CN114256573A

CN114256573A - Microstrip low-pass filter and design method thereof

Info

Publication number: CN114256573A
Application number: CN202111579168.9A
Authority: CN
Inventors: 吴元清; 张�诚; 杨春; 尤建洁
Original assignee: Network Communication and Security Zijinshan Laboratory
Current assignee: Network Communication and Security Zijinshan Laboratory
Priority date: 2021-12-22
Filing date: 2021-12-22
Publication date: 2022-03-29
Anticipated expiration: 2041-12-22
Also published as: CN114256573B

Abstract

The invention discloses a microstrip low-pass filter and a design method thereof, wherein the filter forms a transmission zero near a pass band by introducing a plurality of LC resonance units connected in parallel, thereby overcoming the defect of moderation of sideband transition performance of the traditional step impedance filter, and introduces a plurality of transmission zeros at a parasitic pass band of the filter by introducing an open circuit structure and a parallel trap unit structure, thereby widening a stop band. The filter designed by the invention has the advantages of compact structure, steep transition band and wide stop band.

Description

Microstrip low-pass filter and design method thereof

Technical Field

The invention relates to the technical field of filters, in particular to a microstrip low-pass filter and a design method thereof.

Background

Filters are important components of radio frequency circuitry and are widely used in communication systems. The high-performance filter can obviously reduce the noise coefficient of the system, reduce the out-of-band interference of the system and reduce the error rate of the system, thereby obviously improving the performance of the communication system. With the advent of the fifth generation wireless communication era, the demand of radio frequency communication links for adjacent channels and far-end out-of-band spurious suppression is increasing, and therefore, designing a miniaturized, steep-sideband, wide-stopband high-performance low-pass filter is crucial to improving the performance of a communication system.

The traditional microstrip line structure filter has relatively small volume, good consistency among the filters and easy integration with other active circuits, so the traditional microstrip line structure filter is widely applied to a radio frequency front-end system. But the high-band rejection performance of the conventional microstrip low-pass filter is relatively poor due to the parasitic passband of the filter. The parasitic passband of the traditional stepped impedance line low-pass filter is more 2.5 times of the central frequency, and the filter has a large structural volume and slow transition band drop, so that when the filter is applied to a 5G communication system, the filter is difficult to effectively suppress adjacent channels and high-frequency-band spurious signals, and the actual use requirement of an actual communication system cannot be met.

At present, researchers have proposed various solutions to expand the stop band of the filter, for example, parallel connection of quarter-wavelength open stubs can add additional transmission zero points on the stop band, but the parasitic pass band of the filter is usually very wide, one open stub is often insufficient, and a plurality of quarter-wavelength open stubs need to be connected in parallel, which inevitably increases the design area of the filter; in the article of Compact low impedance filters with a shared and expanded stored and used modulated impedance hairpin units published by IEEE Microwave and Wireless Components letters, the authors of Li L et al propose a scheme of using a loaded step impedance resonator and introducing two transmission zeros to improve the performance, but have the problems of increased filter area, complicated design, and insufficient stop band suppression due to less transmission zeros. The chinese patent with application number 2016110676151, "a wide stop band microstrip line low-pass filter and design method", proposes a scheme of expanding the stop band of the filter by a method of generating cancellation through a pole-zero, but has a problem of complicated modeling simulation in the actual design process of the filter.

In order to improve the sideband transition characteristic of the low-pass filter, more transmission zeros need to be added outside the band to improve the sideband suppression performance of the filter, and the method has the defects of higher design complexity of the filter, larger filter volume, introduction of extra filter in-band loss and the like, is not easy to integrate with an active circuit, is not suitable for various communication and radar radio frequency front-end receiving and transmitting systems from an L waveband to a Ku waveband, and has long product development period and low product performance.

Disclosure of Invention

The technical purpose is as follows: aiming at the technical problems, the invention aims to provide a microstrip low-pass filter and a design method thereof, and solves the problems of large volume and mild sidebands of the existing microstrip low-pass filter.

