A reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity

Abstract

The present invention discloses a reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity, comprising: a resonator-loaded open-ended coupled line, absorption stubs, and a 50-ohm transmission line; the resonator-loaded open-ended coupled line includes a first parallel coupled line, a second parallel coupled line, and an open-ended parallel circuit; the first parallel coupled line includes a first coupled line and a second coupled line; the second parallel coupled line includes a third coupled line and a fourth coupled line; the open-ended parallel circuit includes a first open line and a first transmission line; the absorption stubs include a first absorption stub and a second absorption stub; the filter has a relatively high passband flatness and has a reflectionless characteristic throughout the entire test frequency band (0-4 GHz), that is, the bandwidth of the filter with a return loss less than 10 dB is 0-4 GHz.

Description

Technical Field

[0001] The present invention relates to the field of microwave passive devices, and particularly to a non-reflective broadband bandpass filter. Background Art

[0002] Filters are an essential part of radio frequency and microwave systems. The reflected wave energy of the stopband signal of traditional filters will be reflected back to the source, overlapping with the existing signals to generate many interference signals, which affects the performance of the entire filter. To solve this problem, generally, a non-reciprocal device (such as an isolator) can be placed in front of the filter, but this will reduce the sensitivity of the filter, increase the cost, and increase the volume of the entire system. The emergence of non-reflective filters solves the above problems. In the stopband of non-reflective filters, the untransmitted energy will not return to the source but is consumed by lossy devices. Therefore, the port impedance can be well matched both inside and outside the band.

[0003] With the continuous development of modern wireless communication technologies, the technologies for implementing non-reflective filters or quasi-non-reflective filters by various methods have gradually become research hotspots. Currently, the main structures for implementing non-reflective filters are: odd-even mode circuit structures, balanced circuit structures, and complementary duplex structures. Most odd-even mode circuit structures are implemented using lumped elements and are not suitable for high-frequency circuits; balanced circuit structures generally consist of two similar broadband 3dB quadrature couplers and two identical reflective BPFs, with large circuit sizes and difficult to optimize; complementary duplex structures consist of a "main" bandpass channel and a "sub" bandstop channel, which can achieve good non-reflective characteristics both inside and outside the band. However, currently, non-reflective filters with complementary duplex structures generally have problems such as poor in-band flatness and poor frequency selectivity. Summary of the Invention

[0004] According to the problems existing in the prior art, the present invention discloses a non-reflective broadband bandpass filter with high in-band flatness and high frequency selectivity, including: a resonator-loaded open-ended coupled line, absorbing stubs, and 50-ohm transmission lines;

[0005] The resonator-loaded open-ended coupled line includes a first parallel coupled line, a second parallel coupled line, and an open-ended parallel circuit; the first parallel coupled line includes a first coupled line and a second coupled line; the second parallel coupled line includes a third coupled line and a fourth coupled line; the open-ended parallel circuit includes a first open line and a first transmission line; the absorbing stubs include a first absorbing stub and a second absorbing stub; the first absorbing stub includes a second transmission line, a second open line, a first resistor, a third open line, and a third transmission line; the second absorbing stub includes a fourth transmission line, a fourth open line, a second resistor, a fifth open line, and a fifth transmission line; the 50-ohm transmission lines include a first 50-ohm transmission line and a second 50-ohm transmission line;

[0006] The first 50-ohm transmission line is connected to the left end of the first coupling line, and the second transmission line is connected in parallel with the first 50-Ω transmission line and the first coupling line; the second transmission line is connected to the first resistor, and the second open circuit line is connected in parallel with the second transmission line and the first resistor; the first resistor is connected to the third transmission line, and the third open circuit line is connected in parallel with the first resistor and the third transmission line, and the other end of the third transmission line is grounded; the right end of the second coupling line is connected to the left end of the third coupling line, the first transmission line is connected in parallel with the second coupling line and the fourth coupling line, and the lower end of the first transmission line is connected to the first open circuit line; the right end of the third coupling line is connected to the second 50-ohm transmission line, and the fourth transmission line is connected in parallel with the right end of the third coupling line and the second 50-ohm transmission line; the fourth transmission line is connected to the second resistor, and the fourth open circuit line is connected in parallel with the fourth transmission line and the second resistor; the second resistor is connected to the fifth transmission line, and the fifth open circuit line is connected in parallel with the second resistor and the fifth transmission line; the other end of the fifth transmission line is grounded.

