CN104078727B - Tandem type one side elliptic function line filter - Google Patents

Abstract

A series type single-sided elliptic function transmission line filter circuit proposed by the present invention is composed of transmission line elements such as input and output transmission lines, series cross joints, short-circuit transmission line stubs, cascaded transmission line sections, and series transmission line sections to form a distributed parameter network. The first series cross The joint (2) is arranged opposite to the second series cross joint (7), the opposite ports of the two are connected by a cascade transmission section (3), and the two same-side ports of the two are connected by a series transmission line section (4); The other two ports of the first series cross joint (2) are respectively connected to the input transmission line (1) and the first short-circuit transmission line stub (5), and the other two ports of the second series cross joint (7) are respectively connected to the output transmission line (8) and Second short circuit stub (6). The parameters of each transmission line element in the filter are properly designed to realize the transmission characteristics of one-sided elliptic function. The invention is suitable for the medium and lower bandwidth, low loss and high selectivity filter of microwave and millimeter wave frequency bands.

Description

Series Single-sided Elliptic Function Transmission Line Filter

technical field

The invention relates to a system widely used in microwave and millimeter wave frequency band communication, radar and measuring instruments, etc., to control the frequency response of signals, block the transmission of frequency components of useless signals, and make the frequency components of useful signals pass through almost without attenuation filter.

Background technique

Filters are passive circuits widely used in microwave circuits and systems. They are used to achieve specific frequency selection characteristics. Filters can be divided into high-pass, low-pass, band-pass and band-rejection types according to transmission characteristics. The passband of the filter Properties such as insertion loss and stop band rejection have a great influence on the performance index of the circuit. With the increasingly tight frequency resources and the increasingly crowded spectrum, the radio system puts forward higher requirements for the frequency selection characteristics of the filter, requiring a steeper transition band attenuation characteristics (or high square coefficient), low insertion loss in the passband, Flat passband group delay, large stopband bandwidth and stopband rejection, etc., and with the improvement of system miniaturization requirements, it is necessary to realize the miniaturization and integration of filters as much as possible.

In order to use the network synthesis technology to design the filter, the traditional method is to use a specific function to approximate the required filter transfer characteristics, and realize the specific transfer function through network synthesis. The function types commonly used to approximate filter transfer characteristics are maximum flat type, Chebyshev type, and elliptic function type, where the elliptic function type transfer function has transmission zeros and poles at the finite frequency of the filter's stop band, and thus improves the bandpass Out-of-band rejection performance of the filter. The elliptic function filter has a compact structure, has better transition characteristics and higher out-of-band rejection than the Chebyshev filter, and can achieve superior frequency selection performance. However, the circuit implementation is complicated, and it is usually implemented by introducing cross-coupling between non-adjacent resonant cavities. Although the synthesis and design of this filter with cross-coupling have been widely studied, its processing and debugging are difficult, which limits its engineering application.

In order to achieve good frequency selection characteristics near the working frequency band, band-pass filters and band-stop filters are mainly used at present. However, band-pass filters with steep transition band attenuation characteristics generally require multi-stage filters. Multi-stage filters are composed of a Composed of a series of mutually coupled resonators, the insertion loss is large, the in-band group delay and its fluctuation increase, and the volume is large. Multi-stage band-stop filters have the same disadvantages. A filter with a single high-Q resonant circuit can achieve lower insertion loss, but this single-loop filter has a narrow bandwidth, which is difficult to meet the requirements of practical applications.

In many applications of filters, more attention is paid to the frequency selection characteristics near the operating frequency band of the system, and the requirements for low insertion loss and miniaturization of filters are more important. Typical applications include preselectors, duplexers or multiplexers, A filter for suppressing specific clutter components of the transmit signal and a filter for suppressing transmit leakage at the receive front end. In most cases, these filters only require steep transition band characteristics on one side of the passband, moderate passband and stopband widths, but require insertion loss as low as possible to reduce system loss, and should have a simple structure and The characteristics of miniaturization are convenient for processing and system integration.

Contents of the invention

The purpose of the present invention is to address the shortcomings and application requirements of the above-mentioned prior art, to provide a circuit with simple form, easy integration, miniaturization, moderate passband and stopband bandwidth, and easy to achieve low insertion loss in the passband. One-sided elliptic function transmission characteristic transmission line filter with excellent frequency selectivity.