The technical scheme is as follows: in order to achieve the technical purpose, the invention adopts the following technical scheme:

a microstrip low pass filter characterized by: the microstrip line wiring layer is included;

the microstrip line wiring layer is provided with an input feeder line, an output feeder line, a plurality of folding inductance units and a plurality of LC resonance units;

the folding inductance unit is connected between the input feeder line and the output feeder line in a series mode;

an LC resonance unit is connected between every two adjacent folding inductance units.

Preferably, the LC resonant unit includes a first microstrip line and a first open-loop fan-shaped stub, a dot of the first open-loop fan-shaped stub is connected to one end of the first microstrip line, the other end of the first microstrip line is connected between adjacent folded inductor units, and the LC resonant unit is configured to form a transmission zero in a stop band close to the pass band of the filter.

Preferably, the folding inductance unit is of a right-angle bending structure.

Preferably, an open circuit unit is connected into at least one middle folding inductance unit; the open circuit unit is used for forming a transmission zero point in a parasitic passband.

Preferably, the middle folding inductor unit connected into the open circuit unit comprises a three-side surrounded opening structure, and the three-side surrounded opening structure comprises a first inductor arm and two second inductor arms vertically connected with the first inductor arm;

the open circuit unit comprises a second microstrip line vertically connected with the first inductance arm.

Preferably, a trap unit is connected in parallel to the folded inductor unit connected to the output feeder, and the trap unit includes a second open-circuited fan-shaped stub, and a center point of the second open-circuited fan-shaped stub is connected to the folded inductor unit.

Preferably, the radius of the second open-circuited sector stub is set to 1/4 wavelengths of the signal corresponding to the filter spurious passband resonant frequency.

A design method of a microstrip low-pass filter is characterized by comprising the following steps:

bending the high-impedance microstrip line at right angles to obtain a plurality of folding inductance units; manufacturing a plurality of LC resonance units by adopting a first microstrip line and a first open-circuit fan-shaped stub line;

connecting the folded inductance units in series between the input and output feed lines;

an LC resonance unit is connected between every two adjacent folding inductance units;

the frequency of the transmission zero point formed in the stop band close to the pass band of the filter is controlled by adjusting the radius length of the first open-circuit fan-shaped stub of each LC resonance unit and the length of the first microstrip line.

Preferably, the method further comprises the following steps:

the method comprises the following steps that at least one middle folding inductance unit is made into an opening structure with three surrounded surfaces, wherein the opening structure with three surrounded surfaces comprises a first inductance arm and two second inductance arms which are vertically connected with the first inductance arm;

an open circuit unit is connected into the opening structure surrounded by three sides; the open circuit unit is a second microstrip line vertically connected with the first inductance arm;

and the frequency of the transmission zero point of the corresponding parasitic passband is controlled by adjusting the length and the width of the second microstrip line of the open circuit unit.

Preferably, the method further comprises the following steps:

a trap unit is connected in parallel with the folding inductance unit connected with the output feeder line, and comprises a second open-circuit fan-shaped stub;

the radius of the second open-circuited sector stub of the notch unit is set to a quarter wavelength of a signal corresponding to the resonance frequency of the parasitic pass band.

Has the advantages that: compared with the prior art, the invention has the following technical effects:

according to the invention, the LC resonance units are connected in parallel between each two adjacent folding inductance units, and a plurality of transmission zeros can be formed in the stop band close to the passband through the LC resonance units, and the transmission zeros are very effective in improving the sideband transition characteristic of the filter, so that the transition bandwidth of the filter is greatly reduced, and the problem of slow transition band of the traditional stepped impedance filter is solved; by adopting the LC resonance structure with less orders and only introducing a zero point at the near point of the pass band, the problem of wider transition bandwidth of the filter can be improved, the roll-off characteristic of the filter is improved, and the design volume of the filter is reduced.

According to the invention, the open-circuit unit is introduced, and the notch unit is connected in parallel with the folded inductance unit connected with the output feeder, so that a transmission zero is introduced at the parasitic passband of the filter, on one hand, the parasitic passband of the filter can be effectively suppressed, and on the other hand, the stop band of the filter is greatly expanded.

