstrip line, the fourth microstrip line and the sixth microstrip line are symmetrical about the central axis of the fifth microstrip line, and the fifth open branch and the sixth open branch are symmetrical about the central axis of the eighth microstrip line.

[0009] In some embodiments, the topology has an odd-mode form and an even-mode form.

[0010] In some embodiments, the electrical length of the second microstrip line, the electrical length of the fifth microstrip line, the electrical length of the fifth open-circuit branch, and the electrical length of the sixth open-circuit branch are all one-quarter wavelength corresponding to the stopband center frequency.

[0011] In some embodiments, 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. The sum of the electrical length of the third microstrip line and the electrical length of the second open branch is a quarter wavelength corresponding to the center frequency of the stop band. The sum of the electrical length of the fourth microstrip line and the electrical length of the third open branch is a quarter wavelength corresponding to the center frequency of the stop band. The sum of the electrical length of the sixth microstrip line and the electrical length of the fourth open branch is a quarter wavelength corresponding to the center frequency of the stop band.

[0012] In some embodiments, the electrical length of the seventh microstrip line and the electrical length of the eighth microstrip line are both one eighth of the wavelength corresponding to the center frequency of the stop band.

[0013] In some embodiments, the characteristic impedance of the first microstrip line, the characteristic impedance of the third microstrip line, the characteristic impedance of the fourth microstrip line, the characteristic impedance of the sixth microstrip line, the characteristic impedance of the first open branch, the characteristic impedance of the second open branch, the characteristic impedance of the third open branch, and the characteristic impedance of the fourth open branch are equal. The characteristic impedance of the second microstrip line is equal to the characteristic impedance of the fifth microstrip line. The characteristic impedance of the seventh microstrip line is equal to the characteristic impedance of the eighth microstrip line. The characteristic impedance of the fifth open branch is equal to the characteristic impedance of the sixth open branch. And the characteristic impedance of the first microstrip line, the characteristic impedance of the second microstrip line, the characteristic impedance of the seventh microstrip line, and the characteristic impedance of the fifth open branch are not equal to each other.

[0014] In order to solve the above technical problem, another technical solution adopted by the present application is: to provide a communication device, including the above-mentioned electrically adjustable low-pass filter.

[0015] The beneficial effect of the embodiment of the present application is that, different from the prior art, the embodiment of the present application provides an electrically adjustable low-pass filter. The electrically adjustable low-pass filter includes a substrate and a topological structure. The topological structure includes an input end, an output end, 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, an eighth microstrip line, a first open branch, a second open branch, a third open branch, a fourth open branch, a fifth open branch, a sixth open branch and a varactor diode. Among them, the first end of the first microstrip line, the first end of the second microstrip line, and the first end of the third microstrip line are all connected to the input end, the first open branch is connected to the second end of the first microstrip line, the second open branch is connected to the second end of the third microstrip line, the second end of the second microstrip line is connected to the first end of the seventh microstrip line and the first end of the fifth microstrip line, the second end of the fifth microstrip line, the first end of the fourth microstrip line, and the first end of the sixth microstrip line are all connected to the output end, the third open branch is connected to the second end of the fourth microstrip line, the first end of the fourth open branch is connected to the second end of the sixth microstrip line, the second end of the seventh microstrip line is electrically connected to the first end of the varactor diode, the second end of the varactor diode is connected to the first end of the eighth microstrip line, and the fifth open branch and the sixth open branch are both connected to the second end of the eighth microstrip line. Through the above structural design, the internal structure design of the topological structure is reasonable, which facilitates the miniaturization of the electrically adjustable low-pass filter and increases its use scenarios in current wireless communication systems. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0017] Figure 1 is a schematic diagram of a topological structure provided by one embodiment of the present application;

[0018] Figure 2 It is a schematic diagram of an odd-mode form of an equivalent topological structure provided by the present application;

[0019] Figure 3 It is an even-mode schematic diagram of the equivalent topological structure provided by the present application;

[0020] Figure 4 It is a layout diagram of the topological structure provided in the embodiment of the present application;

[0021] Figure 5It is a schematic diagram of the topological structure provided in the embodiment of the present application;

[0022] Figure 6 is the S parameter simulation result of the electrically adjustable low-pass filter provided in the embodiment of the present application in the State 1 state;

[0023] Figure 7 is the S parameter simulation result of the electrically adjustable low-pass filter provided in the embodiment of the present application in the State 2 state;

[0024] Figure 8 is the S of the electrically adjustable low-pass filter provided in the embodiment of the present application in different working states 21 Parameter simulation results.