In order to achieve the above object, the present invention proposes a series-type single-sided elliptic function transmission line filter, including transmission line elements such as input and output transmission lines, series cross joints, short-circuit transmission line stubs, cascaded transmission line sections and series transmission line sections, characterized in that: The above-mentioned transmission line elements all adopt various high-frequency transmission line forms including parallel double wires, and the series cross connector is a four-port transmission line element, and its function is to realize any four transmission lines of the same type connected in series; in the filter circuit, the input The output transmission line, the series cross joint, the short-circuit transmission line stub, the cascaded transmission line section and the series transmission line section transmission line elements form a distributed parameter network, wherein the first series cross joint 2 and the second series cross joint 7 are arranged oppositely, and the ports between the two opposite ports The two ports on the same side of the two are connected through a series transmission line node 4; the other two ports of the first series cross joint 2 are respectively connected to the input transmission line 1 and the first short-circuit transmission line stub 5, and the second series connection The other two ports of the cross connector 7 are respectively connected to the output transmission line 8 and the second short-circuit transmission line stub 6 . The distributed parameter two-port network with the above connection relationship has multiple resonance characteristics. If the parameters of the transmission line components are properly designed, the network will generate a set of non-zero finite frequency transmission zeros and a set of non-zero finite frequency reflection zeros in the filter operating frequency band. The two are adjacent but not overlapped on the frequency axis, so they have one-sided elliptic function filtering characteristics.

Compared with the prior art, the present invention has the following beneficial effects:

The form of the filter circuit is simple, the operating frequency can reach above the millimeter wave frequency band, and it is easy to realize integration. The filter proposed by the invention only involves basic transmission line elements, has excellent high-frequency performance, and the filter circuit can be accurately modeled and simulated, and is easy to design and implement. There are no restrictions on the transmission line form of the transmission line components used; the series cross joint can adopt various deformation forms such as cascading two series T-shaped joints; the short-circuit transmission line stub in the filter can adopt the form of an open transmission line stub whose electrical length increases or decreases by 90°; The transmission line elements can be in the form of straight lines, broken lines, spiral lines or combinations thereof, and multi-layer dielectric substrates are used to integrate transmission lines and interconnected between layers, which can reduce the size of the filter. At the same time, the filter circuit only uses transmission line components, which have the characteristics of small parasitic parameters, low loss and excellent high-frequency characteristics, so that the circuit can be applied to realize low-loss filters above the millimeter wave frequency band. Therefore, the operating frequency of the filter can reach the frequency band above the millimeter wave. The filter can be in the form of a planar transmission line, which is easy to manufacture and realize filter integration.

The filter has one-sided elliptic function filtering characteristics, moderate bandwidth and low insertion loss in the passband. And the present invention forms a symmetrical two-port distributed parameter network with multiple resonance characteristics through transmission line elements, and the cut-off frequency f _c of the transmission coefficient is near the resonance frequency f _r of the stub of the short-circuit transmission line. By selecting the impedance and electrical length parameters of each transmission line element, more than two non-zero finite frequency reflection zeros with frequencies lower or higher than f _c can be generated, and filter passbands can be formed in the frequency bands around them; two or more can be generated at the same time The non-zero finite frequency transmission zero point whose frequency is higher or lower than f _c and forms a filter stop band in the frequency band around it, so as to realize the low-pass or high-pass filtering characteristic of one-sided elliptic function near f _c . Since the passband and the stopband have more than two reflection zeros and transmission zeros respectively, they have 10% to 20% bandwidth of the passband and stopband. At the same time, since the filter circuit of the present invention has no complicated coupling structure, and the adopted simple transmission line element itself has low loss characteristics, it is easy to realize low insertion loss in the passband.

Filters are easy to miniaturize. Its physical structure is very compact, and the circuit area is less than 1/4 of the interdigitated filter with the same selectivity and the same transmission line form, and it is also more compact than other elliptic function filters.

This filter has excellent selectivity, the attenuation pole in the stop band and the transmission pole in the pass band can be controlled separately, and a very steep frequency-selective characteristic curve can be realized near the cut-off frequency. This filter has a moderate passband and stopband bandwidth of 10-20%, low in-band insertion loss, and is suitable for microwave and millimeter-wave frequency bands with lower than medium bandwidth, low loss, and high selectivity filters.

The good agreement between test results and simulation results verifies the correctness and effectiveness of this filter circuit. Tests show that this filter can not only meet the requirements of miniaturization, but also has steep transition band attenuation characteristics, moderate passband and stopband bandwidth, and has excellent filter characteristics such as low passband insertion loss. Based on the above reasons, this filter is especially suitable for high-selectivity and low-loss filters such as miniaturized and integrated duplexers, preselectors, and near-end spurious suppression filters for transmitted signals.

Description of drawings

Fig. 1 is a schematic circuit diagram of a series-type one-sided elliptic function transmission line filter of the present invention.

Fig. 2 is a schematic diagram of reflection and transmission characteristic curves of a series-type single-sided elliptic function transmission line filter using a rectangular waveguide transmission line.

In the figure: 1 input transmission line, 2 first series cross joint, 3 cascade transmission line section, 4 series transmission line section, 5 first short-circuit transmission line stub, 6 second short-circuit transmission line stub, 7 second series cross joint, 8 output transmission line.

detailed description

See Figure 1. In the embodiment described below, the series-type single-sided elliptic function transmission line filter is composed of six basic transmission line elements, including input and output transmission lines, series cross joints, short-circuit transmission line stubs, cascaded transmission line sections, and series transmission line sections. The transmission lines are expressed in the form of double lines. The specific circuit implementation can adopt various high-frequency transmission line forms including double-conductor transmission lines. The transmission line components are all in the form of various high-frequency transmission lines including parallel double lines. The series cross joint is a four-port transmission line component, and its function is to realize the series connection of any four transmission lines of the same type; in the filter circuit, input and output transmission lines, series cross joints, short-circuit transmission line stubs, cascaded transmission line sections and series transmission line section transmission line components A distributed parameter network is formed, wherein the first series cross joint 2 and the second series cross joint 7 are arranged oppositely, the opposite ports of the two are connected through a cascade transmission node 3, and the two ports on the same side of the two are connected through a series transmission line joint 4 connections; the other two ports of the first series cross joint 2 are respectively connected to the input transmission line 1 and the first short-circuit transmission line stub 5, and the other two ports of the second series cross joint 7 are respectively connected to the output transmission line 8 and the second short-circuit transmission line stub 6.

In the circuit shown in Figure 1, the input signal is fed into the first series cross connector 2 from the input transmission line 1, due to the impedance mismatch, part of it is reflected back to the input transmission line here, and the remaining signal is divided into two transmission lines, one through the series transmission line section 4. The other path passes through the cascaded transmission line node 3, and the two signals are synthesized at the second series cross connector 7, and at the same time, some signals are reflected again due to mismatch, and the remaining synthesized signal is output from the output transmission line 8. The first short-circuit transmission line stub 5 and the second short-circuit transmission line stub 6 are used to adjust the amplitude and phase of the reflected signal at the input and output ends.

The initial parameters can be selected as follows: at a given filter cut-off frequency fc, the cascaded transmission line section ₃ has an electrical length less than 180°, the series transmission line section 4 has an electrical length greater than 270° but less than 360°, the first short circuit The transmission line stub 5 and the second short-circuit transmission line stub 6 are the same, and both have an electrical length greater than 180° but less than 360°. By selecting the electrical length of the transmission line element in the above range and designing the proper characteristic impedance of the transmission line element, the filter can realize the transmission characteristic of the one-sided elliptic function near _fc .

In the filter circuit, the parameters of the first short-circuited transmission line stub 5 and the second short-circuited transmission line stub 6 are the same, and their electrical lengths reach 270° at the frequency f _s1 of the transmission zero point, which is equivalent to a series open circuit and forms a transmission zero point. The input signal is fed into the first series cross joint 2 from the input transmission line 1, part of the signal is reflected here, and the rest of the signal is divided into two transmissions, one through the series transmission line node 4, and the other through the cascaded transmission line node 3, at the frequency transmission zero point On f _s2 , the transmission coefficients of the two paths are equal in magnitude and opposite in phase, and the transmitted signals of the two paths cancel each other out at the second series cross connector 7 to generate a second transmission zero.

When the frequency of the input signal is lower than the transmission zero point f _s1 , the electrical lengths of the first short-circuit transmission line stub 5 and the second short-circuit transmission line stub 6 are between 180° and 270°, which are equivalent to inductive series reactance, while the cascaded transmission line stub 3 and the series The combination of the transmission line section 4 is equivalent to a parallel capacitor, and the combination of these four elements forms a T-shaped low-pass network, which can be equivalent to a right-hand transmission line section. Properly select the parameters of the transmission line, match the impedance of the right-hand transmission line section with the input transmission line 1 and the output transmission line 8 on at least two non-zero frequencies f _p1 and f _p2 less than the transmission zero point f _s1 , form two reflection zero points, and make f _p 1<f _p2 <fs _2< f _s1 , the passband of the filter is formed in the frequency band near f _p1 and f _p2 , and the stop band of the filter is formed in the frequency band near f _s1 and f _s1 , and the pass band and stop band are close to f _s1 Adjacent to the cut-off frequency f _c , so that the filter has a one-sided elliptic function near f _c and low-pass characteristics.

When the input signal frequency is higher than the transmission zero point f _s1 , the electrical lengths of the first short-circuit transmission line stub 5 and the second short-circuit transmission line stub 6 are between 270° and 360°, which are equivalent to capacitive series reactance, while the cascaded transmission line stub 3 and The combination of the series transmission line section 4 is equivalent to a parallel inductance. The combination of these four elements forms a T-shaped high-pass network, which can be equivalent to a left-hand transmission line section. With appropriate selection of transmission line parameters, at least two finite frequencies higher than the transmission zero point f _s1 On f _p3 and f _p4 , the impedance of the left-hand transmission line section is matched with the impedance of input transmission line 1 and output transmission line 8 to form two finite frequency reflection zero points, and make f _s1 <f _s2 <f _p3 <f _p4 . The passband of the filter is formed in the frequency band near f _p1 and f _p2 , and the stop band of the filter is formed in the frequency band near f _s1 and f _s1 . The pass band and the stop band are adjacent at the cut-off frequency f _c close to f _s1 , so that the filter High-pass transfer characteristics with one-sided elliptic function near _fc . .

The circuit design has no restrictions on the transmission line form of the transmission line element used; the first series cross joint 2 and the second series cross joint 7 can adopt various deformation forms such as cascading two series T-shaped joints; the transmission line element can adopt straight line, broken line, spiral Shaped lines or their combination forms, as well as the use of multilayer dielectric substrates to integrate transmission lines and interconnection between layers, etc., to reduce the size of the filter.

Figure 2 describes the reflection and transmission characteristics of a typical metal rectangular waveguide series single-sided elliptic function transmission line filter circuit. From the figure, it can be clearly seen that there are three transmission zeros in the stop band and three reflection zeros in the pass band .

The concrete implementation of the present invention can adopt following steps:

First of all, according to the system application requirements, select the appropriate transmission line implementation form. Considering the integration of filters, planar transmission lines such as coplanar strip lines and slot lines can be selected, and low-loss transmission lines such as waveguides should be selected in consideration of low loss, but it is necessary to consider using the same transmission line form as the pre-stage or post-stage circuit to avoid The increase in volume size brought about by the transmission mode conversion and the introduction of additional losses. Secondly, according to the application requirements, determine the cut-off frequency _{f c} , passband and stopband bandwidth, the passband and stopband bandwidth are normalized by fc to relative bandwidth, and its value should be less than 20%.

Using the microwave circuit computer-aided design software, establish the corresponding filter circuit, set the required transmission characteristic design target, and determine the transmission line parameters of each component through the optimization design program of the software. The filter design can also be completed by manual iterative calculation, the method is as follows: set the two transmission zeros f _s1 and f _s2 in the filter stop band, and the two reflection zeros in the pass band are f p1 and f _p1 for the left elliptic function filter f _p2 , f _p3 , f _p4 for the right elliptic function filter. The equations are established according to the conditions for forming transmission zero point and reflection zero point described in the working principle of the filter, and the impedance and electrical length of each transmission line element are obtained by solving.

After determining the design parameters, the structural model of the filter is established in the electromagnetic simulation software environment, and the final design verification is carried out through electromagnetic field simulation.

Claims (9)

1. a tandem type one side elliptic function line filter, including following transmission line element: input, export transmission line, Series connection cruciform joint, short-circuited transmission line minor matters, cascaded transmission line joint and series transmission lines joint, it is characterised in that: described transmission line Element all uses the high frequency transmission line form including parallel wire, described series connection cruciform joint to be a kind of four port transmission lines Element, its function is connected in series with type transmission line for realizing any four；In filter circuit, input transmission line passes with output Defeated line, series connection cruciform joint, short-circuited transmission line minor matters, cascaded transmission line joint and series transmission lines joint transmission line element constitute distribution Parameter two-port network, wherein, the first series connection cruciform joint (2) is connected with second cruciform joint (7) arranged opposite, and the two is relative Port between by cascaded transmission joint (3) connect, the two two with side ports by series transmission lines joint (4) connect；The One series connection cruciform joint (2) another two port connects input transmission line (1) and the first short-circuited transmission line minor matters (5) respectively, and second The another two port of series connection cruciform joint (7) connects output transmission line (8) and the second short-circuited transmission line minor matters (6) respectively, and first Short-circuited transmission line minor matters (5), the second short-circuited transmission line minor matters (6) are in frequency f_s1Upper electrical length reaches 270, is equivalent to shorted to earth, Form transmission zero f_s1；The distributed constant two-port network with above-mentioned annexation has multiple resonance characteristic, this net Network will produce one group of non-zero finite frequency transmission zero and one group of non-zero finite frequency hop zero point in wave filter working band, The two does not overlaps adjoining on the frequency axis, thus has unilateral elliptic function filter characteristic.

Tandem type one side the most according to claim 1 elliptic function line filter, it is characterised in that: in given filter Ripple device cut-off frequency f_cOn, cascaded transmission line joint (3) has less than 180^oElectrical length, series transmission lines joint (4) has and is more than 270^oBut less than 360^oElectrical length, the first short-circuited transmission line minor matters (5) are identical with the second short-circuited transmission line minor matters (6), and electricity is long Degree is all higher than 180^oBut less than 360^o 。

Tandem type one side the most according to claim 1 elliptic function line filter, it is characterised in that: input signal exists After first series connection cruciform joint (2) place part is reflected, remaining signal is divided into two-way, respectively via cascaded transmission line joint (3) and Series transmission lines joint (4) two paths transmission, in frequency f_s2On, the transmission coefficient amplitude of this two paths is equal, opposite in phase, Two paths of signals after transmission is cancelled out each other at the second series connection cruciform joint (7) place, produces second transmission zero f_s2。

Tandem type one side the most according to claim 3 elliptic function line filter, it is characterised in that: frequency is less than passing Defeated zero point f_s1Time, the first short-circuited transmission line minor matters (5) and the second short-circuited transmission line minor matters (6) electrical length 180～270 it Between, it is equivalent to inductance series reactor, and is equivalent to shunt capacitance after cascaded transmission line joint (3) and series transmission lines joint (4) combination, These four element combinations form T-shaped low-pass network, can be equivalent to a right-handed transmission line joint.

Tandem type one side the most according to claim 4 elliptic function line filter, it is characterised in that: suitably select to pass Defeated line parameter, less than transmission zero f_s1Two nonzero frequency f_p1、f_p2On make this right-handed transmission line joint impedance with input transmission Line (1) and output transmission line (8) coupling, form two reflection zeros, and make f_p1<f_p2<f_s2<f_s1, at f_p1、f_p2Neighbouring frequency band Shaping filter passband, simultaneously at f_s1、f_s1Neighbouring frequency band shaping filter stopband, passband and stopband are close to f_s1Cut-off Frequency f_cUpper adjacent, so that wave filter has f_cNeighbouring right side elliptic function transmission characteristic.

Tandem type one side the most according to claim 1 elliptic function line filter, it is characterised in that: input signal frequency Rate is higher than transmission zero f_s1Time, the first short-circuited transmission line minor matters (5) and the second short-circuited transmission line minor matters (6) electrical length 270～ Between 360, it is equivalent to capacitive series reactance, and is equivalent to also after cascaded transmission line joint (3) and series transmission lines joint (4) combination Connection inductance, these four element combinations form T-shaped high pass network, are equivalent to a left hand transmission line joint.

Tandem type one side the most according to claim 6 elliptic function line filter, it is characterised in that: suitably select to pass Defeated line parameter, can be higher than transmission zero f_s1Two reflection zero frequencies f_p3、f_p4On make this left hand transmission line joint impedance with defeated Enter transmission line (1) and output transmission line (8) impedance matching, form two finite frequency reflection zeros, and make f_s1<f_s2<f_p3< f_p4At f_p1、f_p2Neighbouring frequency band shaping filter passband, simultaneously at f_s1、f_s1Neighbouring frequency band shaping filter stopband, passband and Stopband is close to f_s1Cut-off frequency f_cUpper adjacent, so that wave filter has f_cNeighbouring left side elliptic function transmission spy Property.

Tandem type one side the most according to claim 1 elliptic function line filter, it is characterised in that: have two in stopband Individual or above non-zero finite transmission zero, has two or more non-zero finite reflection zeros in passband.

Tandem type one side the most according to claim 1 elliptic function line filter, it is characterised in that: set wave filter Two transmission zeros f in stopband_s1、f_s2, two reflection zeros in passband are f for left side elliptic function filter_p1、f_p2, right Right side elliptic function filter is f_p3、f_p4。