The invention adopts the folding inductor and the open-circuit fan-shaped stub, can greatly reduce the design area of the filter through proper shape arrangement, has simple and easy design method, compact structure of the designed filter, steep transition band, easy integration with an active circuit, is suitable for various communication and radar radio frequency front end receiving and transmitting systems from L waveband to Ku waveband, can greatly shorten the product development period and improve the product performance.

Drawings

Fig. 1 is a schematic structural diagram of a microstrip low-pass filter according to an embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating a microstrip low-pass filter according to an embodiment of the present invention;

fig. 3 is a schematic diagram of an S-parameter simulation curve provided in an embodiment of the present invention.

Detailed Description

The structure of the filter and the design method thereof according to the present invention will be described in detail with reference to the accompanying drawings.

Example 1

The invention provides a microstrip low-pass filter, comprising: the microstrip line comprises a microstrip line wiring layer, a dielectric substrate and a bottom layer of the microstrip line, wherein the microstrip line wiring layer is a top layer, the dielectric substrate is a middle layer, the middle layer provides proper characteristic impedance for the microstrip line wiring layer on the top layer, and the bottom layer is used as a reference ground plane of the microstrip line wiring layer and provides a return path for the top layer.

The microstrip line wiring layer comprises an input feeder line and an output feeder line which are positioned at two ends, a plurality of folding inductance units connected between the input feeder line and the output feeder line in a series connection mode, and a plurality of LC resonance units arranged between adjacent folding inductance units, wherein the LC resonance units are used for forming transmission zero points in a stop band close to a filter pass band. The folding inductance units are of a right-angle bending structure, and all the folding inductance units are connected in series between the input feeder line and the output feeder line in an end-to-end connection mode;

folding inductance element and LC resonance unit interval set up, if set up first LC resonance unit between first folding inductance element and the second folding inductance element, so on, set up nth LC resonance unit between the inductance unit is folded to the m-1 and the mth, and m equals n + 1.

In the embodiment, a plurality of LC resonance units connected in parallel are adopted, and a plurality of transmission zeros are formed in the stop band close to the pass band through the formed LC resonance structure, and the transmission zeros are very effective in improving the sideband transition characteristic of the filter, so that the transition bandwidth of the filter is greatly reduced, and the problem of slow transition band of the traditional stepped impedance filter is solved.

The folding inductance unit realizes the compactness of the filter volume through folding. The folding inductance unit is made of a high-impedance microstrip line, namely a microstrip line with a narrow line width.

Further, the LC resonance unit includes a first microstrip line and a first open-circuit fan-shaped stub, a dot of the first open-circuit fan-shaped stub is connected to one end of the first microstrip line, and the other end of the first microstrip line is connected between adjacent folded inductance units; the first microstrip line is made of a high-impedance microstrip line, namely a microstrip line with a narrow line width. For example, a high-impedance microstrip line having an impedance of 100 Ω is used.

The LC resonance units form transmission zero points in the stop band close to the passband of the filter, and the transmission zero point frequency in the corresponding stop band is adjusted by adjusting the radius length of the first open-circuit fan-shaped stub of each LC resonance unit and the length of the first microstrip line.

The central angle orientation of the first open-circuit fan-shaped stub does not have requirements, and on the premise of ensuring the performance of the filter, minimum wiring is realized; common values of the central angle are 90 degrees and 60 degrees, and other central angle values can be selected according to the requirement of the design volume of the filter.

Furthermore, an open circuit unit is connected into at least one middle folding inductance unit; the open circuit unit is used for forming a transmission zero point in a parasitic passband.

Except for the two folding inductance units at the two ends, the other folding inductances are the middle folding inductance unit; the middle folding inductance unit connected into the open circuit unit comprises an opening structure surrounded on three sides, and the opening structure surrounded on three sides comprises a first inductance arm and two second inductance arms vertically connected with the first inductance arm. The opening structure surrounded by three sides further minimizes the area of the filter, and the opening direction is not required.