[0025] The reference numerals are as follows:

[0026]

[0027] DETAILED DESCRIPTION

[0028] In order to facilitate the understanding of the present application, the present application 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.

[0029] Unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used in this specification and in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" used in this specification includes any and all combinations of one or more of the related listed items.

[0030] In order to make full use of spectrum resources and suppress out-of-band signals, highly selective and wide-stopband electrically adjustable low-pass filters are required in various microwave communication systems. However, existing high-performance bandwidth-reconfigurable electrically adjustable low-pass filters often have the defect of large size, which greatly limits their use in modern wireless communication systems. For example, there are a variety of existing design technologies for reconfigurable microstrip electrically adjustable 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 electrically adjustable low-pass filters designed by the above traditional methods is large, and more branches are loaded, which makes the electrically adjustable low-pass filters generally have the defect of large size.

[0031] Based on the above technical problems, the present application provides an electrically adjustable low-pass filter, which can be used in more scenarios by reducing the size of the electrically adjustable low-pass filter. The electrically adjustable low-pass filter includes: a substrate and a topological structure 100. The topological structure 100 includes an input end 11, an output end 12, a first microstrip line 21, a second microstrip line 22, a third microstrip line 23, a fourth microstrip line 24, a fifth microstrip line 25, a sixth microstrip line 26, a seventh microstrip line 27, an eighth microstrip line 28, a first open branch 31, a second open branch 32, a third open branch 33, a fourth open branch 34, a fifth open branch 35, a sixth open branch 36 and a varactor diode 37. Among them, the first end of the first microstrip line 21, the first end of the second microstrip line 22, and the first end of the third microstrip line 23 are all connected to the input end 11, the first open branch 31 is connected to the second end of the first microstrip line 21, the second open branch 32 is connected to the second end of the third microstrip line 23, the second end of the second microstrip line 22 is connected to the first end of the seventh microstrip line 27 and the first end of the fifth microstrip line 25, the second end of the fifth microstrip line 25, the first end of the fourth microstrip line 24, and the first end of the sixth microstrip line 26 are all connected to the output end 12, the third open branch 33 is connected to the second end of the fourth microstrip line 24, the first end of the fourth open branch 34 is connected to the second end of the sixth microstrip line 26, the second end of the seventh microstrip line 27 is electrically connected to the first end of the varactor diode 37, the second end of the varactor diode 37 is connected to the first end of the eighth microstrip line 28, and the fifth open branch 35 and the sixth open branch 36 are both connected to the second end of the eighth microstrip line 28. Through the above structural design, the internal structure design of the topological structure 100 is reasonable, which facilitates the miniaturization of the electrically adjustable low-pass filter and increases its application scenarios in current wireless communication systems.

[0032] In order to further improve the performance of the above-mentioned electrically adjustable low-pass filter, reduce the size of the electrically adjustable low-pass filter, and increase its application scenarios in wireless communication systems, the present application also makes the following design.

[0033] In some embodiments, the first microstrip line 21, the second microstrip line 22, the third microstrip line 23, the fourth microstrip line 24, the fifth microstrip line 25, the sixth microstrip line 26, the fifth open branch 35 and the sixth open branch 36 are all arranged in parallel along the first direction. The seventh microstrip line 27, the first open branch 31, the second open branch 32, the third open branch 33, and the fourth open branch 34 are arranged in parallel along the second direction, and the seventh microstrip line 27, the varactor diode 37 and the eighth microstrip line 28 are collinear. The first direction is perpendicular to the second direction. It is still necessary to supplement that the second microstrip line 22 and the fifth microstrip line 25 are collinear, the first microstrip line 21 and the fourth microstrip line 24 are collinear, and the third microstrip line 23 and the sixth microstrip line 26 are collinear. In the first direction, the space on both sides of the second microstrip line 22 and the fifth microstrip line 25 is fully utilized to connect different microstrip lines respectively; and in the second direction, the seventh microstrip line 27, the varactor diode 37 and the eighth microstrip line 28 are connected in a colinear manner in sequence to extend the structure in the second direction, so as to provide the connection space of the fifth open-circuit branch 35 and the sixth open-circuit branch 36, thereby reducing the overall occupied space of the topological structure 100, so as to facilitate the miniaturization design of the electrically adjustable low-pass filter. It is still necessary to explain that the cathode of the varactor diode 37 is connected to the seventh microstrip line 27, and the anode of the varactor diode 37 is connected to the eighth microstrip line 28. The varactor diode 37 (Varactor Diodes) is also called "variable reactance diode", which is made by utilizing the characteristic that the size of the junction capacitance changes with the applied voltage when the PN junction is reverse biased. When the reverse bias voltage increases, the junction capacitance decreases, and vice versa.

