Microstrip filter of CQ frequency-variable coupling structure and design method thereof

Abstract

The invention belongs to the technical field of microwave filter design, and particularly relates to a microstrip filter of a CQ frequency-variable coupling structure and a design method of the microstrip filter. The method comprises the following steps: firstly, according to required filter indexes, such as order, center frequency, bandwidth, return loss and transmission zero position, solving a transverse matrix M of a filter by utilizing a filter comprehensive theory; then performing similarity transformation and scaling transformation on the transverse matrix to eliminate unnecessary coupling terms in the transverse matrix so as to obtain a filter topological structure and a frequency-dependent coupling matrix M'which are easier to implement; a quarter-wavelength short-circuit stepped impedance resonator is selected as a resonance unit of the micro-strip filter simulation model; finally, relevant coupling parameters of the micro-strip filter simulation model are made to approach the theoretical coupling coefficient by adjusting relevant physical structure parameters of the resonator, and then design is completed. By introducing a frequency-dependent coupling mechanism into the structure, the position of a transmission zero point is flexible and controllable, and the spectrum selectivity and the out-of-band rejection capability of the filter can be effectively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave filter design, and in particular relates to a microstrip filter with a CQ frequency-variable coupling structure and a design method thereof. Background Art

[0002] As wireless communication systems evolve from fifth-generation (5G) to sixth-generation (6G), RF front-end filters are becoming increasingly important in wireless communication networks, becoming a key component in spectrum management. With the increasing number of frequency bands and the fragmentation of spectrum resources, filters face more design and implementation challenges. Filters must not only balance high selectivity and wide bandwidth within a compact package, but also exhibit strong suppression to reduce signal interference and crosstalk.

[0003] Among the many filter performance indicators, the introduction and precise control of transmission zeros are considered to be the key means to improve the filter's spectral selectivity and out-of-band suppression capabilities. Transmission zeros can completely suppress signals at specific frequency points, thereby significantly improving the stopband characteristics and enhancing the filter's out-of-band attenuation rate and adjacent channel interference suppression capabilities. Currently, the realization of transmission zeros mainly relies on the rational design of coupling structures (such as source / load coupling, cross-coupling, etc.) or the use of asymmetric topology structures (such as SIR structures, asymmetric feeding, etc.). Although these methods can introduce transmission zeros to improve out-of-band suppression capabilities to a certain extent, they often have problems such as complex structures, high required orders or limited number of transmission zeros, and inflexible position adjustment. These problems make it difficult to meet the comprehensive requirements of high-performance RF systems for high selectivity, wide response, and integrability of filters.

[0004] Frequency-variable coupling structures offer a new approach to the flexible implementation of transmission zeros. By embedding frequency-dependent coupling paths and introducing an adjustable frequency-variable coupling mechanism, frequency-variable coupling can achieve multiple, flexibly positionable transmission zeros in lower-order topologies. Furthermore, the frequency-variable coupling structure's compact design and high degree of modularity can effectively reduce the number of required coupling structures, achieving a balance between miniaturization and high performance of the filter. Compared to traditional methods, this approach based on frequency-variable coupling design can effectively improve the overall performance of the filter without significantly increasing volume and losses, making it suitable for the widespread demand for highly integrated, high-performance filters in modern communication systems. Summary of the Invention

[0005] In view of the above problems or deficiencies, in order to solve the flexible design of multiple transmission zeros outside the filter band and achieve a balance between high selectivity and high suppression of the filter; the present invention provides a microstrip filter with a CQ frequency-variable coupling structure and a design method thereof.

[0006] A microstrip filter with a CQ frequency-variable coupling structure includes four quarter-wavelength short-circuited stepped impedance resonators, namely resonator 1, resonator 2, resonator 3, and resonator 4. The high-impedance end of the resonator has a short-circuit hole connected to the metal on the back of the dielectric substrate, and the low-impedance end is open. The adjacent resonators are coupled, and the filter is fed by direct coupling.

[0007] In its topological structure, the feed input S is connected to the resonator 1, and the feed output L is connected to the resonator 4.

[0008] The resonators 1, 2, 3, and 4 are distributed in four regions of a plane rectangular coordinate system based on a plane rectangular coordinate system: resonator 1 corresponds to the lower left corner of the plane rectangular coordinate system, resonator 2 corresponds to the upper left corner of the plane rectangular coordinate system, resonator 3 corresponds to the upper right corner of the plane rectangular coordinate system, and resonator 4 corresponds to the lower right corner of the plane rectangular coordinate system.