[0007] The electrical lengths and characteristic impedances of the first parallel coupling line and the second parallel coupling line are the same, and the electrical length is 90°; the electrical lengths of the first open circuit line, the first transmission line, the second transmission line, the second open circuit line, the third open circuit line, the third transmission line, the fourth transmission line, the fourth open circuit line, the fifth open circuit line, and the fifth transmission line are all 90°.

[0008] The return loss S 11 and insertion loss S 21 of this filter are expressed as follows:

[0009]

[0010]

[0011] where Γ o is the reflection coefficient under odd-mode excitation, Γ e is the reflection coefficient under even-mode excitation. Under odd-mode excitation, the calculation formula for the reflection coefficient Γ o is:

[0012]

[0013] where Z in1 is the input characteristic impedance of the input port under odd-mode excitation, and the expression of Z in1 is:

[0014]

[0015] In the above formula, Z ino is the characteristic impedance of the port of the first parallel coupling line under odd-mode excitation, Z sub is the characteristic impedance of the absorbing stub port, Zino and Z sub The expressions are as follows:

[0016]

[0017]

[0018] where Z c is the port characteristic impedance seen from the lower end of the second transmission line. The expression of Z c is as follows:

[0019]

[0020] In the case of even-mode excitation, the calculation formula of the reflection coefficient Γ e is as follows:

[0021]

[0022] where Z in2 is the characteristic impedance of the input port under even-mode excitation. The expression of Z in2 is as follows:

[0023]

[0024] where Z ine is the characteristic impedance of the port of the first parallel-coupled line under even-mode excitation, Z sub is the characteristic impedance of the absorption stub port, Z ine and Z sub The expressions are as follows:

[0025]

[0026]

[0027] where Z d is the characteristic impedance seen from the port of the open-circuited shunt circuit at the end in the even-mode circuit, Z c is the port characteristic impedance seen from the lower end of the second transmission line, Z d and Z c The expressions are as follows:

[0028]

[0029]

[0030] Where Z1 is the characteristic impedance of the first open circuit line, Z2 is the characteristic impedance of the first transmission line, Z3 is the characteristic impedance of the second transmission line and the fourth transmission line, Z4 is the characteristic impedance of the second open circuit line and the fourth open circuit line, R is the resistance value of the first resistor and the second resistor, Z5 is the characteristic impedance of the third open circuit line and the fifth open circuit line, Z6 is the characteristic impedance of the third transmission line and the fifth transmission line, Z o and Z e are respectively the odd-mode characteristic impedance and the even-mode characteristic impedance of the resonator-loaded open-ended coupled line, Z0 is the characteristic impedance of the 50-ohm transmission line, and θ is the electrical length of the first parallel coupled line, the second parallel coupled line, the first transmission line, the first open circuit line, the second transmission line, the second open circuit line, the third open circuit line, the third transmission line, the fourth transmission line, the fourth open circuit line, the fifth open circuit line and the fifth transmission line.

[0031] The reflectionless characteristics in the passband and out-of-band are controlled by adjusting the impedances of the transmission lines in each section of the absorption stub and the resistance values of the first resistor and the second resistor; the in-band flatness and frequency selectivity are regulated by adjusting the characteristic impedances of the first transmission line and the first open circuit line.

[0032] The characteristic impedances of the second transmission line, the fourth transmission line, the third open circuit line and the fifth open circuit line are all greater than 100Ω; the characteristic impedances of the first open circuit line, the second open circuit line and the fourth open circuit line are all less than 50Ω.