[0034] In some embodiments, the first microstrip line 21 and the fourth microstrip line 24, the second microstrip line 22 and the fifth microstrip line 25, the third microstrip line 23 and the sixth microstrip line 26, the first open branch 31 and the third open branch 33, the second open branch 32 and the fourth open branch 34, the fifth open branch 35 and the sixth open branch 36 are all symmetrical about the central axis of the seventh microstrip line 27. Through the above symmetrical design, the space of the topological structure 100 is fully utilized, making the topological structure 100 compact, thereby reducing the size of the electrically adjustable low-pass filter.

[0035] In some embodiments, the first open branch 31 and the second open branch 32 are symmetrical about the central axis of the second microstrip line 22, the first microstrip line 21 and the third microstrip line 23 are symmetrical about the central axis of the second microstrip line 22, the third open branch 33 and the fourth open branch 34 are symmetrical about the central axis of the fifth microstrip line 25, the fourth microstrip line 24 and the sixth microstrip line 26 are symmetrical about the central axis of the fifth microstrip line 25, and the fifth open branch 35 and the sixth open branch 36 are symmetrical about the central axis of the eighth microstrip line 28. Through the above symmetrical design, the space of the topological structure 100 is fully utilized, making the topological structure 100 compact, thereby reducing the size of the electrically adjustable low-pass filter.

[0036] In some embodiments, the topological structure 100 has an odd-mode form and an even-mode form. Since the design structure includes a varactor diode 37, the topological structure 100 has two working modes, and its specific working mode will be further described below.

[0037] In some embodiments, the electrical length of the second microstrip line 22, the electrical length of the fifth microstrip line 25, the electrical length of the fifth open-circuit branch 35, and the electrical length of the sixth open-circuit branch 36 are all one-quarter wavelength corresponding to the stopband center frequency, thereby simplifying the design of the electrically tunable low-pass filter.

[0038] In some embodiments, the sum of the electrical length of the first microstrip line 21 and the electrical length of the first open branch 31 is a quarter wavelength corresponding to the center frequency of the stop band. The sum of the electrical length of the third microstrip line 23 and the electrical length of the second open branch 32 is a quarter wavelength corresponding to the center frequency of the stop band. The sum of the electrical length of the fourth microstrip line 24 and the electrical length of the third open branch 33 is a quarter wavelength corresponding to the center frequency of the stop band. The sum of the electrical length of the sixth microstrip line 26 and the electrical length of the fourth open branch 34 is a quarter wavelength corresponding to the center frequency of the stop band. In this way, the design of the electrically adjustable low-pass filter is simplified.

[0039] In some embodiments, the electrical length of the seventh microstrip line 27 and the electrical length of the eighth microstrip line 28 are both one eighth of the wavelength corresponding to the stopband center frequency, so as to simplify the design of the electrically tunable low-pass filter.

[0040] In some embodiments, the characteristic impedance of the first microstrip line 21, the characteristic impedance of the third microstrip line 23, the characteristic impedance of the fourth microstrip line 24, the characteristic impedance of the sixth microstrip line 26, the characteristic impedance of the first open branch 31, the characteristic impedance of the second open branch 32, the characteristic impedance of the third open branch 33, and the characteristic impedance of the fourth open branch 34 are equal. The characteristic impedance of the second microstrip line 22 is equal to the characteristic impedance of the fifth microstrip line 25. The characteristic impedance of the seventh microstrip line 27 is equal to the characteristic impedance of the eighth microstrip line 28. The characteristic impedance of the fifth open branch 35 is equal to the characteristic impedance of the sixth open branch 36. And the characteristic impedance of the first microstrip line 21, the characteristic impedance of the second microstrip line 22, the characteristic impedance of the seventh microstrip line 27, and the characteristic impedance of the fifth open branch 35 are not equal to each other. In this way, the design of the electrically adjustable low-pass filter is simplified.