The open circuit units are used for introducing transmission zero points at the parasitic pass band of the filter, and the frequency of the transmission zero points of the parasitic pass band corresponding to the length and the width of each second microstrip line of each open circuit unit is controlled.

Furthermore, a trap unit is connected in parallel with the last stage of the folding inductance unit, the trap unit comprises a second open-circuit fan-shaped stub, and the center point of the second open-circuit fan-shaped stub is connected to the folding inductance unit;

the radius of the second open-circuit fan-shaped stub of the notch unit is set to be a quarter wavelength of a signal corresponding to the resonance frequency of the parasitic passband, and the second open-circuit fan-shaped stub is used for introducing a transmission zero at the parasitic passband of the filter, so that the parasitic passband of the filter can be further suppressed, and the stop band of the filter is greatly expanded.

Example 2

A design method of a microstrip low-pass filter comprises the following steps:

arranging LC resonance units between adjacent folded inductance units;

Further, the design method of the microstrip low-pass filter further includes:

Setting the resonant frequency of the open cell at the spurious passband frequency introduces at least one transmission zero within the spurious passband of the filter.

Further, the design method of the microstrip low-pass filter further includes:

a trap unit is connected in parallel to the last stage of folding inductance unit, and comprises a second open-circuit fan-shaped stub;

The notch unit is used for introducing a transmission zero at the parasitic passband of the filter, so that the parasitic passband of the filter can be effectively suppressed, and the stop band of the filter is greatly expanded.

Example 3

In this embodiment, a ninth-order low-pass filter is taken as an example to describe, and 5 folded inductance units are designed, where fig. 1 shows a top microstrip line wiring layer, which includes: the circuit comprises an input feeder line 1, a first folding inductance unit 2, a second folding inductance unit 3, a third folding inductance unit 4, a fourth folding inductance unit 5, a fifth folding inductance unit 6, a first LC resonance unit 7, a second LC resonance unit 8, a third LC resonance unit 9, a fourth LC resonance unit 10, a first open-circuit unit 11, a second open-circuit unit 12, a third open-circuit unit 13, a trap unit 14 and an output feeder line 15.

A first LC resonance unit 7 is arranged between the first folding inductance unit 2 and the second folding inductance unit 3, a second LC resonance unit 8 is arranged between the second folding inductance unit 3 and the third folding inductance unit 4, a third LC resonance unit 9 is arranged on the upper side between the third folding inductance unit 4 and the fourth folding inductance unit 5, and a fourth LC resonance unit 10 is arranged between the fourth folding inductance unit 5 and the fifth folding inductance unit 6; each LC resonance unit comprises a first microstrip line and a first open-circuit fan-shaped stub line.

The first open-circuit unit 11 is disposed between two inductance arms of the second folded inductance unit parallel to the Y axis, the second open-circuit unit 12 is disposed between two inductance arms of the third folded inductance unit parallel to the Y axis, and the third open-circuit unit 13 is disposed between two inductance arms of the fourth folded inductance unit parallel to the Y axis. A trap unit 14 is provided on the fifth folding inductance unit 6.

The filter of the embodiment forms transmission zeros near the passband by introducing four parallel LC resonance units, and the four transmission zero frequencies are controlled by adjusting the radii of the open-circuit fan-shaped stub lines of the first, second, third and fourth LC resonance units and the length of the first microstrip line, so that the defect that the sideband transition performance of the traditional step impedance filter is alleviated is overcome; by introducing three open-circuit units and notch units, four transmission zeros are introduced at the parasitic passband of the filter, the resonant frequency of the open-circuit units is adjusted to be one quarter wavelength of a signal corresponding to the parasitic resonant frequency by adjusting the lengths and widths of the first open-circuit unit, the second open-circuit unit and the third open-circuit unit, and the stop band can be expanded to be more than quadruple frequency of the central frequency.

