CN111740190A - Broadband band-stop filter based on transmission line parallel multi-section open circuit stub line and design method thereof - Google Patents
Broadband band-stop filter based on transmission line parallel multi-section open circuit stub line and design method thereof Download PDFInfo
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Abstract
The invention discloses a broadband band-stop filter based on a transmission line parallel multi-section open circuit stub and a design method thereof, wherein the filter comprises: two source end impedance units; n sections of first transmission lines are connected in series between the two source end impedance units; a first connection region is formed between the source end impedance and the adjacent first transmission line, and a second connection region is formed between two adjacent sections of the first transmission lines; two sets of first open-circuit stubs are arranged in one-to-one correspondence to the first connection regions, and one ends of the first open-circuit stubs are connected to the first connection regions; one or more groups of second open-circuit stub lines which are arranged in one-to-one correspondence with the second connection regions, and one ends of the second open-circuit stub lines are connected to the second connection regions; wherein the first open stub comprises M sections of a second transmission line connected in series, and the second open stub comprises P sections of a third transmission line connected in series; and satisfies: n is more than or equal to 2, M is more than or equal to 1, P is more than or equal to 1, and M is more than or equal to P.
Description
Technical Field
The invention relates to the technical field of manufacturing of microstrip line devices of radio frequency circuits, in particular to a broadband band-stop filter based on a transmission line connected with a plurality of open-circuit stub lines in parallel and a design method thereof.
Background
Band-stop filters are widely used as basic building blocks for radio frequency/microwave devices. Its main function is to make the signal in a given frequency band unable to pass (or be attenuated or suppressed), while in the remaining frequency range the signal passes well. Therefore, the band-stop filter is mainly used for eliminating single interference frequency, and has wide application in the technical field of communication. With the rapid development of modern wireless communication technology, the demand for band stop filters has increased rapidly. At the same time, the performance requirements for band stop filters are also increasing.
This makes wideband bandstop filters (WBSF) a hot issue of research, and a good wideband bandstop filter needs to have several good features: 1. the achievable bandwidth range of the stop band is large; 2. the pass-band loss is small; 3. the stop band signal suppression effect is good; 4. the selectivity is good.
At present, there are two main methods for designing and implementing a band-stop filter: one method is to implement a band-stop filter based on a coupled line structure, and the other is a conventional method, in which an open stub is inserted between lateral transmission lines to construct a band-stop filter.
The band-stop filter constructed by using the coupled line structure in the first method has the advantage that more transmission zeros can be generated, but the band-stop filter generally has a narrower band-stop bandwidth and a more complex circuit structure, so that the performance of the filter is limited. The second traditional design method is simple in circuit structure and convenient to implement, but a basic structure of a classical transverse transmission line with a single open stub inserted between the transmission lines cannot generate a new transmission zero except for a central frequency, and other similar design methods do not have comprehensive method guidance to generate quantitative transmission zero and reflection zero.
Disclosure of Invention
One of the objectives of the present invention is to provide a broadband band-stop filter based on a transmission line connecting multiple open-circuit branch lines in parallel, which can quantitatively increase the number of transmission zeros and reflection zeros, and has a wider stop band bandwidth and better roll-off characteristics.
The invention also aims to provide a design method of the broadband band-stop filter based on the transmission line parallel multi-section open-circuit branch lines, which calculates and determines the impedance value of each section of transmission line according to the circuit transmission function and the constraint condition and can ensure that the designed filter meets the required performance requirement.
The technical scheme provided by the invention is as follows:
a broadband band-stop filter based on a transmission line parallel multi-section open circuit stub comprises:
two source end impedance units;
n sections of first transmission lines are connected in series between the two source end impedance units;
a first connection region is formed between the source end impedance and the adjacent first transmission line, and a second connection region is formed between two adjacent sections of the first transmission lines;
two sets of first open-circuit stubs are arranged in one-to-one correspondence to the first connection regions, and one ends of the first open-circuit stubs are connected to the first connection regions;
one or more groups of second open-circuit stub lines which are arranged in one-to-one correspondence with the second connection regions, and one ends of the second open-circuit stub lines are connected to the second connection regions;
wherein the first open stub comprises M sections of a second transmission line connected in series, and the second open stub comprises P sections of a third transmission line connected in series;
and satisfies: n is more than or equal to 2, M is more than or equal to 1, P is more than or equal to 1, and M is more than or equal to P.
A design method of a broadband band-stop filter based on a transmission line connected with a plurality of open-circuit branch lines in parallel comprises the following steps:
step one, determining the number of sections N of a first transmission line, the number of sections M of a second transmission line, the number of sections P of a third transmission line, a stop band rejection SR and a return loss RL in a filter circuit according to the actual circuit size and performance requirements;
step two, calculating an integral transmission matrix of the filter circuit, and obtaining a transmission function of the filter circuit according to the integral transmission matrix;
and step three, determining the constraint condition of the transmission function, and determining the impedance values of the first transmission line, the second transmission line and the third transmission line according to the transmission function and the constraint condition.
Preferably, in the second step, the overall transmission matrix of the filter circuit is:
wherein,is the overall transmission matrix of the first set of open truncates,is the overall transmission matrix of the second open stub set,a transmission matrix of N sections of the first transmission line.
Preferably, in the second step, the transfer function of the filter circuit is obtained by the following relation:
wherein S is21Is the forward transfer function of the filter circuit, AT、BT、CTAnd DTRespectively, elements of the overall transmission matrix of the filter circuit.
Preferably, when P ═ 1,
wherein θ is the electrical length of the transmission line, and all the open-circuit stub lines and the transverse transmission line have the same electrical length; n is the number of the first transmission line; m is the number of the second transmission line; w is a characteristic constant function;andis a factor characteristic impedance function; h isM,NIs the overall characteristic impedance function.
Preferably, in the third step, the constraint condition of the transfer function is:
wherein D is1、D2… … are the poles of the S function of the filter circuit;… … is the input reflection function S of the filter circuit11The electrical length at each of the pole points,… … is the forward transfer function S of the filter circuit21Electrical length at each pole; RL is the return loss of the circuit and SR is the stop-band rejection of the circuit;RLis S11The height of the corrugations of (a) is,SRis S21The height of the corrugations.
The invention has the beneficial effects that:
the broadband band-stop filter based on the transmission line parallel multi-section open-circuit branch line can quantitatively increase the number of transmission zero points and reflection zero points, and has wider stop band bandwidth and better roll-off characteristic.
According to the design method of the broadband band-stop filter based on the transmission line parallel multi-section open circuit stub, the impedance value of each section of transmission line is calculated and determined according to the circuit transmission function and the constraint condition, and the designed filter can meet the required performance requirement.
Drawings
Fig. 1 is a circuit model diagram of a broadband band-stop filter based on a transmission line connected with a plurality of open-circuit branch lines in parallel according to the present invention.
Fig. 2 is a circuit model diagram of a wideband band-stop filter in embodiment 1 of the present invention.
Fig. 3 is a circuit model diagram of a wideband band-stop filter in embodiment 2 of the present invention.
Fig. 4 is an ADS simulation diagram of the circuit model in embodiment 1 of the present invention.
Fig. 5 is an ADS simulation diagram of a circuit model in embodiment 2 of the present invention.
Fig. 6 is a circuit model diagram of a wideband band-stop filter in embodiment 3 of the present invention.
Fig. 7 is an ADS simulation diagram of a circuit model according to embodiment 3 of the present invention.
Detailed Description
The present invention is further described in detail below with reference to the attached drawings so that those skilled in the art can implement the invention by referring to the description text.
The invention provides a broadband band elimination filter based on a transmission line parallel multi-section open circuit branch line, wherein the filter is of a symmetrical structure and is provided with multiple transmission zeros and multiple reflection zeros. As shown in fig. 1, the broadband band-stop filter based on the transmission line parallel multi-section open-circuit stub comprises: two source end impedances Z0(equivalent of two load resistors) and a middle transverse N-section transmission line (Z)1、Z2… …) and N +1 sets of open stubs (Z) interspersed between the transverse transmission liness11、Zs12、…Zs1M,Zs21、Zs22、…Zs2P) (ii) a The number of the transmission line nodes in the open stub needs to be selected according to the practical working requirement and the rule given in the invention. Because the designed broadband band-stop filter is of a symmetrical structure, only the left half part of the circuit is marked. The impedance values of the two source end impedance units are Z0(ii) a The impedance values of the N sections of transverse first transmission lines are respectively as follows: z1,Z2,Z3… …; the impedance values of the second transmission lines in series connection with M sections in the first open circuit cutoff group are respectively as follows: zs11,Zs12,Zs13,……,Zs1M(ii) a The impedance values of the third transmission lines in series with P sections in the second open circuit intercept group are respectivelyComprises the following steps: zs21,Zs22,Zs23,……,Zs2P。
For further discussion of the influence of M, N, and P on the circuit structure and the circuit performance, and for convenience of circuit calculation and simplification of the circuit structure as much as possible to explain the most basic rule, the number of transmission line nodes in the (N-1) groups of open-circuit stub sets except the open-circuit stub on the leftmost side and the rightmost side is first selected as one node, that is, P is 1. And analyzing the influence of the M and N changes on the circuit structure and performance to obtain the rule of increasing the transmission zero point and the reflection zero point.
When M is fixed and N is continuously increased, the number of transmission zero points is unchanged, the number of reflection zero points is continuously increased, and the number of reflection zero points is 2(N +1), which is only related to N, namely, the reflection zero points can be regularly increased by increasing the number of sections of the transverse transmission line, and meanwhile, the transverse size of the circuit is increased.
When N is fixed and M is continuously increased, the number of reflection zero points is unchanged, the number of transmission zero points is continuously increased, and the number of transmission zero points is (M-W +1), where W is related to the value of M only, and W ═ sgn (MOD (M, 2)). That is, increasing the number of sections of the longitudinal transmission line can regularly increase the transmission zero point while the longitudinal size of the circuit increases.
M and N are unchanged, and when only P is increased, transmission zero points can also be increased, and every time P is increased by 1, the transmission zero points are symmetrically increased by 2, and the number of reflection zero points is unchanged.
Example 1(M ═ 4, N ═ 2, P ═ 1)
As shown in fig. 2, the source end impedance Z of the left end0One end of which is connected with the leftmost section Z of the transverse transmission line1One end of the transmission line Z and the first section of the transmission line Z of the leftmost open circuit intercept groups11And the other end is grounded; transmission line Zs11Is connected to the second transmission line Z in the leftmost open-circuit cut-off groups12One end of (a); transmission line Zs12Is connected to the third transmission line Z in the leftmost open-circuit cut-off line groups13One end of (a); transmission line Zs13Is connected to the fourth transmission line Z in the leftmost open-circuit cut-off line groups14(ii) a First section transverse transmission line Z1Is connected to a second set of open stubs Zs2And a second section of transverse transmission line Z1One end of (circuit left-right mirror symmetry); second section transverse transmission line Z1The other end of the transmission line Z is connected with the first section of the transmission line Z of the rightmost open circuit cut-off groups11And source end impedance Z of right end0One end of (circuit left-right mirror symmetry); source end impedance Z of right end0The other end is grounded; transmission line Zs11The other end of the transmission line Z is connected with the second section of transmission line Z in the rightmost open-circuit cut-off line groups12One end of (a); transmission line Zs12Is connected with the third transmission line Z in the rightmost open-circuit cut-off line groups13One end of (a); transmission line Zs13Is connected to the fourth transmission line Z in the leftmost open-circuit cut-off line groups14(circuit left-right mirror symmetry).
Example 2(M ═ 2, N ═ 4, P ═ 1)
As shown in fig. 3, the source end impedance Z of the left end0One end of which is connected with the leftmost section Z of the transverse transmission line1One end of the transmission line Z and the first section of the transmission line Z of the leftmost open circuit intercept groups11And the other end is grounded; transmission line Zs11Is connected to the second transmission line Z in the leftmost open-circuit cut-off groups12(ii) a First section transverse transmission line Z1Is connected to a second set of open stubs Zs2And a second section of transverse transmission line Z2One end of (a); second section transverse transmission line Z2Is connected with the third group of open stub Zs3Transverse to the third section of the transmission line Z2One end of (circuit left-right mirror symmetry); third section transverse transmission line Z2Is connected with the fourth group of open stub Zs2And a fourth section of transverse transmission line Z1One end of (circuit left-right mirror symmetry); fourth section transverse transmission line Z1The other end of the transmission line Z is connected with the first section of the transmission line Z of the rightmost open circuit cut-off groups11Is connected to the source impedance Z0One end of (circuit left-right mirror symmetry); source end impedance Z0The other end is grounded; transmission line Zs11The other end of the transmission line Z is connected with the second section of transmission line Z in the rightmost open-circuit cut-off line groups12(circuit left-right mirror symmetry).
Example 3(M ═ 3, N ═ 2, P ═ 2)
As shown in fig. 6Source end impedance Z of left end0One end of which is connected with the leftmost section Z of the transverse transmission line1One end of the transmission line Z and the first section of the transmission line Z of the leftmost open circuit intercept groups11And the other end is grounded; transmission line Zs11Is connected to the second transmission line Z in the leftmost open-circuit cut-off groups12One end of (a); transmission line Zs12Is connected to the third transmission line Z in the leftmost open-circuit cut-off line groups13(ii) a First section transverse transmission line Z1The other end of the first transmission line Z is connected with the second group of open stub liness21One end of (2) and a second section of transverse transmission line Z1One end of (circuit left-right mirror symmetry); transmission line Zs21Is terminated with a second section of transmission line Z in a second set of open stubss22(ii) a Second section transverse transmission line Z1The other end of the transmission line Z is connected with the first section of the transmission line Z of the rightmost open circuit cut-off groups11Source impedance Z of one end and the right end of0(circuit left-right mirror symmetry); source end impedance Z of right end0The other end is grounded; transmission line Zs11The other end of the transmission line Z is connected with the second section of transmission line Z in the rightmost open-circuit cut-off line groups12One end of (a); transmission line Zs12Is connected with the third transmission line Z in the rightmost open-circuit cut-off line groups13(circuit left-right mirror symmetry).
The invention also provides a design method of the broadband band-stop filter based on the transmission line parallel multi-section open circuit stub, which comprises the following steps:
step one, determining the working center frequency F required by a filter circuit according to the working occasion of the filter0Bandwidth of the pass band and the stop band, and stop band rejection SR and return loss RL.
And step two, selecting a proper number N of transverse transmission line sections according to requirements, namely selecting the number P of transmission line sections in other open circuit section line groups except the open circuit section line group at the two sides and the number M of transmission line sections in the open circuit section line groups at the two sides. As can be seen from the above-mentioned rules, M and P determine the number of transmission zeros. When N and P are unchanged, every time M is increased by 2, two transmission zero points are symmetrically increased; when M and N are not changed, every time P is increased by 1, the transmission zero point is symmetrically increased by two. And N determines the number of the reflection zeros, and when M and P are kept unchanged, N is increased by 1 every time, and the reflection zeros are symmetrically increased by two. Meanwhile, when M, N and P are changed, the circuit structure is changed, the number of impedances is changed, the number of degrees of freedom is different, and the number of degrees of freedom needs to meet the proposed performance requirements. In addition, the circuit size selection M, N, P can be used to determine the specific circuit structure.
And step three, calculating the ABCD matrix and the transmission function of the circuit.
The ABCD matrix of the ith transverse transmission line in the filter circuit structure is as follows:
the ABCD matrix of the single-section open-circuit stub is as follows:
the ABCD matrix of the M open circuit intercept groups is as follows:
the ABCD matrix of the entire filter circuit structure is:
wherein,a transmission matrix that is a first open stub,a transmission matrix that is a second open stub,a transmission matrix of N sections of the first transmission line. i is a natural number 0, 1, … …;the filter is a polynomial only consisting of all normalized characteristic impedances of the filter, and different M and i values correspond to different polynomials; gMAlso a polynomial consisting of only all normalized characteristic impedances of the filter, different M corresponding to different polynomials.
Wherein,S21for the forward transmission coefficient in the circuit structure, stop-band rejection is generally referred to in band stop filters, and S is the case of equal ripple21The value at the ripple extreme point is SR; s11Is the input reflection coefficient in the circuit structure, also called input return loss, which is S in the case of equiripple11The value at the extreme point of ripple is RL; fcircuitIs S11And S21By the ratio function of FcircuitCan simplify the calculation and conveniently obtain S11And S21The pole zero of (2).
When P is 1, a unified expression can be generalized, and given here, M and N can be directly substituted by:
wherein i is a natural number 0, 1, … …;b2i,the filter is a polynomial only consisting of all normalized characteristic impedances of the filter, and different M, N and i values correspond to different polynomials; a isM,N,bM,NAnd cM,NAlso a polynomial consisting of only all normalized characteristic impedances of the filter, different M, N corresponding to different polynomials.
Wherein,and hM,NIs a polynomial determined by all normalized characteristic impedances of the filter.
And step four, enabling the obtained circuit transfer function to meet constraint conditions so as to meet the required performance requirements. If the calculation of the third step is combined according to the given theoretical analysis; assuming that the forward transfer function S of the circuit is obtained21With 2m equiripples, the forward transfer function of the circuit is input to the reflection function S11There are 2N equal ripples (the number of M and N is determined by the value of M and N), then:
according to the transmission function F calculated in the step threecircuitThe method comprises the following steps:
wherein D is1、D2… … are the poles of the S function of the filter circuit;… … is the input reflection function S of the filter circuit11The electrical length at each of the pole points,… … is the forward transfer function S of the filter circuit21Electrical length at each pole; RL is the return loss of the circuit and SR is the stop-band rejection of the circuit;RLis S11The height of the corrugations of (a) is,SRis S21The height of the corrugations. The set of equations gives the control S11And S21The ripple height at the ripple extreme point makes it meet the requirements of return loss and stop band suppression.
And step five, searching a proper group of transmission line impedance values according to the impedance constraint condition so as to obtain the required band-stop filter.
And sixthly, calculating the actual parameters of the device by using electromagnetic field simulation software to obtain the corresponding line width and line length, then performing electromagnetic field simulation by using Sonnet software, and adjusting the parameters to obtain the optimal waveform. And then manufacturing an actual device according to the optimal data obtained by simulation.
Under the requirement of different circuit performances, other different circuit structures and circuit performance effects can be obtained by repeating the same six steps.
Three circuit configurations are selected for illustration, which are representative:
in example 1 (as shown in fig. 2), the center frequency of the filter circuit was determined to be 1GHz, the stop band rejection SR was determined to be 25dB, and the return loss RL was determined to be 20 dB. To facilitate bandwidth calculation, define θcIs S11And S21The electrical length of the frequency at the equal ripple dB value can be used to measure the equal ripple bandwidth, for S11In other words, θcThe larger the passband bandwidth; to S21In other words, θcThe smaller the stopband bandwidth. In the case of the example 1, the following,is at an angle of 25 DEG,at 21.5 deg., M is 4, N is 2, and P is 1.
The unified expression when M is 4, N is 2, and P is 1 is given as:
a4,2=Zs11Zs2;
b0 up=Z1;
b2 up=Z1-2Zs2;
c8 up=(Z1+2Zs2)(Z1+Zs11)2(Zs11+Zs12)2(Zs12+Zs13)2(Zs13+Zs14)2;
c4 down=-(Zs11+Zs12)(Zs12+Zs13)(Zs13+Zs14)。
wherein, a4,2,b4,2,c4,2Is the overall characteristic impedance coefficient; is a factor of the characteristic impedance coefficient; zs11,……,Zs14A second transmission line impedance value Z of M series in the first open-circuit cutoff groups21A third transmission line impedance value, Z, of the P-node series connection in the second open-circuit stub set1Is the transverse first transmission line impedance value.
Calculating in MATLAB according to the previous steps, verifying whether the obtained characteristic impedance value of the model can generate expected ripples in ADS software after obtaining a proper impedance value group, wherein the specific circuit normalization parameters are as shown in Table 1:
table 1 table of parameters of filter in example 1
The simulation result of the circuit ADS obtained under the set of impedance values is shown in fig. 4.
In the implementation ofIn example 2 (as shown in fig. 3), when M is 2 and N is 4, the filter has 3 transmission zeros and 10 reflection zeros; passband bandwidth: thetac s1136.1 °, stop band bandwidth: theta c s2140 °; the stop band rejection is 50dB and the return loss is 20 dB. The circuit has good stop band suppression and more reflection zeros. The normalization parameters in the circuit are shown in table 2:
table 2 parameter table of filter in example 2
Under the set of impedance values, the simulation result of the circuit ADS is shown in FIG. 5.
In example 3 (as shown in fig. 6), when M is 3 and N is 2, the filter has 5 transmission zeros and 6 reflection zeros; passband bandwidth: thetac s1127.3 °, stop band bandwidth: thetac s2135 °; the stop band rejection is 30dB and the return loss is 20 dB. The circuit has more transmission zero points and ensures S21Is equi-corrugated. The normalization parameters in the circuit are shown in table 3:
table 3 parameter table of filter in example 3
Under the set of impedance values, the simulation result of the circuit ADS is shown in FIG. 7.
While embodiments of the invention have been described above, it is not limited to the applications set forth in the description and the embodiments, which are fully applicable in various fields of endeavor to which the invention pertains, and further modifications may readily be made by those skilled in the art, it being understood that the invention is not limited to the details shown and described herein without departing from the general concept defined by the appended claims and their equivalents.
Claims (6)
1. A broadband band-stop filter based on a transmission line parallel multi-section open circuit stub is characterized by comprising:
two source end impedance units;
n sections of first transmission lines are connected in series between the two source end impedance units;
a first connection region is formed between the source end impedance and the adjacent first transmission line, and a second connection region is formed between two adjacent sections of the first transmission lines;
two sets of first open-circuit stubs are arranged in one-to-one correspondence to the first connection regions, and one ends of the first open-circuit stubs are connected to the first connection regions;
one or more groups of second open-circuit stub lines which are arranged in one-to-one correspondence with the second connection regions, and one ends of the second open-circuit stub lines are connected to the second connection regions;
wherein the first open stub comprises M sections of a second transmission line connected in series, and the second open stub comprises P sections of a third transmission line connected in series;
and satisfies: n is more than or equal to 2, M is more than or equal to 1, P is more than or equal to 1, and M is more than or equal to P.
2. A design method of a broadband band-stop filter based on a transmission line connected with a plurality of open-circuit stub lines in parallel is characterized by comprising the following steps:
step one, determining the number of sections N of a first transmission line, the number of sections M of a second transmission line, the number of sections P of a third transmission line, a stop band rejection SR and a return loss RL in a filter circuit according to the actual circuit size and performance requirements;
step two, calculating an integral transmission matrix of the filter circuit, and obtaining a transmission function of the filter circuit according to the integral transmission matrix;
and step three, determining the constraint condition of the transmission function, and determining the impedance values of the first transmission line, the second transmission line and the third transmission line according to the transmission function and the constraint condition.
3. The method for designing a wideband band-stop filter based on multiple open-circuit stubs in parallel of transmission lines according to claim 2, wherein in the second step, the overall transmission matrix of the filter circuit is:
4. The method for designing a wideband band-stop filter based on multiple open-circuit stubs in parallel of a transmission line according to claim 3, wherein in the second step, the transmission function of the filter circuit is obtained by the following relation:
wherein S is21Is the forward transfer function of the filter circuit, AT、BT、CTAnd DTRespectively, elements of the overall transmission matrix of the filter circuit.
5. The design method of the broadband band-stop filter based on the transmission line parallel multi-section open-circuit stub as claimed in claim 4, wherein when P is 1,
wherein θ is the electrical length of the transmission line, and all the open-circuit stub lines and the transverse transmission line have the same electrical length; n is the number of the first transmission line; m is the number of the second transmission line; w is a characteristic constant function;andis a factor characteristic impedance function; h isM,NIs the overall characteristic impedance function.
6. The method for designing a wideband band-stop filter based on transmission line parallel multi-section open-circuit stub as claimed in claim 4 or 5, wherein in the third step, the constraint condition of the transmission function is:
wherein D is1、D2… … are the poles of the S function of the filter circuit;… … is the input reflection function S of the filter circuit11The electrical length at each of the pole points,… … is the forward transfer function S of the filter circuit21Electrical length at each pole; RL is the return loss of the circuit and SR is the stop-band rejection of the circuit;RLis S11The height of the corrugations of (a) is,SRis S21The height of the corrugations.
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