[0041] Based on the above embodiments and one or more combinations of the embodiments, the odd-mode form and even-mode form of the topological structure 100 are further described. When the capacitance of the varactor diode 37 is infinite, the varactor diode 37 has no effect on the performance of the filter designed based on the topological structure 100. At this time, the varactor diode 37 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. For the odd-mode form of the topological structure 100, please refer to Figure 2 For the even mode form of topology 100, see Figure 3 When Y ino =∞, it can be concluded that the topology 100 has two odd-mode transmission poles, and their corresponding frequencies are:

[0042] f op1 = 0, and

[0043] f op2 =f 0 ;

[0044] Here, f 0 is the center frequency of the band-stop filter.

[0045] Here, the electrical length of the ninth microstrip line and the electrical length of the tenth microstrip line are both one-eighth wavelength corresponding to the stopband center frequency; the characteristic impedance of the ninth microstrip line and the characteristic impedance of the tenth microstrip line are both 2Z3. When Yi ne=∞, it can be concluded that the topological structure 100 has an even-mode transmission pole, and its corresponding frequency is

[0046]

[0047] It should be noted that the characteristic impedance of the first microstrip line 21, the characteristic impedance of the third microstrip line 23, the characteristic impedance of the fourth microstrip line 24, the characteristic impedance of the sixth microstrip line 26, the characteristic impedance of the first open branch 31, the characteristic impedance of the second open branch 32, the characteristic impedance of the third open branch 33, and the characteristic impedance of the fourth open branch 34 are equal, and are all Z 1 The characteristic impedance of the second microstrip line 22 is equal to the characteristic impedance of the fifth microstrip line 25, both of which are Z 2 The characteristic impedance of the seventh microstrip line 27 and the characteristic impedance of the eighth microstrip line 28 are equal and are both Z 3 The characteristic impedance of the fifth open branch 35 and the characteristic impedance of the sixth open branch 36 are equal, both are Z 4 .

[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 =f 0

[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. 1 , Z 2 , Z 3 , Z 4 How does the value of change, the relative position of these transmission zero poles, that is, 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 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 an electrically adjustable 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.

[0053] In addition, further research on the equivalent topological structure 100 shows that, for the electrically adjustable low-pass filter designed based on the topological structure 100, when the capacitance of the varactor diode 37 is reduced, the maximum operating frequency of the passband will move upward, but the maximum operating frequency of the stopband will remain almost unchanged. Therefore, by controlling the capacitance of the varactor diode 37 through an independent DC power supply, the bandwidth of the electrically adjustable low-pass filter 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 4 and Figure 5 The layout diagram of the bandwidth reconfigurable filter designed based on the topology 100 is shown in FIG. Figure 4 It can be seen that the varactor diode 37D V An independent power supply and DC grounding hole are required to form the required DC voltage drop, so as to control its capacitance. Among them, R1 is the protection varactor diode 37D V resistance; L1 and L2 are RF blocking inductors 70, the purpose of which is to prevent the power circuit and the grounding circuit from participating in the operation of the RF circuit; the first pad 40 is a pad for protecting the resistor and the RF blocking inductor 70; the second pad 50 is a pad for an independent power supply, which is used to introduce the voltage of a DC power supply; the DC grounding hole 60 is used to realize the grounding function.

[0055] The electrically adjustable low-pass filter is designed on a circuit board with a dielectric loss of 0.0022 and a thickness of 0.813 mm, and the size of the circuit board is 23.9 mm*15.8 mm. Figure 5 A typical set of size parameters is:

[0056] l 1H =8.1mm,l 1V =1.8mm,l 2 =9.3mm,l 3 =5.05mm,l 4 =5.75mm,l 5 =9.5mm,w 1 =0

[0057] .4mm,w 2 =0.1mm,w 3 =1.6mm, w 4 =1.7mm, w 5 =1.7mm,s 1 =0.5mm.

[0058] It is understandable that the above parameter l 1H , l 1V , l 2 , l 3 , l 4, l 5 、w 1 、w 2 、w 3 、w 4 、w 5 and 1 It is not limited to the above values ​​and can be adjusted according to needs.

