A microstrip bandpass filter with quasi-absorption and arbitrary frequency-variable coupling structure

Abstract

The present invention belongs to the field of microwave filters, and is specifically a microstrip bandpass filter containing a quasi-absorption and arbitrary frequency-variable coupling structure. The present invention forms a filter network with a complex transfer function by coupling multiple resonant cavities of a main parallel-coupled microstrip line between an input port and an output port. Each resonant cavity in the parallel-coupled structure contributes a transmission extreme value, and the superposition of multiple extreme values ​​improves the quality factor of the filter. The signal attenuates and filters out the reflected signal at the input and output ends through the quasi-absorption microstrip line structure; when the signal passes through the arbitrary frequency-variable structure, an additional transmission zero is introduced by two 1 / 4 wavelength open-circuit microstrip lines with characteristic frequencies of f1 and f2, and the attenuation efficiency of the signal out of the band is accelerated by adjusting f1 and f2; and the passband bandwidth of the present invention can be adjusted by changing the coupling coefficient of the parallel-coupled microstrip line. The present invention takes into account both out-of-band reflection absorption and out-of-band attenuation rate, and is suitable for 5G communication frequency bands, with simple design and easy integration.

Description

Technical Field

[0001] The invention belongs to the technical field of microwave filters, and in particular relates to a microstrip line bandpass filter having a quasi-absorption and arbitrary frequency-variable coupling structure. Background Art

[0002] Traditional reflective filters filter out unwanted signals in the stopband by providing reactive impedance (the signal to be filtered is reflected back at the reactive impedance to achieve the filtering function), but the reflective properties of the filter may cause system-level problems in nonlinear circuits such as power amplifiers and mixers, thereby affecting the performance of the overall circuit. The reason why the reflective properties of the filter may cause system-level problems in nonlinear circuits is that nonlinear circuits are sensitive to out-of-band impedance, especially at harmonics, so the reflective filter may cause unpredictable system performance degradation due to its reflective properties (such as increased efficiency loss, excessive pseudo signal generation, increased dynamic range loss, etc.). Traditional solutions to this problem include using attenuators or non-reciprocal components such as isolators and circulators to reduce the reflection of unwanted stray signals, but the obvious cost of such methods is relatively large signal loss, high cost, uncontrollable volume and system weight.

[0003] Using an absorption filter to solve this problem can avoid the degradation of the overall system performance to the greatest extent. In an absorption filter, the port impedance is well matched both in-band and out-of-band. When the in-band signal passes through the filter with minimal insertion loss, the out-of-band signal will not be reflected inside the filter, but will be attenuated and can be approximated to be completely absorbed. There are many types of absorption filters, such as Figure 3 The quasi-absorption structure shown can achieve a good level of reflection properties. We use the quasi-absorption structure to describe here because its reflection signal, although very small, is not zero, and we think it can be ignored.

[0004] Although the quasi-absorption structure can avoid the impact of out-of-band reflection on system performance, it will lead to slow out-of-band attenuation of the filter. Most existing technologies reduce this adverse effect by cascading multiple filter units or mixing other structures such as cavity filters. However, while improving the out-of-band suppression capability, it also increases unfavorable factors such as filter area and power consumption. Summary of the invention

[0005] In view of the above-mentioned problems or shortcomings, the present invention provides a microstrip bandpass filter with a quasi-absorption and arbitrary frequency-variable coupling structure, which can adapt to the higher requirements of 5G communication for system internal noise elimination and filter out-of-band suppression rate, and has a simple design and high integration.

[0006] The technical solution adopted by the present invention is:

[0007] A microstrip line bandpass filter with quasi-absorption and arbitrary frequency-variable coupling structures comprises an input-end quasi-absorption microstrip line structure, an output-end quasi-absorption microstrip line structure, an arbitrary frequency-variable microstrip line structure and a main body parallel coupling microstrip line structure.

[0008] The quasi-absorbing microstrip line structure has two absorbing short sections for absorbing out-of-band reflections. Each absorbing short section is formed by connecting a matching load resistor and a matching impedance, one end of the two matching load resistors is respectively connected to the left and right ports of the microstrip line on the same side of the parallel coupled microstrip line, and the other end is connected to the corresponding matching impedance, and the other end of the matching impedance is grounded.