[0033] Due to the adoption of the above technical solution, the present invention provides a reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity. This filter has reflectionless characteristics throughout the entire test frequency band (0 - 4 GHz), that is, the bandwidth with a return loss less than 10 dB of this filter is 0 - 4 GHz. In addition, the 3-dB relative bandwidth in the passband of this filter is 102% (1.01 - 3.05 GHz), having broadband filtering characteristics. At the same time, this filter also has high frequency selectivity, and its rectangularity coefficient K 30dB is 1.09. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present application 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 drawings described below are only some embodiments recorded in the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0035] Figure 1 is a schematic structural diagram of the reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity described in the present invention.

[0036] Figure 2It is the odd-mode equivalent circuit diagram of the reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity described in the present invention.

[0037] Figure 3 It is the even-mode equivalent circuit diagram of the reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity described in the present invention.

[0038] Figure 4 It is the S-parameter curve of the reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity described in the present invention.

[0039] In the figure: 1. Resonator-loaded terminated open-circuit coupled line, 11. First parallel coupled line, 12. Second parallel coupled line, 13. Terminated open-circuit shunt circuit, 111. First coupled line, 112. Second coupled line, 121. Third coupled line, 122. Fourth coupled line, 131. First open-circuit line, 132. First transmission line, 2. Absorbing stub, 21. First absorbing stub, 22. Second absorbing stub, 211. Second transmission line, 212. Second open-circuit line, 213. First resistor, 214. Third open-circuit line, 215. Third transmission line, 221. Fourth transmission line, 222. Fourth open-circuit line, 223. Second resistor, 224. Fifth open-circuit line, 225. Fifth transmission line, 3. 50-ohm transmission line, 31. First 50-ohm transmission line, 32. Second 50-ohm transmission line. Detailed implementation manners

[0040] To make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific implementation manners and with reference to the accompanying drawings. It should be understood that these descriptions are merely exemplary and are not intended to limit the scope of the present invention. In addition, in the following descriptions, the descriptions of well-known structures and technologies are omitted to avoid unnecessarily obscuring the concepts of the present invention.

[0041] Figure 1 It is the structural schematic diagram of a reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity described in the present invention. The reflectionless bandpass filter in this example may include: a resonator-loaded terminated open-circuit coupled line 1, an absorbing stub 2, and a 50-ohm transmission line 3;

[0042] Furthermore, the resonator-loaded open-ended coupled line 1 includes a first parallel coupled line 11, a second parallel coupled line 12, and an open-ended shunt circuit 13; the first parallel coupled line 11 includes a first coupled line 111 and a second coupled line 112; the second parallel coupled line 12 includes a third coupled line 121 and a fourth coupled line 122; the open-ended shunt circuit 13 includes a first transmission line 132 and a first open-circuited line 131; the absorbing stub 2 includes a first absorbing stub 21 and a second absorbing stub 22; the first absorbing stub 21 includes a second transmission line 211, a second open-circuited line 212, a first resistor 213, a third open-circuited line 214, and a third transmission line 215; the second absorbing stub 22 includes a fourth transmission line 221, a fourth open-circuited line 222, a second resistor 223, a fifth open-circuited line 224, and a fifth transmission line 225; the 50-ohm transmission line 3 includes a first 50-ohm transmission line 31 and a second 50-ohm transmission line 32.

[0043] Furthermore, the left end of the first 50-ohm transmission line 31 is connected to the left end of the first coupled line 111, and the second transmission line 211 is connected in parallel with the first 50-Ω transmission line 31 and the first coupled line 111; the second transmission line 211 is connected to the first resistor 213, and the second open-circuited line 212 is arranged in parallel with the second transmission line 211 and the first resistor 213; the first resistor 213 is connected to the third transmission line 215, and the third open-circuited line 214 is connected in parallel with the first resistor 213 and the third transmission line 215, and the third transmission line 215 is connected to the ground; the right end of the second coupled line 112 is connected to the left end of the third coupled line 122, the first transmission line 132 is arranged in parallel with the second coupled line 112 and the fourth coupled line 122, and the lower end of the first transmission line 132 is connected to the first open-circuited line 131; the right end of the third coupled line 121 is connected to the second 50-ohm transmission line 32, and the fourth transmission line 221 is arranged in parallel with the right end of the third coupled line 121 and the second 50-ohm transmission line 32; the fourth transmission line 221 is connected to the second resistor 223, and the fourth open-circuited line 222 is arranged in parallel with the fourth transmission line 221 and the second resistor 223; the second resistor 223 is connected to the fifth transmission line 225, and the fifth open-circuited line 224 is arranged in parallel with the second resistor 223 and the fifth transmission line 225; the other end of the fifth transmission line 225 is grounded.