[0059] Fig. 6 shows the S parameter simulation results of the bandpass filter in State 1. Among them, "State 1" means that the capacitance of the varactor diode 37 is 1.8pF 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 1.739GHz, and the maximum loss in the band is 0.5dB; the stopband with an isolation greater than 20dB ranges from 2.566GHz to 8.0GHz; there are two transmission poles in the passband, located at 0 and 1.56GHz, respectively, to ensure flatness in the band; there are three transmission zeros in the stopband, located at 2.979GHz, 5.958GHz, and 7.937GHz, respectively, to ensure high selectivity, wide stopband, and high isolation.

[0060] Please further combine Figure 7 , where "State 2" means that the capacitance of the varactor diode 37 is 95pF 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 1.08GHz, and the maximum loss in the band is 0.488dB; the stopband with an isolation greater than 20dB ranges from 1.787GHz to 8.0GHz; there are two transmission poles in the passband, located at 0 and 0.799GHz, respectively, to ensure flatness in the band; there are three transmission zeros in the stopband, located at 1.999GHz, 5.064GHz, and 7.879GHz, respectively, to ensure high selectivity, wide stopband, and high isolation.

[0061] Depend on Figures 6 to 7 It can be seen from the simulation results that the electrically tunable low-pass filter designed based on the topology structure 100 proposed in the present invention can have its maximum operating frequency continuously shifted down from 1.739 GHz to 1.08 GHz under the control of an independent DC power supply.

[0062] Depend on Figure 8 It can be seen that the electrically tunable low-pass filter designed based on the topological structure 100 has a bandwidth that can be independently and continuously reconfigured under different working conditions, and has high selectivity, wide stopband and high isolation.

[0063] In an embodiment of the present application, an electrically adjustable low-pass filter is provided. The electrically adjustable low-pass filter includes a substrate and a topological structure 100. The topological structure 100 includes an input terminal 11, an output terminal 12, a first microstrip line 21, a second microstrip line 22, a third microstrip line 23, a fourth microstrip line 24, a fifth microstrip line 25, a sixth microstrip line 26, a seventh microstrip line 27, an eighth microstrip line 28, a first open branch 31, a second open branch 32, a third open branch 33, a fourth open branch 34, a fifth open branch 35, a sixth open branch 36 and a varactor diode 37. Among them, the first end of the first microstrip line 21, the first end of the second microstrip line 22, and the first end of the third microstrip line 23 are all connected to the input end 11, the first open branch 31 is connected to the second end of the first microstrip line 21, the second open branch 32 is connected to the second end of the third microstrip line 23, the second end of the second microstrip line 22 is connected to the first end of the seventh microstrip line 27 and the first end of the fifth microstrip line 25, the second end of the fifth microstrip line 25, the first end of the fourth microstrip line 24, and the first end of the sixth microstrip line 26 are all connected to the output end 12, the third open branch 33 is connected to the second end of the fourth microstrip line 24, the first end of the fourth open branch 34 is connected to the second end of the sixth microstrip line 26, the second end of the seventh microstrip line 27 is electrically connected to the first end of the varactor diode 37, the second end of the varactor diode 37 is connected to the first end of the eighth microstrip line 28, and the fifth open branch 35 and the sixth open branch 36 are both connected to the second end of the eighth microstrip line 28. Through the above structural design, the internal structure design of the topological structure 100 is reasonable, which facilitates the miniaturization of the electrically adjustable low-pass filter and increases its application scenarios in current wireless communication systems.

[0064] Based on the same inventive concept, the present application also provides a communication device embodiment, the communication device includes the above-mentioned electrically adjustable low-pass filter. The structure and function of the electrically adjustable low-pass filter can be referred to the above-mentioned embodiment, which will not be described one by one here.

[0065] It should be noted that the preferred embodiments of the present application are given in the specification and drawings of the present application. However, the present application 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 limitations on the content of the present application. The purpose of providing these embodiments is to make the understanding of the disclosure of the present application 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 present application; 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 present application.