[0009] Resonators 1 and 4 have the same shape and are mirror images about the X-axis, forming the first set of resonators. They form a frequency-variable coupling structure between them, generating a low-frequency transmission zero. Resonators 2 and 3 have the same shape and are mirror images about the X-axis, forming the second set of resonators. They form another frequency-variable coupling structure between them, generating a high-frequency transmission zero.

[0010] The difference between the two sets of resonators is that the low and high impedance widths of the first set of resonators are narrower than those of the second set of resonators, respectively; and the low and high impedance lengths of the first set of resonators are longer than those of the second set of resonators. Because longer resonators have lower self-resonance frequencies, the first set of resonators is used to create low-frequency transmission zeros, while the second set of resonators is used to create high-frequency transmission zeros.

[0011] The frequency-variable coupling structure forms magnetic coupling at the short-circuited ends of the resonators, while the gaps between adjacent resonators form electrical coupling. Adjacent resonators: Resonators 1 and 2, and resonators 3 and 4 each form a cross-coupling structure. These two cross-coupling structures generate a third transmission zero, the highest-frequency transmission zero. There is no coupling between non-adjacent resonators 1 and 3, and resonators 2 and 4. The feed inputs at both ends are connected to resonators 1 and 4, respectively.

[0012] A design method for a microstrip filter with a CQ frequency-variable coupling structure includes the following steps:

[0013] A. Determine the filter parameters, including filter order, center frequency, bandwidth, return loss, and transmission zero position.

[0014] B. Based on the index parameters, the filter's transverse coupling matrix is ​​calculated through filter theory synthesis.

[0015] C. The filter topology is selected as the frequency-variable CQ structure. Similarity transformation and scaling transformation are used to eliminate unnecessary items in the lateral coupling matrix to obtain the frequency-variable coupling matrix.

[0016] D. Calculate the coupling bandwidth between resonators and the theoretical value of the external quality factor based on the frequency-dependent coupling matrix of the filter.

[0017] E. Establish a microstrip filter model of the above CQ frequency-variable coupling structure.

[0018] F. Simulate each resonator in the filter model established in step E, optimize the performance of the microstrip filter by adjusting the size and relative position of the resonators, and finally complete the design of the microstrip filter with a CQ frequency-variable coupling structure.

[0019] Furthermore, in step F, the resonant frequency of the resonator is optimized by adjusting the length of the resonator.

[0020] Furthermore, in step F, resonators 1 and 4 or resonators 2 and 3 are simulated, and the coupling bandwidth of the frequency-variable coupling is optimized by adjusting the size of the short-circuit branches of the same resonator to change the size of the magnetic coupling and adjusting the spacing between the same resonators to change the size of the electric coupling.

[0021] Furthermore, in step F, simulation is performed on resonators 1 and 2 or resonators 3 and 4, and the coupling bandwidth of the cross-coupling is optimized by adjusting the spacing between different resonators.

[0022] Furthermore, the step F also includes optimizing the external quality factor of the filter by adjusting the feeding position of the filter.

[0023] The microstrip filter designed based on the frequency-variable coupling structure of the present invention has the following advantages:

[0024] 1. The present invention applies the CQ frequency-variable coupling structure to the microstrip filter. On the traditional CQ cross-coupling structure, a frequency-variable coupling design is superimposed, so that each frequency-variable coupling structure can independently generate and control the position of a transmission zero point, thereby realizing the flexible layout of multiple transmission zero points and effectively improving the filter's spectral selectivity and out-of-band suppression capability.

[0025] 2. Compared with traditional cross-coupling or source / load coupling methods, the CQ frequency-variable coupling structure in the present invention has a high degree of modularity and requires fewer coupling structures, avoiding complex and redundant coupling path design, which is conducive to the miniaturization and integration of the filter and is suitable for scenarios with limited RF front-end space.

[0026] In summary, the present invention first uses filter synthesis theory to calculate the transverse matrix M of the filter based on the required indicators of the microstrip filter, such as order, center frequency, bandwidth, return loss and transmission zero position; then performs similarity transformation and scaling transformation on the transverse matrix to eliminate unnecessary coupling terms, thereby obtaining a filter topology structure and frequency-variant coupling matrix M' that is easier to implement; and selects a quarter-wavelength short-circuit step impedance resonator as the resonant unit of the microstrip filter simulation model; finally, by adjusting the relevant physical structure parameters of the resonator, the relevant coupling parameters of the filter simulation model are close to the theoretical coupling coefficient, thereby completing the design. By introducing a frequency-dependent coupling mechanism into the structure, the present invention makes the transmission zero position flexible and controllable, which can effectively improve the spectral selectivity and out-of-band suppression capability of the filter; not only improves the out-of-band suppression performance while ensuring the compact size of the filter, but also simplifies the implementation process of the coupling path, and is suitable for complex communication requirements such as multi-band and wide stopband, and has certain theoretical value and engineering guidance significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 2 is a schematic diagram of a microstrip filter structure of a CQ frequency-variable coupling structure according to an embodiment;

[0028] Figure 2 2. This is a schematic diagram of S parameters obtained by theoretical synthesis of a microstrip filter with a CQ frequency-variable coupling structure according to an embodiment;

[0029] Figure 3 1 is a schematic diagram of a frequency-variable coupling topology structure of a microstrip filter of a CQ frequency-variable coupling structure according to an embodiment;

[0030] Figure 4 is a basic structural diagram of a quarter-wavelength short-circuit stepped impedance resonator according to an embodiment;

[0031] Figure 5 1 is a schematic diagram of the simulated and measured S parameters of the microstrip filter of the CQ frequency-variable coupling structure of the embodiment. DETAILED DESCRIPTION

[0032] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0033] A design method for a microstrip filter with a CQ frequency-variable coupling structure includes the following steps:

[0034] A. Determine the index parameters of the microstrip filter, including filter order, center frequency, bandwidth, return loss, and the location of the transmission zero point;

[0035] First, the CQ frequency-variable coupled microstrip filter to be designed has a center frequency of 2400MHz, a passband width of 100MHz, an in-band return loss of less than 15dB, and three transmission zeros located at 2260MHz, 2670MHz, and 2890MHz.

[0036] B. Based on the index parameters determined in step A, calculate the filter's transverse coupling matrix through filter theory synthesis.

[0037] According to the comprehensive theory of the filter, the S parameters can be obtained as follows Figure 2 As shown, the lateral coupling matrix M is shown in Table 1.

[0038] Table 1:

[0039]

[0040] C. The filter topology is selected as the frequency-variable CQ structure. Similarity transformation and scaling transformation are used to eliminate unnecessary items in the lateral coupling matrix to obtain the frequency-variable coupling matrix.

[0041] Because the coupling structure corresponding to the transverse coupling matrix is ​​too complex and difficult to implement physically, it is necessary to simplify the coupling matrix to eliminate unnecessary coupling terms, thereby obtaining a simpler and easier-to-implement filter topology. Typically, this coupling matrix simplification is achieved through a matrix similarity transformation. This method preserves the matrix eigenvalues ​​and therefore does not change the filter's eigenfrequency or S-parameter response.

[0042] Assume that the transverse coupling matrix before simplification is M0, construct the rotation matrix R, and the matrix after similarity transformation is:

[0043] M1=R T M0R (1)

[0044] For the N+2×N+2 M0 matrix, construct an identity matrix I n , and then replace the i-th row and i-th column, the j-th row and j-th column, the i-th row and j-th column, and the j-th row and i-th column as follows:

[0045]

[0046] The rotation point is [i, j] (i, j = 1, 2, ..., N) and the rotation angle is θ

[0047] R ii =cosθ,R jj =cosθ,R ij =-sinθ,R ji = sinθ (3)

[0048] The N+2 order coupled equivalent circuit admittance matrix of the N order filter can be expressed as

[0049] [Y]=[M]+Ω[C]-j[G] (4)

[0050] Where M corresponds to the coupling coefficient in the actual circuit, C is the capacitance matrix, and G is the external quality factor corresponding to the input and output terminals.

[0051] The transformation of the Y matrix is:

[0052]

[0053] Where n = 0, 1, 2 ... represents the corresponding parameters of the nth transformation operation. Here, it is assumed that the matrix G has only G SS =G LL =1 (assuming the matrix is ​​numbered S, 1, 2, ..., N, L), the remaining elements are all zero and will not change in the similarity transformation, so only the M matrix and the C matrix need to be transformed accordingly; and the C matrix is ​​a diagonal matrix, and C SS =C LL =1, C 11 =C 22 =...=C NN =1.

[0054] The T matrix here consists of two steps:

[0055] 1) T matrix is ​​the rotation matrix R

[0056] M1=R T M0R (6)

[0057] C1=R T C0R (7)

[0058] 2) T matrix is ​​the scaling matrix U

[0059] M'=U T M1U (8)

[0060] C'=U T C1U (9)

[0061] The U matrix is:

[0062]

[0063] According to the above method, the simplified CQ frequency-variable coupling matrix is ​​shown in Table 2, and its topology is as follows: Figure 3 shown.

[0064] Table 2:

[0065]

[0066]

[0067] E. Establish a microstrip filter model with CQ frequency-variable coupling structure.

[0068] The quarter-wavelength short-circuit step impedance resonator is used as the resonant structure of the microstrip filter, such as Figure 4 The figure shows the basic structure of a quarter-wavelength short-circuited stepped impedance resonator, where Z1, Z2, θ1, and θ1 are the characteristic impedance and corresponding electrical length of the microstrip line in the resonator. When the transmission line terminal is short-circuited, the input admittance is:

[0069]

[0070] When the input admittance Y in =0, the resonance condition of the quarter-wavelength short-circuit step impedance resonator is:

[0071] Z2-Z1 tanθ1 tanθ2=0 (13)

[0072] Right now

[0073]

[0074] where K Z is the microstrip line impedance ratio, by adjusting K Z The resonator size and the position of the parasitic passband can be effectively controlled, and the total electrical length θ of the resonant structure TA for:

[0075]

[0076] Therefore, by controlling the electrical length θ1 and the impedance ratio K Z The resonator length and stray resonance frequency can be effectively controlled.

[0077] Finally, a microstrip filter model with CQ frequency-variable coupling structure is established, as shown in Figure 1 As shown, the dielectric constant of the substrate used in the embodiment is 2.2 and the thickness of the substrate is 0.508 mm.

[0078] F. Simulate each resonator in the filter model established in step E, optimize the performance of the microstrip filter by adjusting the size and relative position of the resonators, and finally complete the design of the microstrip filter with a CQ frequency-variable coupling structure.

[0079] The simulation steps are as follows:

[0080] 1) Simulate a single resonator. By adjusting the size of the resonator, the self-resonant frequency of the resonator can be changed so that the simulated value of the resonant frequency is close to the theoretical value.

[0081] The resonant frequency f of a single resonator res The relationship with the center frequency f0 is:

[0082]

[0083] Where FBW is the fractional bandwidth

[0084]

[0085] 2) Simulate resonators 1 and 4 or resonators 2 and 3. By adjusting the size of the short-circuit branches of the same resonator, the size of the magnetic coupling can be changed. By adjusting the spacing between the same resonators, the size of the electric coupling can be changed. This makes the simulated coupling bandwidth value of the frequency-variable coupling close to the theoretical coupling bandwidth value, and the transmission zero point position is correct.

[0086] Theoretical value of coupling bandwidth CBW and coupling coefficient m i,j The relationship between them is:

[0087] CBW=m i,j ·BW (18)

[0088] Where BW is the absolute bandwidth:

[0089] BW=f2-f1 (19)

[0090] 3) By adjusting the spacing between different resonators, the simulated value of the cross-coupling coupling bandwidth is made close to the theoretical value.

[0091] 4) By adjusting the feeding position of the filter, the simulated value of the filter's external quality factor is made close to the theoretical value.

[0092] External quality factor theoretical value Q e and coupling coefficient m i,j The relationship between them is:

[0093]

[0094] The final designed CQ frequency-variable coupling structure microstrip filter has the following simulation and test S parameters: Figure 5 shown.

[0095] As can be seen from the above examples, the simulation test results are in good agreement with the theoretically synthesized S parameters. The CQ frequency-variable coupling structure microstrip filter design method provided by the present invention introduces a frequency-dependent coupling mechanism into the structure, making the distribution of transmission zeros more flexible and the filter response characteristics easier to control. This method not only improves the out-of-band suppression performance while ensuring the compact size of the filter, but also simplifies the implementation process of the coupling path. It is suitable for complex communication requirements such as multi-band and wide stopband, and has certain theoretical value and engineering guidance significance.

Claims

1. A microstrip filter with a CQ frequency-variable coupling structure, characterized in that: The filter comprises four quarter-wavelength short-circuited step impedance resonators, namely resonator 1, resonator 2, resonator 3 and resonator 4. The high-impedance end of the resonator has a short-circuit hole connected to the metal on the back of the dielectric substrate, and the low-impedance end is open. The adjacent resonators are coupled to each other, and the filter is fed by direct coupling. In its topology, the feed input S is connected to resonator 1, and the feed output L is connected to resonator 4; The resonator 1, resonator 2, resonator 3, and resonator 4 are distributed in four regions of a plane rectangular coordinate system based on a plane rectangular coordinate system: resonator 1 corresponds to the lower left corner of the plane rectangular coordinate system, resonator 2 corresponds to the upper left corner of the plane rectangular coordinate system, resonator 3 corresponds to the upper right corner of the plane rectangular coordinate system, and resonator 4 corresponds to the lower right corner of the plane rectangular coordinate system; Resonators 1 and 4 have the same shape and are mirror images about the X-axis. They form the first set of resonators, forming a frequency-variable coupling structure to generate a low-frequency transmission zero. Resonators 2 and 3 have the same shape and are mirror images about the X-axis, forming a second set of resonators. Another frequency-variable coupling structure is formed between the two to generate a high-frequency transmission zero point. The difference between the two groups of resonators is that the low and high impedance widths of the first group of resonators are narrower than the low and high impedance widths of the second group of resonators; The low and high impedance lengths of the first group of resonators are longer than the low and high impedance lengths of the second group of resonators; The frequency-variable coupling structure forms magnetic coupling at the short-circuited ends of the resonators, while the gaps between adjacent resonators form electrical coupling. A cross-coupling structure is formed between adjacent different resonators: resonators 1 and 2, and resonators 3 and 4. The two cross-coupling structures generate a third transmission zero, namely the highest-frequency transmission zero. There is no coupling between non-adjacent resonators 1 and 3, and resonators 2 and 4. The feeding input positions at both ends are connected to resonators 1 and 4 respectively.

2. The design method of the microstrip filter of the CQ frequency-variable coupling structure as claimed in claim 1, characterized in that: The following steps are involved: A. Determine the index parameters of the microstrip filter, including filter order, center frequency, bandwidth, return loss, and the location of the transmission zero point; B. Based on the index parameters determined in step A, calculate the filter's transverse coupling matrix through filter theory synthesis; C. The filter topology is selected as a frequency-variable CQ structure. Similarity transformation and scaling transformation are used to eliminate unnecessary items in the lateral coupling matrix to obtain a frequency-variable coupling matrix. D. Calculate the coupling bandwidth between resonators and the theoretical value of the external quality factor based on the frequency-dependent coupling matrix of the filter; E. Establish a microstrip filter model of the above CQ frequency-variable coupling structure; F. Simulate each resonator in the filter model established in step E, optimize the performance of the microstrip filter by adjusting the size and relative position of the resonators, and finally complete the design of the microstrip filter with a CQ frequency-variable coupling structure.

3. The design method of the microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 2, characterized in that: In step F, the resonant frequency of the resonator is optimized by adjusting the length of the resonator.

4. The design method of the microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 2, characterized in that: In step F, resonators 1 and 4 or resonators 2 and 3 are simulated, and the coupling bandwidth of the frequency-variable coupling is optimized by adjusting the size of the short-circuit branches of the same resonator to change the size of the magnetic coupling and adjusting the spacing between the same resonators to change the size of the electric coupling.

5. The design method of the microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 2, characterized in that: In step F, simulation is performed on resonators 1 and 2 or resonators 3 and 4, and the coupling bandwidth of the cross-coupling is optimized by adjusting the spacing between different resonators.

6. The method for designing a microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 2, wherein: The step F also includes optimizing the external quality factor of the filter by adjusting the feeding position of the filter.

7. The design method of the microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 2, characterized in that: In step C, the similarity transformation and scaling transformation of the frequency-dependent coupling matrix are: [Y n+1 ]=[T n+1 ] T [Y n ][T n+1 ] =[T n+1 ] T [M n ][T n+1 ]+Ω[T n+1 ] T [C n ][T n+1 ]-j[G] Where n = 0, 1, 2 ... represents the corresponding parameters of the nth transformation operation; suppose the matrix G numbered S, 1, 2, ..., N, L has only G SS =G LL =1, and the rest of the elements are zero, which will not change in the similarity transformation; therefore, only the M matrix and the C matrix need to be transformed accordingly; and the C matrix is ​​a diagonal matrix, and C SS =C LL =1, C 11 =C 22 =…=C NN =1; The T matrix here consists of two steps: 1) The T matrix is ​​the rotation matrix R: M1 = R T M0R, C1=R T C0R; 2) The T matrix is ​​the scaling matrix U: M' = U T M1U, C'=U T C1U; The U matrix is 8. The design method of the microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 3, characterized in that: The resonant frequency f of a single resonator res The relationship with the center frequency f0 is: Where FBW is the fractional bandwidth 9. The method for designing a microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 4, wherein: The theoretical value of the coupling bandwidth CBW and the coupling coefficient m i，j The relationship between them is: CBW=m i，j BW, where BW is the absolute bandwidth FBW = f2 - f1.

10. The method for designing a microstrip filter with a CQ frequency-variable coupling structure as claimed in claim 6, wherein: The external quality factor theoretical value Q e and coupling coefficient m i，j The relationship between them is:

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