[0044] Furthermore, the first parallel coupled line 11 and the second parallel coupled line 12 have the same electrical length and characteristic impedance, where the electrical length is 90°; the electrical lengths of the first open-circuited line 131, the first transmission line 132, the second transmission line 211, the second open-circuited line 212, the third open-circuited line 214, the third transmission line 215, the fourth transmission line 221, the fourth open-circuited line 222, the fifth open-circuited line 224, and the fifth transmission line 225 are all the same, and the electrical length is 90°.

[0045] Specifically, in this example, the reflectionless characteristics of the input and output ports can be achieved. The two-port network is analyzed using the odd-even mode analysis method to calculate the circuit parameters.

[0046] Under odd-mode excitation, the voltage on the symmetry plane of the reflectionless filter is zero, equivalent to a short circuit. Figure 2 (a) shows the odd-mode equivalent circuit of the reflectionless bandpass filter. Z ino is the odd-mode characteristic impedance of the first parallel-coupled line 11), Z sub is the input characteristic impedance of the first absorption stub 21, Z in1 is the characteristic impedance of the input port, θ is the electrical length of the second transmission line 211, the second open circuit line 212, the third open circuit line 214, the third transmission line 215, and the first parallel-coupled line 11.

[0047] Under even-mode excitation, the current on the symmetry plane of the reflectionless filter is zero, equivalent to an open circuit. Figure 2 (b) shows the even-mode equivalent circuit of the reflectionless bandpass filter. Z d is the even-mode characteristic impedance of the terminal open-circuit parallel circuit 3, Z ine is the even-mode characteristic impedance of the first parallel-coupled line 11, Z sub is the input characteristic impedance of the first absorption stub 21, Z in2 is the characteristic impedance of the input port, θ is the electrical length of the second transmission line 211, the second open circuit line 212, the third open circuit line 214, the third transmission line 215, and the first parallel-coupled line 11, and the electrical length in the even-mode case of the first transmission line 132 and the first open circuit line 131.

[0048] According to Figure 2 and Figure 3 of the two-port network, the odd-even mode circuit analysis of the reflectionless broadband bandpass filter is carried out, and the relevant parameters are solved. The design formula of the broadband bandpass filter with reflectionless characteristics of the present invention can be obtained. The solution steps are as follows:

[0049] Step 1: According to the odd-even mode analysis method, the reflectionless bandpass filter is decomposed into odd-mode and even-mode circuits for analysis.

[0050] Step 2: Under odd-mode excitation, the characteristic impedance of each node of the odd-mode circuit is solved. The odd-mode characteristic impedance Z of the first parallel-coupled line 11 ino The solution process is as follows:

[0051]

[0052] where I1 = I3 = 0, V4 = 0, and there are:

[0053]

[0054] Obtain the odd - mode characteristic impedance \(Z\) of the first - stage parallel coupled line 11 ino :

[0055]

[0056] In addition, the input characteristic impedance \(Z\) of the absorption stub 2 in the odd - mode and even - mode circuits sub is the same, and the solution process is as follows:

[0057]

[0058]

[0059]

[0060] Furthermore, obtain the port input impedance \(Z\) under odd - mode excitation in1 The expression is:

[0061]

[0062] Furthermore, obtain the reflection coefficient \(\Gamma\) under odd - mode excitation o is:

[0063]

[0064] Step 3: Under even - mode excitation, solve the characteristic impedance of each fulcrum of the even - mode circuit. The even - mode characteristic impedance \(Z\) of the open - circuit parallel circuit 3 at the terminal d The expression is:

[0065]

[0066] The even - mode characteristic impedance \(Z\) of the first - stage parallel coupled line 11 ine The solution process is as follows:

[0067]

[0068] where \(I_1 = I_3=0\) and there is

[0069]

[0070] Obtain the even - mode characteristic impedance of the first - stage parallel coupled line 11:

[0071]

[0072] The characteristic impedance \(Z\) of the absorption stub port sub is the same as that of the odd - mode circuit, and the expression is:

[0073] Furthermore, obtain the port input impedance \(Z\) under even - mode excitation in2The expression is:

[0074]

[0075] Furthermore, the reflection coefficient Γ under even-mode excitation can be obtained. e It is:

[0076]

[0077] Step 4: According to the reflection coefficients of the odd- and even-mode circuit ports obtained in Step 2 and Step 3, the expressions for the return loss (S 11 ) and insertion loss (S 21 ) of the entire reflectionless broadband bandpass filter can be obtained:

[0078]

[0079]

[0080] In a specific embodiment of the present invention, the center frequency of the reflectionless broadband bandpass filter with flat in-band response and high frequency selectivity is 2.0 GHz, as Figure 4 shown. The filter has a relatively flat passband and exhibits reflectionless characteristics throughout the entire test frequency band (0 - 4 GHz), that is, the bandwidth with a return loss less than 10 dB is 0 - 4 GHz. In addition, the 3-dB relative bandwidth within the filter band is 102% (1.01 - 3.05 GHz), demonstrating broadband filtering characteristics. At the same time, the filter also has high frequency selectivity, and its rectangularity coefficient K 30dB is 1.09.

[0081] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, making equivalent substitutions or changes, should be covered within the protection scope of the present invention.

Claims

1. A reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity, characterized in that Comprising: A resonator-loaded open-ended coupled line (1), an absorbing stub (2), and a 50-ohm transmission line (3); The resonator-loaded open-ended coupled line (1) includes a first parallel coupled line (11), a second parallel coupled line (12), and an open-ended parallel circuit (13); the first parallel coupled line (11) includes a first coupled line (111) and a second coupled line (112); the second parallel coupled line (12) includes a third coupled line (121) and a fourth coupled line (122); the open-ended parallel circuit (13) includes a first open line (131) and a first transmission line (132); the absorbing stub (2) includes a first absorbing stub (21) and a second absorbing stub (22); the first absorbing stub (21) includes a second transmission line (211), a second open line (212), a first resistor (213), a third open line (214), and a third transmission line (215); the second absorbing stub (22) includes a fourth transmission line (221), a fourth open line (222), a second resistor (223), a fifth open line (224), and a fifth transmission line (225); the 50-ohm transmission line (3) includes a first 50-ohm transmission line (31) and a second 50-ohm transmission line (32); The first 50-ohm transmission line (31) is connected to the left end of the first coupled line (111), and the second transmission line (211) is connected in parallel with the first 50Ω transmission line (31) and the first coupled line (111); the second transmission line (211) is connected to the first resistor (213), and the second open line (212) is connected in parallel with the second transmission line (211) and the first resistor (213); the first resistor (213) is connected to the third transmission line (215), and the third open line (214) is connected in parallel with the first resistor (213) and the third transmission line (215), and the other end of the third transmission line (215) is grounded; the right end of the second coupled line (112) is connected to the left end of the third coupled line (122), the first transmission line (132) is connected in parallel with the second coupled line (112) and the fourth coupled line (122), and the lower end of the first transmission line (132) is connected to the first open line (131); the right end of the third coupled line (121) is connected to the second 50-ohm transmission line (32), and the fourth transmission line (221) is connected in parallel with the right end of the third coupled line (121) and the second 50-ohm transmission line (32); the fourth transmission line (221) is connected to the second resistor (223), and the fourth open line (222) is connected in parallel with the fourth transmission line (221) and the second resistor (223); the second resistor (223) is connected to the fifth transmission line (225), and the fifth open line (224) is connected in parallel with the second resistor (223) and the fifth transmission line (225); the other end of the fifth transmission line (225) is grounded; The first parallel coupled line (11) and the second parallel coupled line (12) have the same electrical length and characteristic impedance, where the electrical length is 90°; the electrical lengths of the first open circuit line (131), the first transmission line (132), the second transmission line (211), the second open circuit line (212), the third open circuit line (214), the third transmission line (215), the fourth transmission line (221), the fourth open circuit line (222), the fifth open circuit line (224) and the fifth transmission line (225) are all 90°.