Claims

1. An electrically tunable low-pass filter, characterized in that: include: substrate; A topological structure, comprising an input end, an output end, 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, an eighth microstrip line, a first open branch node, a second open branch node, a third open branch node, a fourth open branch node, a fifth open branch node, a sixth open branch node and a varactor diode; Among them, the first end of the first microstrip line, the first end of the second microstrip line, and the first end of the third microstrip line are all connected to the input end, the first open branch is connected to the second end of the first microstrip line, the second open branch is connected to the second end of the third microstrip line, the second end of the second microstrip line is connected to the first end of the seventh microstrip line and the first end of the fifth microstrip line, the second end of the fifth microstrip line, the first end of the fourth microstrip line, and the first end of the sixth microstrip line are all connected to the output end, the third open branch is connected to the second end of the fourth microstrip line, the first end of the fourth open branch is connected to the second end of the sixth microstrip line, the second end of the seventh microstrip line is electrically connected to the first end of the varactor diode, the second end of the varactor diode is connected to the first end of the eighth microstrip line, and the fifth open branch and the sixth open branch are both connected to the second end of the eighth microstrip line.

2. The electrically tunable low-pass filter according to claim 1, characterized in that: The first microstrip line, the second microstrip line, the third microstrip line, the fourth microstrip line, the fifth microstrip line, the sixth microstrip line, the fifth open-circuit branch node and the sixth open-circuit branch node are all arranged in parallel along the first direction; The seventh microstrip line, the first open branch, the second open branch, the third open branch, and the fourth open branch are arranged in parallel along the second direction, and the seventh microstrip line, the varactor diode, and the eighth microstrip line are collinear; The first direction and the second direction are perpendicular.

3. The electrically tunable low-pass filter according to claim 2, characterized in that: The first microstrip line and the fourth microstrip line, the second microstrip line and the fifth microstrip line, the third microstrip line and the sixth microstrip line, the first open branch and the third open branch, the second open branch and the fourth open branch, the fifth open branch and the sixth open branch are all symmetrical about the central axis of the seventh microstrip line.

4. The electrically tunable low-pass filter according to claim 3, characterized in that: The first open branch and the second open branch are symmetrical about the central axis of the second microstrip line, the first microstrip line and the third microstrip line are symmetrical about the central axis of the second microstrip line, the third open branch and the fourth open branch are symmetrical about the central axis of the fifth microstrip line, the fourth microstrip line and the sixth microstrip line are symmetrical about the central axis of the fifth microstrip line, and the fifth open branch and the sixth open branch are symmetrical about the central axis of the eighth microstrip line.

5. The electrically tunable low-pass filter according to claim 1, characterized in that: The topology has an odd-mode form and an even-mode form.

6. The electrically tunable low-pass filter according to any one of claims 1 to 5, characterized in that: The electrical length of the second microstrip line, the electrical length of the fifth microstrip line, the electrical length of the fifth open-circuit branch, and the electrical length of the sixth open-circuit branch are all one-quarter wavelength corresponding to the stopband center frequency.

7. The electrically tunable low-pass filter according to claim 6, characterized in that: The sum of the electrical length of the first microstrip line and the electrical length of the first open-circuit branch is a quarter wavelength corresponding to the stopband center frequency; The sum of the electrical length of the third microstrip line and the electrical length of the second open-circuit branch is a quarter wavelength corresponding to the stopband center frequency; The sum of the electrical length of the fourth microstrip line and the electrical length of the third open-circuit branch is a quarter wavelength corresponding to the stopband center frequency; The sum of the electrical length of the sixth microstrip line and the electrical length of the fourth open-circuit branch is a quarter wavelength corresponding to the center frequency of the stop band.

8. The electrically tunable low-pass filter according to claim 7, characterized in that: The electrical length of the seventh microstrip line and the electrical length of the eighth microstrip line are both one eighth of the wavelength corresponding to the center frequency of the stop band.

9. The electrically tunable low-pass filter according to any one of claims 1 to 5, characterized in that: The characteristic impedance of the first microstrip line, the characteristic impedance of the third microstrip line, the characteristic impedance of the fourth microstrip line, the characteristic impedance of the sixth microstrip line, the characteristic impedance of the first open-circuit branch, the characteristic impedance of the second open-circuit branch, the characteristic impedance of the third open-circuit branch, and the characteristic impedance of the fourth open-circuit branch are equal; The characteristic impedance of the second microstrip line is equal to the characteristic impedance of the fifth microstrip line; The characteristic impedance of the seventh microstrip line is equal to the characteristic impedance of the eighth microstrip line; The characteristic impedance of the fifth open-circuit branch is equal to the characteristic impedance of the sixth open-circuit branch; Furthermore, the characteristic impedance of the first microstrip line, the characteristic impedance of the second microstrip line, the characteristic impedance of the seventh microstrip line, and the characteristic impedance of the fifth open-circuit branch are not equal to each other.

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

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