The design method of the microstrip low-pass filter comprises the following steps:

step 1, bending a high-impedance line, namely a narrow part of a microstrip line at a right angle to obtain a plurality of folding inductance units; forming a folding inductance unit with a plurality of inductance arms after the right-angle bending, wherein the inductance arms of the folding inductance unit are parallel to an X axis or a Y axis; connecting the folded inductance units in series between the input and output feed lines;

step 2, manufacturing a plurality of LC resonance units by adopting a first microstrip line and a first open-circuit fan-shaped stub line;

arranging LC resonance units between adjacent folded inductance units;

the folding inductance units and the LC resonance units are arranged at intervals, namely, the first LC resonance unit is arranged between the first folding inductance unit and the second folding inductance unit, and the like, and the fourth LC resonance unit is arranged between the fourth folding inductance unit and the fifth folding inductance unit. The length of the microstrip line is represented by L, the width of the microstrip line is represented by W, and the radius of the fan-shaped stub line is represented by R, specifically, as shown in FIG. 2, the filter realizes the control of the frequency of the transmission zero outside the passband by changing the sizes of the first LC resonance units L1 and R1, the second LC resonance units L4 and R2, the third LC resonance units L8 and R3 and the fourth LC resonance units L10 and R4, the four transmission zeros are generated at four frequency points of 1.7GHz, 1.88GHz, 2.1GHz and 2.48GHz, and the filter has steeper sideband characteristics because the four transmission zeros are near the passband, thereby improving the disadvantage of the gentleness transition band of the traditional stepped impedance filter;

The advantage of gradual impedance change of the open-circuit fan-shaped stub is utilized to replace a traditional rectangular micro-strip capacitor, and the highest applicable frequency range of the fan-shaped micro-strip capacitor is farther, so that the parasitic passband of the filter is pushed back;

step 3, the middle folding inductance unit is made into an opening structure with three surrounded surfaces, and the opening structure with three surrounded surfaces comprises a first inductance arm and two second inductance arms vertically connected with the first inductance arm; an open circuit unit is connected into the opening structure surrounded by three sides; the open circuit unit is a second microstrip line vertically connected with the first inductance arm;

an open circuit unit is added between the inductance arms of the folding inductance units parallel to the Y axis, the open circuit unit has maximum attenuation at the resonance frequency, but has extremely low insertion loss at other frequencies, so that when the filter is designed, the resonance frequency of the open circuit unit is set at the parasitic passband frequency, namely, a plurality of transmission zeros can be introduced into the parasitic passband of the filter, and the expansion of the stop band of the filter is realized; specifically, as shown in fig. 2, the resonant frequencies of the open cells are determined by the first open cells L3, W4, the second open cells L6, W6, and the third open cells L9, W8, and by adjusting the length and width dimensions of the open cells, the present embodiment forms three transmission zeros at 2.84GHz, 3.36GHz, and 5.96GHz, thereby expanding the stop band of the filter;

step 4, according to the practical application condition of the filter and the related index requirements, a notch unit is connected in parallel with the last stage of folding inductance unit, and the notch unit comprises a second open-circuit fan-shaped stub; the radius of the second open-circuited sector stub of the notch unit is set to a quarter wavelength of a signal corresponding to the resonance frequency of the parasitic pass band.

The radius (R5) of the open-circuited sectoral stub is determined according to the frequency of the parasitic passband in the simulation result, and in this embodiment, according to the simulation result, the final R5 is selected as a quarter wavelength corresponding to the frequency of the parasitic passband being 5.2GHz, and as can be known from the quarter wavelength impedance transformation characteristic, the filter exhibits a short-circuit characteristic at the position corresponding to 5.2GHz, thereby further effectively suppressing the parasitic passband of the filter.