2. The reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity according to claim 1, characterized in that: The return loss S of this filter 11 and the insertion loss S 21 are expressed as follows: where Γ o is the reflection coefficient under odd-mode excitation, and Γ e is the reflection coefficient under even-mode excitation. Under odd-mode excitation, the calculation formula for the reflection coefficient Γ o is as follows: Among which Z in1 is the input characteristic impedance of the input port under odd-mode excitation, and the expression of Z in1 is as follows: In the above formula, Z ino is the characteristic impedance of the port of the first - stage parallel - coupled line (11) under odd - mode excitation, Z sub is the characteristic impedance of the port of the absorption stub (2), Z ino and Z sub The expressions of are as follows: Among which Z c is the port characteristic impedance as seen from the lower end of the second transmission line (211), and the expression of Z c is as follows: In the case of even-mode excitation, the reflection coefficient Γ e is calculated by the formula: Among which Z in2 is the characteristic impedance of the input port under even-mode excitation, and the expression of Z in2 is as follows: Among which Z ine is the characteristic impedance of the port of the first - stage parallel - coupled line (11) under even - mode excitation, Z sub is the characteristic impedance of the port of the absorbing stub, Z ine and Z sub The expressions of are as follows: where Z d is the characteristic impedance seen from the port of the open - circuited parallel circuit (13) in the even - mode circuit, Z c is the port characteristic impedance seen from the lower end of the second transmission line (211), Z d and Z c have the following expressions respectively: Where Z1 is the characteristic impedance of the first open circuit line (131), Z2 is the characteristic impedance of the first transmission line (132), Z3 is the characteristic impedance of the second transmission line (211) and the fourth transmission line (221), Z4 is the characteristic impedance of the second open circuit line (212) and the fourth open circuit line (222), R is the resistance value of the first resistor (213) and the second resistor (223), Z5 is the characteristic impedance of the third open circuit line (214) and the fifth open circuit line (224), Z6 is the characteristic impedance of the third transmission line (215) and the fifth transmission line (225), Z o and Z e are the odd-mode characteristic impedance and the even-mode characteristic impedance of the resonator-loaded open-circuit coupled line (1) respectively, Z0 is the characteristic impedance of the 50-ohm transmission line (3), and θ is the electrical length of the first parallel coupled line (11), the second parallel coupled line (12), the first transmission line (132), the first open circuit line (131), the second transmission line (211), the second open circuit line (212), the third open circuit line (214), the third transmission line (215), the fourth transmission line (221), the fourth open circuit line (222), the fifth open circuit line (224) and the fifth transmission line (225).

3. The reflectionless broadband bandpass filter with in-band flatness and high frequency selectivity according to claim 1, characterized in that: The reflectionless characteristics in the band and out of the band are controlled by adjusting the impedances of the transmission lines in each section of the absorption stub (2) and the resistance values of the first resistor (213) and the second resistor (223); the in-band flatness and frequency selectivity are regulated by adjusting the characteristic impedances of the first transmission line (132) and the first open circuit line (131).

4. The non-reflective broadband bandpass filter with in-band flatness and high frequency selectivity according to claim 1, characterized in that: The characteristic impedances of the second transmission line (211), the fourth transmission line (221), the third open circuit line (214) and the fifth open circuit line (224) are all greater than 100Ω; the characteristic impedances of the first open circuit line (131), the second open circuit line (212) and the fourth open circuit line (222) are all less than 50Ω.

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