[0009] Furthermore, the matching impedance is implemented using a microstrip line to improve the overall device integration.

[0010] The main parallel coupled microstrip line structure is divided into two identical parts, each of which is a parallel coupled microstrip line cascaded in n stages, which is respectively used to connect to the two ends of the arbitrary frequency-variable microstrip line structure and the input and output quasi-absorption microstrip line structures; counting the n-stage parallel coupled microstrip lines cascaded from the arbitrary frequency-variable microstrip line structure to the two ends, the two parallel coupled microstrip lines of each stage have exactly the same structural parameters, thereby satisfying the symmetry of the entire structure, n≥2.

[0011] Furthermore, the number n of parallel coupled microstrip lines of the main body is adjusted according to the performance requirements of the filter. The higher the performance requirements, the more parallel coupled microstrip lines are cascaded (the larger n is), and the cascade number n is calculated from the Chebyshev frequency response curve.

[0012] The arbitrary frequency-variable microstrip line structure comprises: two 1 / 4 wavelength microstrip lines (corresponding to two series resonators), two open microstrip line short sections, and an input-output microstrip line coupling part realized by parallel coupling microstrip lines. The input-end parallel coupling microstrip line is connected to one port of the first 1 / 4 wavelength microstrip line, the other port of the first 1 / 4 wavelength microstrip line is connected to the two open microstrip line short sections and one port of the second 1 / 4 wavelength microstrip line, the other ends of the two open microstrip line short sections are open, and the other port of the second 1 / 4 wavelength microstrip line is connected to the output-end parallel coupling microstrip line.

[0013] Furthermore, the two open microstrip line short sections both use open-circuit 1 / 4 wavelength microstrip lines, and their respective characteristic frequencies are f1 and f2, which are determined by the passband width of the filter and the out-of-band attenuation requirements.

[0014] Furthermore, the characteristic frequency of the 1 / 4 wavelength microstrip line is adjusted by the microstrip line size parameters.

[0015] The principle of the present invention is: the parallel coupled microstrip line structure couples multiple resonant cavities (main parallel coupled microstrip lines) between the input port and the output port to form a filter network with a complex transfer function. Each resonant cavity in the parallel coupled structure contributes a transmission extreme value, and the superposition of multiple extreme values ​​improves the quality factor of the filter. The signal is input from the first-stage parallel coupled microstrip line input port, and the signal that needs to be filtered usually generates reflection at this port. Since the first-stage parallel coupled microstrip line is connected with an absorption short section, the reflected signal enters the absorption short section and is consumed by the matching load and the microstrip line (matching impedance), thereby greatly attenuating. Similarly, the reflection generated when the signal passes through the output port will also be attenuated by the absorption short section. The signal will be processed by the main parallel coupled microstrip line structure in the filter body. When passing through any frequency-variable structure, two 1 / 4 wavelength open-circuit microstrip lines with characteristic frequencies of f1 and f2 will introduce additional transmission zeros. By adjusting f1 and f2 to suitable positions outside the passband (generally set in the band-stop regions on both sides of the passband, specifically determined by the passband frequency), the attenuation efficiency of the signal outside the band can be accelerated. The characteristic frequency can be adjusted by the width and length of the open microstrip line, so as to achieve the characteristic of adjustable frequency. And the passband bandwidth of the present invention can also be adjusted by changing the coupling coefficient of the parallel coupled microstrip line.

[0016] Compared with the traditional microstrip line filter, the parallel coupled microstrip line adopted in the present invention can not only achieve a higher quality factor and a narrower passband and stopband bandwidth, but also can achieve the advantages of adaptive adjustment and multi-mode selection by changing the microstrip line width, length, shape and other parameters, and can adjust and control its characteristic impedance by the geometric dimensions of the microstrip line and the dielectric constant of the dielectric material. The quasi-absorption structure replaces the reflection by absorbing the noise signal in the system, which can avoid the influence of the system performance caused by the out-of-band reflection, but will cause the filter to attenuate slowly out of the band, so an arbitrary frequency-variable coupling structure is introduced to accelerate the out-of-band attenuation rate, so that it can absorb the out-of-band reflection and avoid the influence of the out-of-band reflection, and can quickly attenuate the out-of-band signal outside the passband.

[0017] In summary, the present invention has the following advantages and effects: (1) The present invention can absorb most of the out-of-band reflection signals, avoid system problems caused by out-of-band reflections, and significantly improve the out-of-band attenuation rate while improving the out-of-band reflection absorption capability; (2) The out-of-band transmission zero point of the present invention is adjustable, has design flexibility, can adapt to different bandwidth ranges, and can be applied to most 5G communication frequency bands; (3) The circuit structure of the present invention has good symmetry, a small size, good compatibility, and is easy to integrate, and is simple in design and easy to manufacture. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a structural schematic diagram of a comparative parallel coupled microstrip line filter;

[0019] Figure 2 It is the simulation result diagram of the comparative filter;

[0020] Figure 3 A schematic diagram of a quasi-absorbing microstrip line structure of the present invention;

[0021] Figure 4 It is a schematic diagram of the principle of the arbitrary frequency-variable microstrip line structure of the present invention;

[0022] Figure 5 It is the actual structural diagram of the arbitrary frequency-variable microstrip line structure of the present invention;

[0023] Figure 6 is a schematic structural diagram of a filter in an embodiment;

[0024] Figure 7 Graph showing simulation results of the filter of the embodiment. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0026] The parallel coupled microstrip line structure in this embodiment is symmetrical. The specific structure is as follows: Figure 6 As shown, considering the insufficient performance of the filter composed of a single-stage parallel coupled microstrip line, a multi-stage parallel coupled microstrip line structure is adopted, and its parameter calculation theory is as follows:

[0027] Normalized frequency:

[0028]

[0029] This formula can be combined with the Chebyshev frequency response curve to obtain the actual required parallel coupled microstrip line order. In the formula, Ω is the normalized frequency, f 0 is the center frequency, f H is the passband high frequency cutoff frequency, f L is the passband low frequency cutoff frequency.

[0030] Normalized bandwidth:

[0031]

[0032] Microstrip line odd and even mode impedance:

[0033] Z o | i,i+1 =Z 0 [1-Z 0 J i,i+1 +(Z 0 J i,i+1 ) 2 ]

[0034] Z E |i,i+1 =Z 0 [1+Z 0 J i,i+1 +(Z 0 J i,i+1 ) 2 ]

[0035] The above formula can be used to calculate the even and odd mode impedance of each level of parallel coupled microstrip line, and then the actual size parameters of each level of microstrip line can be calculated according to the LINECALE in the ADS tool. H is the upper frequency limit, ω L is the lower frequency limit, ω 0 is the center frequency, Z o is the odd mode characteristic impedance, Z E is the even mode characteristic impedance, Z 0 is the characteristic impedance of the microstrip line, J i,i+1 is the coupling coefficient between adjacent microstrip lines.

[0036] The quasi-absorbing microstrip line structure used in this embodiment is Figure 3 As shown, each quasi-absorption structure contains two absorbing short sections, each of which is composed of a load resistor connected to a microstrip line. The two short sections are respectively loaded at the two ends of a parallel coupled microstrip line. The load resistor and microstrip line parameters are mainly calculated by the following formula:

[0037]

[0038] The parameters involved in the formula include: is the four-port S parameter of the quasi-absorbing structure, is the equivalent impedance of the absorbing short section, M and N are intermediate variables related to the coupled microstrip line parameters, R is the load resistance on the short section, and Z s is the characteristic impedance of the microstrip line, and θ is the electrical length of the absorbing short section.

[0039] The schematic diagram of the arbitrary frequency-variable coupled microstrip line structure in this embodiment is as follows Figure 4 As shown, its basic core is the relationship between the zeros and poles of the short-circuit admittance or scattering parameters of the filter network and the eigenvalues ​​of the coupling matrix and its two main matrices. Figure 4 The two parallel impedances in the schematic can be realized as a distributed element circuit network consisting of the following elements by the coupling matrix parameters, such as Figure 5As shown in the figure: two 1 / 4 wavelength open transmission lines correspond to two series resonators, two open microstrip line short sections, and the input and output microstrip line coupling parts realized by parallel coupled microstrip lines. These two 1 / 4 wavelength microstrip lines will each generate transmission zeros at the characteristic frequencies f1 and f2, which can correspond to the left and right zeros required in the design of bandpass microstrip line filters. Its impedance is calculated by the following formula:

[0040]

[0041] The complete circuit structure of this embodiment is as follows Figure 6 As shown: the first-stage parallel coupled microstrip line of the main body is connected to the quasi-absorbing microstrip line short section at the input end on both sides; the last-stage microstrip line of the main body is connected to the quasi-absorbing microstrip line short section at the output end on both sides; each absorbing short section is composed of a matching load resistor and a microstrip line connection, one end is connected to the parallel coupled microstrip line side, and the other end is grounded; the input and output parallel coupled lines in the distributed elements of any frequency-variable structure can be equivalent to the two-stage parallel coupled microstrip lines in the main body, described from left to right, the input parallel coupled microstrip line is connected to the left port of the first 1 / 4 wavelength microstrip line, the right port of the first 1 / 4 wavelength microstrip line is connected to two open microstrip line short sections and the left port of the second 1 / 4 wavelength microstrip line, the other ends of the two open microstrip line short sections are open, and the right port of the second 1 / 4 wavelength microstrip line is connected to the output parallel coupled microstrip line. The input and output parallel coupled microstrip lines of this part are the middle two stages of the main body parallel coupled microstrip line.

[0042] In this example, the signal is input from the input port of the first-stage parallel coupled microstrip line, and the frequency components within the passband are coupled to the next-stage microstrip line; most of the frequency components outside the passband are blocked, and part of the reflected signal enters the quasi-absorption short section at the input end, and is absorbed through the matching load and microstrip line impedance consumption; the incoming signal is further filtered in the subsequent parallel coupled microstrip line; when the signal reaches the arbitrary frequency-changing structure, the out-of-band signal is further accelerated to attenuate by the two transmission zeros introduced by the arbitrary frequency-changing structure; when the signal reaches the output port, the frequency components within the passband are output through the output port of the last-stage parallel coupled microstrip line, and the residual reflected signal is absorbed by the quasi-absorption short section connected to the output end.

[0043] The PCB circuit board used in the simulation of this embodiment is Rogers5880, whose dielectric constant Er is 2.2, magnetic permeability Mur is 1, board thickness H is 1.6mm, metal layer thickness T is 0.035mm, and loss angle TanD is 0.0009. Rogers5880 is a commonly used high-frequency circuit board. The comparative example uses the same material. The matching load resistance of this embodiment is 40Ω (which can be adjusted according to the passband requirements).

[0044] The RF simulation software ADS Figure 1The simulation results of the circuit shown (comparative example) are as follows Figure 2 shown; Figure 6 The simulation results of the circuit shown are as follows Figure 7 As shown. Among them, S11 and S21 are filter characteristic parameters, S11 is the filter return loss, and S21 is the filter insertion loss. Its characteristic curve reflects the in-band and out-of-band performance of the filter.

[0045] Depend on Figure 2 From the S(2,1) curve, we can see that the single parallel coupled microstrip bandpass filter has large in-band ripples and slow out-of-band attenuation. It cannot achieve at least 30dB attenuation at the center frequency plus or minus 0.6GHz, and its performance is poor. Figure 7 From the S(2,1) curve, it can be seen that the terminal open-circuit microstrip line added to the parallel coupled microstrip line successfully generates two transmission zeros at the positions f1 and f2 on both sides of the center frequency, so that it can achieve an attenuation of nearly 35dB at the center frequency plus or minus 0.6GHz, and there is a deep notch at 3.15GHz and 4.35GHz, with an attenuation of more than 200dB, showing extremely high selectivity, and can maintain an attenuation of 50dB in a larger stopband range, while the usual design requirement is only greater than 30dB, and its ripple coefficient is within a controllable range, and basically no ripples are generated in the passband, and the passband response is excellent. From the S(1,1) reflection curve, it can be seen that: in its passband range of 3.3GHz-4.2GHz, the structure of the embodiment of the present invention can basically achieve no reflection or small reflection (the return loss in most areas is greater than 20dB), and outside the passband, the S11 curve is close to 0dB, and its out-of-band suppression performance can be considered to be ideal.

[0046] It can be seen from the above embodiments that the present invention forms a filter network with a complex transfer function by coupling multiple resonant cavities of the main parallel coupled microstrip line between the input port and the output port. Each resonant cavity in the parallel coupled structure contributes a transmission extreme value, and the superposition of multiple extreme values ​​improves the quality factor of the filter. The signal is attenuated and filtered out of the reflected signal through the quasi-absorption microstrip line structure at the input and output ends; the signal is processed by the main parallel coupled microstrip line structure in the filter body, and when passing through any frequency-changing structure, two 1 / 4 wavelength open-circuit microstrip lines with characteristic frequencies of f1 and f2 will introduce additional transmission zeros. By adjusting f1 and f2 to suitable positions outside the passband, the attenuation efficiency of the signal outside the band can be accelerated; and the passband bandwidth of the filter of the present invention can be adjusted by changing the coupling coefficient of the parallel coupled microstrip line. Finally, the present invention significantly improves the out-of-band attenuation rate while improving the out-of-band reflection absorption capacity, and has good selectivity, low insertion loss, high return loss and good stop-band suppression; the out-of-band transmission zero point is adjustable, the design is flexible, and it can adapt to different bandwidth ranges and can be used for most 5G communication frequency bands; it also has the advantages of good circuit structure symmetry, small size, good compatibility, easy integration, simple design, and easy manufacturing.

Claims

1. A microstrip bandpass filter with a quasi-absorption and arbitrary frequency-variable coupling structure, characterized in that: It includes an input-end quasi-absorbing microstrip line structure, an output-end quasi-absorbing microstrip line structure, an arbitrary frequency-variable microstrip line structure, and a main body parallel coupled microstrip line structure; The quasi-absorbing microstrip line structure has two absorbing short sections for absorbing out-of-band reflections; each absorbing short section is formed by connecting a matching load resistor and a matching impedance, one end of the two matching load resistors is respectively connected to the left and right ports of the microstrip line on the same side of the parallel coupled microstrip line, and the other end is connected to the corresponding matching impedance, and the other end of the matching impedance is grounded; The main parallel coupled microstrip line structure is divided into two identical parts, each of which is a parallel coupled microstrip line cascaded in n stages, and is respectively used to connect to the two ends of the arbitrary frequency-variable microstrip line structure and the input and output quasi-absorption microstrip line structures; counting the n-stage parallel coupled microstrip lines cascaded from the arbitrary frequency-variable microstrip line structure to the two ends, the two parallel coupled microstrip lines of each stage have exactly the same structural parameters, thereby satisfying the symmetry of the entire structure, n≥2; The arbitrary frequency-variable microstrip line structure comprises: two 1 / 4 wavelength microstrip lines, two open microstrip line short sections, and an input and output microstrip line coupling part realized by parallel coupling microstrip lines; The input end parallel coupled microstrip line is connected to one port of the first 1 / 4 wavelength microstrip line, the other port of the first 1 / 4 wavelength microstrip line is connected to two open microstrip line short sections and one port of the second 1 / 4 wavelength microstrip line, the other ends of the two open microstrip line short sections are open, and the other port of the second 1 / 4 wavelength microstrip line is connected to the output end parallel coupled microstrip line; The two open microstrip line short sections both use open-circuit 1 / 4 wavelength microstrip lines, and their respective characteristic frequencies are f1 and f2, which are determined by the passband width and out-of-band attenuation requirements of the filter.

2. The microstrip line bandpass filter having a quasi-absorption and arbitrary frequency-variable coupling structure as claimed in claim 1, characterized in that: The number n of the main parallel coupled microstrip lines is adjusted according to the filter performance requirements. The higher the performance requirements, the larger n is. The cascade number n is calculated from the Chebyshev frequency response curve.

3. The microstrip line bandpass filter having a quasi-absorption and arbitrary frequency-variable coupling structure as claimed in claim 1, characterized in that: The matching impedance is realized by using a microstrip line.

4. The microstrip line bandpass filter having a quasi-absorption and arbitrary frequency-variable coupling structure as claimed in claim 1, characterized in that: The matching load resistor has a resistance of 40Ω.

5. The microstrip line bandpass filter having a quasi-absorption and arbitrary frequency-variable coupling structure as claimed in claim 1, characterized in that: The characteristic frequency of the 1 / 4 wavelength microstrip line is adjusted by the microstrip line size parameters.

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