The compact filter with the steep sideband characteristic designed by the invention is modeled and optimized in simulation through three-dimensional electromagnetic simulation software HFSS, the actual size of the used filter is 32.2mm 28mm, and the medium substrate selected in simulation adopts FR4 plate with the dielectric constant of 4.2 and the plate thickness of 1.6 mm. The dimension labels of the microstrip lines of the filter are shown in fig. 2, the specific dimensions are shown in table 1, and the unit is mm:

TABLE 1

W1＝1.42	W2＝0.2	W3＝3.65	W4＝0.45	W5＝3.46
					W6＝0.37	W7＝3.72	W8＝0.25	W9＝5.95	L1＝4
L2＝5.3	L3＝14.2	L4＝9.19	L5＝4.15	L6＝8.06
					L7＝5.3	L8＝6.5	L9＝11.9	L10＝4.35
R1＝5.7	R2＝5.45	R3＝6.5	R4＝5.2	R5＝6.72
					β₁＝90°	β₂＝60°

The transmission characteristic of the filter is shown in fig. 3, the S parameters comprise S11 and S21, S11 represents reflection characteristic, S21 represents transmission characteristic, the simulated cut-off frequency of the filter is 1.492GHz, the in-band return loss is greater than 15dB, the insertion loss is less than 0.4dB, the insertion loss at the cut-off frequency is less than 2dB, the attenuation of the filter at the position 180M away is more than 40dB, the stop band can be expanded to more than four times of the central frequency, when the stop band is 1.7-4.5 GHz, the rejection of the filter is greater than 35dB, and the rejection of the whole stop band is greater than 25 dB.

If different requirements are required for the sideband characteristic and the stop band characteristic of the filter in an actual scene, the filter can be optimally designed according to the design method introduced by the invention, and the stop band width characteristic of the filter can be further reduced or improved by reducing or increasing the open circuit units of the filter. The sideband roll-off characteristic of the filter can be further reduced or improved by removing or adding a primary LC resonance unit structure and a corresponding folding inductance unit. The filter size can also be further optimized by the above-described optimization measures.

The invention provides a filter design method which is simple in design method, steep in transition band, wide in stop band and compact in structure. The filter meeting the system requirements can be easily designed by the design method. Any modification, equivalent replacement, simple improvement, replacement of the working frequency and the like of the various equivalent forms of the present invention, which are made in the spirit of the present invention, should be included in the protection scope of the present invention.

It should be understood that these examples are only for illustrating the design method of the present invention and are not intended to limit the scope of the present invention. The descriptions of dimensions, angles, thicknesses, lengths, materials, dielectric constants, frequencies, powers, capacitances, inductances, etc. described in the embodiments of the present invention are merely one way of expressing them in the examples. One skilled in the art may combine features of different embodiments and different embodiments described in this specification.

Claims

1. A microstrip low pass filter characterized by: the microstrip line wiring layer is included;

2. The microstrip low pass filter according to claim 1, wherein: the LC resonance unit comprises a first microstrip line and a first open-circuit fan-shaped stub line, a round point of the first open-circuit fan-shaped stub line is connected with one end of the first microstrip line, and the other end of the first microstrip line is connected between the adjacent folding inductance units.

3. The microstrip low pass filter according to claim 1, wherein: the folding inductance unit is of a right-angle bending structure.

4. The microstrip low pass filter according to claim 1, wherein: at least one middle folded inductor unit is connected with an open circuit unit.

5. The microstrip low pass filter according to claim 4, wherein: the middle folding inductance unit connected into the open circuit unit comprises an opening structure surrounded by three surfaces, wherein the opening structure surrounded by three surfaces comprises a first inductance arm and two second inductance arms vertically connected with the first inductance arm;

6. The microstrip low pass filter according to claim 1, wherein: and the folding inductance unit connected with the output feeder is connected in parallel with a trap unit, the trap unit comprises a second open-circuit fan-shaped stub, and the center point of the second open-circuit fan-shaped stub is connected to the folding inductance unit.

7. The microstrip low pass filter according to claim 6, wherein: the radius of the second open-circuited sector stub is set to 1/4 wavelengths of the signal corresponding to the filter's spurious passband resonant frequency.

8. A design method of a microstrip low-pass filter is characterized by comprising the following steps:

9. The design method of the microstrip low-pass filter according to claim 8, characterized in that: further comprising:

10. The design method of the microstrip low-pass filter according to claim 8, characterized in that: further comprising: