JP2008219519A - Filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact filter. <P>SOLUTION: The filter 10 has a line 20 which connects an input terminal 21 and an output terminal 22, a resonator 30 which is connected to the line 20, and a reactance element 40 which is connected to the line 20. The resonator 30 includes a pair of resonators 31 and 32 which are inter-digital connected, and a short-circuit terminal of the resonator 31 is connected to the line 20. At passing frequencies of the filter 10, the resonator 30 and the reactance element 40 form an equivalent parallel resonant circuit between the line 20 and the ground. At a cutoff frequency of the filter 10, the resonator 30 forms an equivalent series resonant circuit between the line 20 and the ground. Thereby, the degree of connection of the pair of resonators 31 and 32 is strengthened, so that the physical size of the resonator 30 is greatly reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a filter comprising an interdigitally coupled resonator.

Due to recent developments in information and communication technology, there is a demand for high-performance band-pass filters that are small, have low insertion loss, and have a steep skirt characteristic. Design and development of band-pass filters having an attenuation pole in the stop band are being carried out. . Conventionally, as a band-pass filter that satisfies these requirements, for example, an elliptic function-type polarized band-pass filter used for mobile communication or the like is known. Patent Document 1 discloses a high-frequency filter that uses a half-wavelength resonator to form attenuation poles on the low-frequency side and the high-frequency side of the passband and has excellent frequency selectivity. Non-Patent Document 1 proposes a synthesis theory of a polar filter for forming an attenuation pole using an open stub with a quarter wavelength termination and obtaining a steep skirt characteristic.
JP-A-11-340706 IEICE Transactions C vol. J89-C No.6 pp.372-384

  However, in the high-frequency filter described in Patent Document 1, the external size of the resonator requires a physical length corresponding to a half wavelength, and thus there are technical limitations in realizing a significant reduction in size. In the polarized filter described in Non-Patent Document 1, an open stub having a physical length corresponding to a quarter wavelength is required, and thus there is a technical restriction in realizing a significant reduction in size.

  Therefore, an object of the present invention is to solve the above-described problems and to realize a significant downsizing of the filter.

  In order to solve the above problem, a filter according to the present invention includes a line connecting an input terminal and an output terminal, a resonator connected to the line, and a reactance element connected between the line and the ground. Prepare. The resonator includes a pair of interdigitally coupled first resonators, and a short-circuit end of one of the pair of first resonators is connected to the line.

  At the pass frequency of the filter, the resonator and the reactance element equivalently form a parallel resonant circuit between the line and the ground. “Equivalently form a parallel resonant circuit” means that the resonator and the reactance element as a whole are equivalent to the parallel resonant circuit formed between the line and the ground at the pass frequency of the filter. To do. At the pass frequency, the parallel resonant circuit is in a state of parallel resonance, and the input impedance of the input terminal becomes infinite, so that the signal input to the input terminal is output as it is to the output terminal via the line.

  On the other hand, at the cutoff frequency of the filter, the resonator equivalently forms a series resonant circuit between the line and the ground. “Forming a series resonant circuit equivalently” means that the resonator is equivalent to a series resonant circuit formed between the line and the ground at the cutoff frequency of the filter. At the cutoff frequency, the series resonance circuit is in a series resonance state, and the input impedance of the input terminal becomes zero. Therefore, the signal input to the input terminal is totally reflected and output from the input terminal as it is.

  Here, the pair of first digitally coupled resonators is designed to resonate at the cutoff frequency of the filter to form an attenuation pole. By increasing the degree of coupling between the pair of interdigitally coupled first resonators, the physical size can be reduced to 1/4 wavelength or less.

  As a reactance element, for example, a capacitor (including an open stub shorter than a quarter wavelength), an inductance element (including a short stub shorter than a quarter wavelength), a pair of second resonators that are interdigitally coupled, and the like are applied. can do. By combining the resonator and these reactance elements, an attenuation pole can be formed on either or both of the low frequency side and the high frequency side of the pass band, and a filter having excellent frequency selectivity can be provided.

  The resonator may further include one or more pairs of interdigitally coupled third resonators. The pair of first resonators and the one or more pairs of third resonators are interdigitally coupled. By interdigitally coupling a plurality of sets of resonators and further increasing the degree of coupling, the resonators can be further miniaturized.

Here, the angular frequency is ω, the angular frequency corresponding to the passing frequency is ω ₀ , the electrical angle of the reactance element is θ ₁ , the electrical angle of the reactance element corresponding to the passing frequency is θ ₁₀ , and the electrical angle of the resonator is θ ₂ The electrical angle of the resonator corresponding to the pass frequency is θ ₂₀ , the susceptance of the filter is B (θ ₁ , θ ₂ ), and the equivalent circuit of the parallel resonant circuit is composed of a parallel connection circuit of a capacitor C ₀ and an inductance L _0. Assuming that the circuit constants of the reactance element and the resonator are ω = ω ₀ , θ ₁ = θ ₁₀ , θ ₂ = θ ₂₀ , B (θ ₁ , θ ₂ ) = ω ₀ C ₀ −1 / If it is within a range of ± 10% of the value satisfying ω ₀ L ₀ = 0 and ω∂B / ∂ω = 2ω ₀ C ₀ , practically sufficient filter characteristics can be obtained.

  According to the present invention, it is possible to realize a filter having a steep skirt characteristic having an attenuation pole by using an interdigitally coupled resonator, and to realize a significant downsizing of the filter.

  Embodiments according to the present invention will be described below with reference to the drawings. About the same element, the same code | symbol shall be attached | subjected and the overlapping description is abbreviate | omitted.

FIG. 1 shows a circuit configuration of a polarized bandpass filter 10 according to the present embodiment.
The polarized bandpass filter 10 includes a line 20 connecting the input terminal 21 and the output terminal 22, a resonator 30 connected to the line 20 at the connection point 23, and a reactance element 40 connected to the line 20 at the connection point 23. Prepare. The reactance element 40 is connected between the line 20 and the ground. Each of these components is constituted by a TEM line. The TEM line can be constituted by, for example, a conductor pattern such as a strip line or a through conductor formed inside the dielectric substrate. A TEM line is a transmission line that transmits an electromagnetic wave (TEM wave) in which both an electric field and a magnetic field exist only in a cross section perpendicular to the traveling direction of the electromagnetic wave.

  The resonator 30 includes a pair of quarter-wave resonators 31 and 32 that are interdigitally coupled. Interdigital coupling is a pair of quarter-wave resonators 31 and 32 having one end as an open end and the other end as a short-circuited end, and the open end of one quarter-wave resonator 31 and the other quarter-wave resonator. The short-circuited end of the resonator 32 is opposed to the short-circuited end of one quarter-wave resonator 31 and the open end of the other quarter-wave resonator 32 is opposed to each other. This means that the wavelength resonators 31 and 32 are electromagnetically coupled. In the example shown in the figure, the short-circuit end of one quarter-wave resonator 31 is connected to the line 20 at the connection point 23, and the short-circuit end of the other quarter-wave resonator 32 is connected to the ground. is doing.

  Here, the short-circuit ends of the pair of quarter-wave resonators 31 and 32 do not necessarily need to be physically connected to the ground, and an alternating current is obtained when the electric field distribution of the short-circuit ends is viewed at the operating frequency at the time of resonance. It is only necessary to be connected to a location where the potential is zero. Even if the pair of quarter-wave resonators 31 and 32 is not connected to a physical ground electrode at the resonance frequency at the time of resonance, the short-circuited end thereof becomes an AC zero potential. Is a portion where the AC potential is zero in terms of the operating frequency at the time of resonance.

The pair of quarter-wave resonators 31 and 32 has a strong interdigital coupling as will be described later, so that the first resonance mode that resonates at the first resonance frequency f ₁ and the first resonance frequency. and a second resonance mode that resonates at a _second resonance frequency f ₂ lower than f ₁ . More specifically, when the resonance frequency of each of the quarter-wave resonators 31 and 32 when the interdigital coupling is not performed is f ₃ , the first resonance frequency higher than the resonance frequency f ₃ of the single wave resonator. It has a first resonance mode that resonates at f ₁ and a second resonance mode that resonates at a _second resonance frequency f ₂ that is lower than the resonance frequency f ₃ alone. Then, the resonator 30, the second resonance frequency f ₂ is set as the operating frequency.

  As a technique for coupling two resonators composed of TEM lines, there are generally two types of comb line coupling and interdigital coupling. Of these, interdigital coupling is known to provide very strong coupling. Interdigital coupling means two resonances such that the open end of one resonator faces the short-circuited end of the other resonator, and the short-circuited end of one resonator faces the open-end of the other resonator. This is a coupling method that results in a structure in which containers are arranged opposite to each other.

  In the pair of quarter-wave resonators 31 and 32 that are interdigitally coupled, the resonance state can be divided into two unique resonance modes. FIG. 2 shows a first resonance mode in the pair of quarter-wave resonators 31 and 32, and FIG. 3 shows a second resonance mode in the pair of quarter-wave resonators 31 and 32. 2 and 3, the curve indicated by the broken line indicates the distribution of the electric field E in each resonator.

  In the first resonance mode, in each of the pair of quarter-wave resonators 31 and 32, the current i flows from the open end side to the short-circuit end side, and the direction of the current i flowing through each resonator is opposite. . In the first resonance mode, electromagnetic waves are excited in phase by the pair of quarter-wave resonators 31 and 32.

  On the other hand, in the second resonance mode, the current i flows from the open end side to the short-circuit end side in one quarter-wave resonator 31, and the other quarter-wave resonator 32 opens from the short-circuit end side. The current i flows to the end side, and the direction of the current i flowing through each resonator is the same direction. That is, in this second resonance mode, as can be seen from the distribution of the electric field E, the pair of quarter-wave resonators 31 and 32 excites electromagnetic waves in opposite phases. In this second resonance mode, the phase of the electric field E is shifted by 180 ° at positions that are rotationally symmetric with respect to the physical rotational symmetry axis 30A of the entire pair of quarter-wave resonators 31 and 32.

Here, the resonance frequency of the first resonance mode is represented by f ₁ in the following equation (1), and the resonance frequency of the second resonance mode is represented by f ₂ in the following equation (2).

Where V _c is the speed of light, ε _r is the effective relative permittivity, μ _r is the effective relative permeability, l is the physical length of the resonator, Z _e is the characteristic impedance of the even mode, and Z _o is the characteristic impedance of the odd mode. Show. In a symmetric coupling transmission line, the transmission mode propagating to the transmission line is decomposed into two independent modes (not interfering with each other) of an even mode and an odd mode.

  Hereinafter, the characteristic impedance in the corner mode and the odd mode will be described with reference to FIGS. 4A shows the distribution of the electric field E in the odd mode of the coupling transmission line, and FIG. 4B shows the distribution of the electric field E in the even mode. 4A and 4B, a ground layer 50 is formed on the outer periphery, and symmetrical conductor lines 51 and 52 are formed inside. 4A and 4B show the electric field distribution in a cross section orthogonal to the transmission direction of the coupling transmission line, and the signal transmission direction is a direction orthogonal to the paper surface.

As shown in FIG. 4A, in the odd mode, the electric field intersects perpendicularly with the symmetry plane of the conductor lines 51 and 52, and the symmetry plane becomes a virtual electric wall 53E. FIG. 5A shows a transmission line equivalent to FIG. As shown in FIG. 5A, by replacing the symmetry plane with an actual electrical wall 53E (zero potential wall, ground), a structure equivalent to a line with only one conductor line 51 can be obtained. The characteristic impedance of the line shown in FIG. 5A becomes the odd-mode characteristic impedance Z _{o in} the above equations (1) and (2).

On the other hand, in the even mode, as shown in FIG. 4B, the electric field is balanced with respect to the symmetry plane of the conductor lines 51 and 52, and the magnetic field intersects perpendicularly with respect to the symmetry plane. In the even mode, the plane of symmetry is a virtual magnetic wall 53H. FIG. 5B shows a transmission line equivalent to FIG. As shown in FIG. 5B, a structure equivalent to a line of only one conductor line 51 can be obtained by replacing the symmetry plane with an actual magnetic wall 53H (an infinite impedance wall). The characteristic impedance of the line shown in FIG. 5B is the characteristic impedance Z _e of the even mode in the above equations (1) and (2).

Here, generally, the characteristic impedance Z of the transmission line is expressed by the following equation (3): the capacitance C with respect to the ground per unit length of the signal line and the inductance component L per unit length of the signal line. It can be expressed by the square root of the ratio.

Characteristic impedance Z _o in the odd mode from line structure of FIG. 5 (A), since the plane of symmetry capacitance C is increased with respect to the ground (electric wall 53E) becomes ground, from equation (3), the value of Z _o Get smaller. On the other hand, the characteristic impedance Z _e in the even mode is smaller than the capacitance C because the symmetry plane is the magnetic wall 53H from the line structure of FIG. 5B, and the value of Z _e is increased from the equation (3). .

Based on this, the equations (1) and (2), which are resonance frequencies of the resonance modes of the pair of quarter-wave resonators 31 and 32 that are interdigitally coupled, will be examined. Since the arctangent function is a monotonically increasing function, the larger the portion related to tan ⁻¹ in the equations (1) and (2), the greater the resonance frequency, and the smaller the portion, The resonance frequency is reduced. That is, as the value of the characteristic impedance Z _o in the odd mode decreases, the value of the characteristic impedance Z _e in the even mode increases, and the difference between them increases, the first resonance from the equation (1) The resonance frequency f _{1 of the} mode increases, and the resonance frequency f ₂ of the second resonance mode decreases from the equation (2).

Therefore, if the ratio of the symmetry planes of the coupled transmission lines is increased, the first resonance frequency f ₁ and the second resonance frequency f ₂ are separated from each other as shown in FIG. Here, FIG. 6 shows a distribution state of resonance frequencies in a pair of quarter-wave resonators 31 and 32 that are interdigitally coupled. A pair of quarter-wave resonators 31 and 32 are strongly coupled by interdigital coupling, whereby a resonance frequency determined by the length of a physical quarter wavelength (each quarter wavelength when not interdigitally coupled). resonance frequency) f ₃ of the resonator itself is separated into two. That is, between the first resonance mode that resonates at first resonant frequency f ₁ is higher than the resonance frequency f _3, and a second resonance mode that resonates at the resonance frequency f ₃ second resonance frequency f ₂ lower than Two modes appear.

Here, increasing the ratio of the symmetry plane of the transmission path corresponds to increasing the capacity C in the odd mode from the equation (3). Increasing the capacitance C corresponds to increasing the degree of coupling of the lines. Accordingly, in the pair of quarter-wave resonators 31 and 32 that are interdigitally coupled, the stronger the coupling between the resonators, the larger the first resonance frequency f ₁ and the second resonance frequency f ₂ are. Will be separated. In this case, by setting the second resonance frequency f ₂ to be the resonance frequency of the resonator 30, there is an advantage that the resonator 30 can be made smaller than when the resonance frequency of the resonator 30 is set to f _3. is there. It is desirable that the first resonance frequency f ₁ is sufficiently higher than the pass frequency band of the filter 30. This is because when the pass frequency band overlaps with the _first resonance frequency f ₁ , the frequency characteristics of the polarized band pass filter 10 deteriorate.

  As shown in FIG. 7, the resonator 30 includes a plurality of pairs of quarter-wave resonators, and a plurality of quarter-wave resonators 31, 32, 33,..., 3n (n is 4). (Even numbers above). In this case, a pair of adjacent quarter wavelength resonators are interdigitally coupled, and as a result, a plurality of pairs of quarter wavelength resonators are formed by a pair of adjacent quarter wavelength resonators. For example, a first pair of quarter-wave resonators are formed by the quarter-wave resonators 31 and 32, and a second pair of quarter-wave resonators are formed by the quarter-wave resonators 32 and 33. Is done. In this manner, by forming a plurality of pairs of quarter wavelength resonators, the physical length of each quarter wavelength resonator can be designed to be shorter, and the size can be further reduced.

  Note that any one of the multiple-stage quarter-wave resonators 31, 32, 33,..., 3n is not necessarily physically connected to the ground, but is connected to the connection point 23. May be. The reason is that the connection point 23 is alternatingly zero potential when the resonator 30 resonates.

Next, the frequency characteristics of the polarized bandpass filter 10 will be examined. Yukyokugata bandpass filter 10 is small and has a steep skirt characteristic with low insertion loss, center frequency in the pass band is f _0, cutoff number frequency in cutoff range is f _t. When the reactance component of the input impedance of the resonator 30 viewed from the connection point 23 is X and the electrical angle of the resonator 30 is θ, the following equations (4) to (5) are established. Here, f is the frequency, μ _r is the effective relative permeability, and l is the physical length of the resonator 30.

If the electrical angle at resonance is θ _t , the following equation (6) is established as the resonance condition of the resonator 30. Here, C ₀ indicates a capacity per unit length with respect to the ground of each of the quarter wavelength resonators 31 and 32, and C _int indicates a capacity per unit length between the quarter wavelength resonators 31 and 32. .

When the equation (6) is modified, the following equations (7) to (10) are established.

When equation (4) is modified using equations (7) to (10), the following equation (11) is established.

When the susceptance of the resonator 30 is Y (θ), the following equation (12) is established by modifying the equation (11).

Here, in order to make the skirt characteristic from the pass band to the attenuation band of the polarized bandpass filter 10 steep, the conditions for forming the attenuation pole on the low frequency side or the high frequency side of the pass band are examined. To do. In equation (11), if θ = θ _t is substituted, X = 0. This indicates that the input impedance of the resonator 30 becomes 0 when the resonator 30 resonates in the second resonance mode. FIG. 8 shows an equivalent circuit of the polarized bandpass filter 10 when the resonator 30 resonates in the second resonance mode, and the input terminal 21 has whatever value the input impedance of the reactance element 40 is. The input impedance viewed from the above becomes zero. When the reference impedance of the input terminal 21 is Z _P and the input impedance is Z _in , the reflection coefficient Γ is defined by the following equation (13).

When the resonator 30 resonates in the second resonance mode, Z _in = 0 is substituted _{in the} equation (13), and Γ = (0−Z _p ) / (0 + Z _p ) = − 1. The reflection coefficient Γ = −1 means that the signal input to the input terminal 21 is totally reflected. When this phenomenon is viewed from the viewpoint of the filter characteristics of the polarized bandpass filter 10, the polarized bandpass filter 10 has a filter characteristic having an attenuation pole at the resonance frequency f ₂ in the second resonance mode of the resonator 30. Means that Here, cut-off number of frequency f _t of Yukyokugata bandpass filter 10 is equal to the resonance frequency f _2.

When θ = θ _t , that is, when the resonator 30 resonates in the second resonance mode, the resonator 30 viewed from the connection point 23 is in series resonance between the connection point 23 and the ground. .

Next, conditions for the polar bandpass filter 10 to use a high-frequency signal (center frequency f ₀ ) having a predetermined bandwidth as a passband will be examined. As an equivalent circuit of the polarized bandpass filter 10 at the center frequency f ₀ , a parallel resonance circuit (a circuit in which a capacitor C ₀ and an inductance L ₀ are connected in parallel) as shown in FIG. 9 is considered, and the center frequency f ₀ is considered. If the circuit constant is set so that the parallel resonant circuit resonates in parallel, the polarized bandpass filter 10 can pass the signal having the center frequency f ₀ .

Here, the susceptance of the resonator 30 is Y ₁ (θ ₁ ), the susceptance of the reactance element 40 is Y ₂ (θ ₂ ), and the combined susceptance of the polarized bandpass filter 10 is B (θ ₁ , θ ₂ ). For example, the following expressions (14) to (15) are established. Here, ω is an angular frequency, θ ₁ is an electrical angle of the resonator 30, and θ ₂ is an electrical angle of the reactance element 40.

When the angular frequency corresponding to the center frequency f ₀ is ω ₀ , the following expressions (16) and (17) are established from the expressions (14) and (15).

(16) and (17) If equation is established at the same time, Yukyokugata bandpass filter 10, the center frequency in the pass band and f _0, having a filter characteristic that the cut-off number frequency in cutoff range and f _t. In particular, the attenuation pole formed by the resonator 30 having a sharp resonance characteristic has an extremely steep attenuation gradient, and is therefore suitable when it is desired to increase the attenuation gradient in the transition region from the passband to the cutoff band.

Note that if the degree of coupling between the pair of quarter-wave resonators 31 and 32 is increased, C _int increases, so that it can be understood that the electrical angle θ _t becomes smaller from the equation (6). This means that the physical length of the resonator 30 can be significantly shortened (can be reduced in size) from the quarter wavelength according to the equation (4).

  Hereinafter, the case where a capacitor, an open stub shorter than a quarter wavelength, an inductance, a short stub shorter than a quarter wavelength, and a pair of interdigitally coupled quarter-wave resonators is applied as the reactance element 40 will be described. 1 to 5 will be described.

  More specifically, in the first to second embodiments, a capacitive element (capacitor or open stub shorter than a quarter wavelength) is used as the reactance element 40, which is more than the pass band of the polarized bandpass filter 10. An example in which an attenuation pole is formed on the low frequency side and a steep skirt characteristic on the low frequency side is realized will be described.

  In the third to fourth embodiments, an inductive element (inductance or a short stub shorter than ¼ wavelength) is used as the reactance element 40, and the attenuation pole is positioned higher in frequency than the pass band of the polarized bandpass filter 10. An example of forming a steep skirt characteristic on the high frequency side will be described.

  In the fifth embodiment, a pair of quarter-wave resonators coupled interdigitally is used as the reactance element 40, and attenuation poles are formed on the low frequency side and the high frequency side of the pass band of the polarized bandpass filter 10, respectively. An example of realizing steep skirt characteristics on the low frequency side and the high frequency side will be described.

FIG. 10 shows a circuit configuration of a polarized bandpass filter 10A according to the first embodiment.
In the first embodiment, the capacitor 40A is connected to the connection point 23 as a reactance element. When the capacitance of the capacitor 40A is C _add and the combined susceptance of the polarized bandpass filter 10A is B, the following equation (18) is established.

By applying the equations (16) and (17) to the equation (18) to obtain C _add , Z _e and Z _o , equations (19) to (21) are obtained.

Here, in order to verify the effectiveness of the present embodiment, an attempt was made to compare and verify the filter characteristics of the Chebychev bandpass filter 60 shown in FIG. 11 and the filter characteristics of the polarized bandpass filter 70 shown in FIG. . The polarized band-pass filter 70 has a circuit configuration in which the parallel resonant circuit 60A surrounded by a broken-line frame of the Chebychev band-pass filter 60 is replaced with a polarized band-pass filter 10A. The Chebychev bandpass filter 60 has a characteristic in which the attenuation gradient in the vicinity of the cut-off frequency is increased by giving ripples (swells) to the pass band, and is suitable for a case where it is desired to increase the attenuation gradient to some extent. . Here, a Chebychev bandpass filter 60 having a center frequency f ₀ = 1 GHz, a bandwidth = 0.1 GHz, and a ripple of 0.01 dB is used. The circuit constants of the Chebychev bandpass filter 60 including a lumped constant circuit are as follows: L1 = 1.267679507 nH, C1 = 20.027419 pF, L2 = 77.212625 nH, C2 = 0.3288812 pF, L3 = 1.267679507 nH, C3 = 20. 027419 pF.

Here, assuming that the cutoff frequency is 0.8 GHz, the effective relative permittivity is 7.3, the effective relative permeability is 1, and the physical length of the resonator 30 is 10 mm, C _add , Z _{When e} and Z _o are obtained, C _add = 6.910440217 pF, Z _e = 205.6924484, Z _o = 10.928334138.

  FIG. 13 is a graph showing the comparison results of the filter characteristics. The horizontal axis indicates the frequency [GHz], and the vertical axis indicates the attenuation [dB]. The graph indicated by the alternate long and short dash line is the amount of attenuation of the input reflection coefficient of the port 61 of the Chebychev bandpass filter 60 (the value obtained by dividing the signal output from the port 61 by the signal input to the port 61) in dB. is there. The graph indicated by the two-dot chain line represents the attenuation amount of the forward transmission coefficient (the value obtained by dividing the signal output from the port 62 by the signal input to the port 61) in dB in the Chebychev bandpass filter 60. Is. The graph indicated by the broken line is a graph showing the attenuation amount of the input reflection coefficient (the value obtained by dividing the signal output from the port 71 by the signal input to the port 71) in dB in the polarized bandpass filter 70. is there. The graph shown by the solid line is the amount of attenuation of the forward transfer coefficient (the value obtained by dividing the signal output from the port 72 by the signal input to the port 71) expressed in dB in the pole-type bandpass filter 70. It is.

  As shown in this graph, in the polarized band-pass filter 70, the attenuation pole due to the resonance action of the resonator 30 is formed at a point with a cutoff frequency of 0.8 GHz. Therefore, the transition band from the pass band to the attenuation band It is possible to increase the attenuation gradient at. In addition, the polarized bandpass filter 70 can further reduce the size of the resonator 30 by increasing the degree of coupling between the pair of quarter-wave resonators 31 and 32 that are interdigitally coupled. it can. That is, the polarized bandpass filter 70 can obtain a steep skirt characteristic with a small size and low loss.

14 is a graph showing filter characteristics when the capacitance of the capacitor 40A of the polarized bandpass filter 70 is lowered by 10%. FIG. 15 is a graph showing the capacitance of the capacitor 40A of the polarized bandpass filter 70 by 10%. is a graph showing filter characteristics when raised, FIG. 16 is a graph showing the filter characteristics when the lower 10% Z _e of the resonator 30 of the polarized bandpass filters 70, 17 Yukyokugata FIG. 18 is a graph showing filter characteristics when Z _e of the resonator 30 of the band-pass filter 70 is increased by 10%, and FIG. 18 shows a graph when Z _o of the resonator 30 of the polarized band-pass filter 70 is decreased by 10%. FIG. 19 is a graph showing filter characteristics when Z _o of the resonator 30 of the polarized bandpass filter 70 is increased by 10%. As shown in these graphs, practically sufficient filter characteristics can be obtained even if there is an error of ± 10% in the calculation results of the equations (19) to (21).

FIG. 20 shows a circuit configuration of a polarized bandpass filter 10B according to the second embodiment.
In the second embodiment, the open stub 40B is connected to the connection point 23 as a reactance element. The open stub 40B functions as a capacitor when its physical length is shorter than a quarter wavelength. When the characteristic admittance of the open stub 40B is Y _C and the combined susceptance of the polarized bandpass filter 10B is B, the following equation (22) is established. Here, θ ₁ represents the electrical angle of the resonator 30, and θ ₂ represents the electrical angle of the open stub 40B.

Here, for each of the resonator 30 and the open stub 40B, the electrical angles at the time of resonance are θ ₁₀ and θ _20, and the equations (16) and (17) are applied to the equation (22), and Y _C , Z _{When e} and Z _o are obtained, equations (23) to (25) are obtained.

  Here, in order to verify the effectiveness of the present embodiment, an attempt was made to compare and verify the filter characteristics of the Chebychev bandpass filter 60 shown in FIG. 11 and the filter characteristics of the polarized bandpass filter 80 shown in FIG. . The polarized bandpass filter 80 has a circuit configuration in which the parallel resonant circuit 60A surrounded by a broken line frame of the Chebychev bandpass filter 60 is replaced with a polarized bandpass filter 10B.

Here, the cutoff frequency is 0.8 GHz, the effective relative permittivity is 7.3, the effective relative permeability is 1, the physical length of the resonator 30 is 10 mm, and the physical length of the open stub 40B is 2 mm. When Y _C , Z _e , and Z _o are obtained from the equation (25), Y _C = 0.3811862, Z _e = 205.7684469, Z _o = 10.13237914.

  FIG. 22 is a graph showing the comparison results of the filter characteristics. The horizontal axis indicates the frequency [GHz], and the vertical axis indicates the attenuation [dB]. The graph indicated by the alternate long and short dash line is the amount of attenuation of the input reflection coefficient of the port 61 of the Chebychev bandpass filter 60 expressed in dB. A graph indicated by a two-dot chain line is a dB display of the attenuation amount of the forward transmission coefficient of the port 61 of the Chebychev bandpass filter 60. The graph indicated by the broken line is a graph showing the attenuation amount of the input reflection coefficient of the port 81 of the polarized bandpass filter 80 (the value obtained by dividing the signal output from the port 81 by the signal input to the port 81) in dB. is there. The graph shown by the solid line is the amount of attenuation of the forward transfer coefficient (the value obtained by dividing the signal output from the port 82 by the signal input to the port 81) expressed in dB in the polarized bandpass filter 80. It is.

  As shown in this graph, in the polarized bandpass filter 80, the attenuation pole due to the resonance action of the resonator 30 is formed at a point with a cutoff frequency of 0.8 GHz. It is possible to increase the attenuation gradient at. In addition, the polarized band-pass filter 80 can further reduce the size of the resonator 30 by increasing the degree of coupling between the pair of quarter-wave resonators 31 and 32 that are interdigitally coupled. it can. That is, the polarized bandpass filter 80 can obtain a steep skirt characteristic with a small size and low loss.

  Although not shown in the graph, the present inventor's simulation shows that practically sufficient filter characteristics can be obtained even if there is an error of ± 10% in the calculation results of the equations (23) to (25). It has been confirmed.

FIG. 23 shows a circuit configuration of a polarized bandpass filter 10C according to the third embodiment.
In the third embodiment, the inductance element 40C is connected to the connection point 23 as a reactance element. When the inductance of the inductance element 40C is L _add and the combined susceptance of the polarized bandpass filter 10C is B, the following equation (26) is established.

By applying the equations (16) and (17) to the equation (26) to obtain L _add , Z _e , and Z _o , equations (27) to (29) are obtained.

  Here, in order to verify the effectiveness of the present embodiment, an attempt was made to compare and verify the filter characteristics of the Chebychev bandpass filter 60 shown in FIG. 11 and the filter characteristics of the polarized bandpass filter 90 shown in FIG. . The polarized bandpass filter 90 has a circuit configuration in which the parallel resonant circuit 60A surrounded by a broken line frame of the Chebychev bandpass filter 60 is replaced with a polarized bandpass filter 10C.

Here, assuming that the cutoff frequency is 1.2 GHz, the effective relative permittivity is 7.3, the effective relative permeability is 1, and the physical length of the resonator 30 is 10 mm, L _add , Z according to the expressions (27) to (29) _{When e} and Z _o are obtained, L _add = 3.9955755702 nH, Z _e = 188.6639626, Z _o = 23.57934289.

  FIG. 25 is a graph showing the comparison results of the filter characteristics. The horizontal axis indicates the frequency [GHz], and the vertical axis indicates the attenuation [dB]. The graph indicated by the alternate long and short dash line is the amount of attenuation of the input reflection coefficient of the port 61 of the Chebychev bandpass filter 60 expressed in dB. A graph indicated by a two-dot chain line is a dB display of the attenuation amount of the forward transmission coefficient of the port 61 of the Chebychev bandpass filter 60. The graph indicated by the broken line is a graph showing the attenuation amount of the input reflection coefficient (the value obtained by dividing the signal output from the port 91 by the signal input to the port 91) in dB in the pole-type bandpass filter 90. is there. The graph shown by the solid line is the amount of attenuation of the forward transfer coefficient of the port 91 of the polarized bandpass filter 90 (the value obtained by dividing the signal output from the port 92 by the signal input to the port 91) in dB. It is.

  As shown in this graph, in the polarized band-pass filter 90, the attenuation pole due to the resonance action of the resonator 30 is formed at a point having a cutoff frequency of 1.2 GHz, so that the transition band from the pass band to the attenuation band is obtained. It is possible to increase the attenuation gradient at. In addition, the polarized band-pass filter 80 can further reduce the size of the resonator 30 by increasing the degree of coupling between the pair of quarter-wave resonators 31 and 32 that are interdigitally coupled. it can. That is, the polarized bandpass filter 80 can obtain a steep skirt characteristic with a small size and low loss.

  Although not shown in the graph, the simulation of the present inventor shows that practically sufficient filter characteristics can be obtained even if there is an error of ± 10% in the calculation results of equations (27) to (29). It has been confirmed.

FIG. 26 shows a circuit configuration of a polarized bandpass filter 10D according to the fourth embodiment.
In Example 4, the short stub 40D is connected to the connection point 23 as a reactance element. The short stub 40D functions as an inductance element when its physical length is shorter than ¼ wavelength. When the characteristic admittance of the short stub 40D is Y _C and the susceptance of the polarized bandpass filter 10D is B, the following equation (30) is established. Here, θ ₁ represents the electrical angle of the resonator 30, and θ ₂ represents the electrical angle of the short stub 40D.

Here, with respect to each of the resonator 30 and the short stub 40D, the electrical angles at the time of resonance are θ ₁₀ and θ _20, and the equations (16) and (17) are applied to the equation (30), and Y _C , Z _{When e} and Z _o are obtained, equations (31) to (33) are obtained.

  Here, in order to verify the effectiveness of the present embodiment, an attempt was made to compare and verify the filter characteristics of the Chebychev bandpass filter 60 shown in FIG. 11 and the filter characteristics of the polarized bandpass filter 100 shown in FIG. . The polarized bandpass filter 100 has a circuit configuration in which the parallel resonant circuit 60A surrounded by a broken-line frame of the Chebychev bandpass filter 60 is replaced with a polarized bandpass filter 10D.

Here, the cutoff frequency is 1.2 GHz, the effective relative permittivity is 7.3, the effective relative permeability is 1, the physical length of the resonator 30 is 10 mm, and the physical length of the short stub 40D is 2 mm. When Y _C , Z _e , and Z _o are obtained from the equation (33), Y _C = 0.00452492, Z _e = 188.9920899, and Z _o = 23.61145507.

  FIG. 28 is a graph showing the comparison results of the filter characteristics. The horizontal axis indicates the frequency [GHz], and the vertical axis indicates the attenuation [dB]. The graph indicated by the alternate long and short dash line is the amount of attenuation of the input reflection coefficient of the port 61 of the Chebychev bandpass filter 60 expressed in dB. A graph indicated by a two-dot chain line is a dB display of the attenuation amount of the forward transmission coefficient of the port 61 of the Chebychev bandpass filter 60. The graph indicated by the broken line is a graph showing the attenuation of the input reflection coefficient (the value obtained by dividing the signal output from the port 101 by the signal input to the port 101) in dB in the polarized bandpass filter 100. is there. The graph shown by the solid line is the amount of attenuation of the forward transfer coefficient (the value obtained by dividing the signal output from the port 102 by the signal input to the port 101) expressed in dB in the polarized bandpass filter 100. It is.

  As shown in this graph, in the polarized band-pass filter 100, the attenuation pole due to the resonance action of the resonator 30 is formed at a point with a cutoff frequency of 1.2 GHz. Therefore, the transition band from the pass band to the attenuation band. It is possible to increase the attenuation gradient at. In addition, the polarized bandpass filter 100 can further reduce the size of the resonator 30 by increasing the degree of coupling between the pair of quarter-wave resonators 31 and 32 that are interdigitally coupled. it can. That is, the polarized bandpass filter 100 can obtain a steep skirt characteristic with a small size and low loss.

  Although not shown in the graph, according to the simulation of the present inventor, a practically sufficient filter characteristic can be obtained even if there is an error of ± 10% in the calculation results of the equations (31) to (33). It has been confirmed.

FIG. 29 shows a circuit configuration of a polarized bandpass filter 10E according to the fifth embodiment.
In the fifth embodiment, a pair of quarter-wave resonators 40E that are interdigitally coupled are connected to the connection point 23 as reactance elements. When the electrical angle of the resonator 30 is θ ₁ , the electrical angle of the resonator 40E is θ _2, and the susceptance of the polarized bandpass filter 10E is B, the following equation (34) is established.

Here, θ _t1 and θ _t2 are electrical angles when each of the resonator 30 and the resonator 40E series-resonates with the ground as viewed from the connection point 23, and the equation (16) and ( 17) is applied to obtain Z _e1 , Z _o1 , Z _e2 , and Z _o2 , equations (35) to (38) are obtained. Here, Z _e1 represents the characteristic impedance of the even mode of the resonator 30, Z _o1 represents the characteristic impedance of the odd mode of the resonator 30, Z _e2 represents the characteristic impedance of the even mode of the resonator 40E, and Z _o2 Indicates the characteristic impedance of the odd mode of the resonator 40E.

  Here, in order to verify the effectiveness of the present embodiment, an attempt was made to compare and verify the filter characteristics of the Chebychev bandpass filter 60 shown in FIG. 11 and the filter characteristics of the polarized bandpass filter 110 shown in FIG. . The polarized bandpass filter 110 has a circuit configuration in which the parallel resonant circuit 60A surrounded by a broken line frame of the Chebychev bandpass filter 60 is replaced with a polarized bandpass filter 10E.

Here, the cutoff frequency of the polarized bandpass filter 110 due to the series resonance action of the resonator 40E is 0.8 GHz, the cutoff frequency of the polarized bandpass filter 110 due to the series resonance action of the resonator 30 is 1.2 GHz, and effective. Assuming that the relative permittivity is 7.3, the effective relative permeability is 1, the physical length of the resonator 40E is 10 mm, and the physical length of the resonator 30 is 10 mm, Z _e1 , Z _{o1 are obtained} by the equations (35) to (38). , Z _e2 and Z _o2 are Z _e1 = 358.9428074, Z _o1 = 19.07045963, Z _e2 = 322.262567, Z _o2 = 40.32157507.

  FIG. 31 is a graph showing the comparison results of the filter characteristics. The horizontal axis indicates the frequency [GHz], and the vertical axis indicates the attenuation [dB]. The graph indicated by the alternate long and short dash line is the amount of attenuation of the input reflection coefficient of the port 61 of the Chebychev bandpass filter 60 expressed in dB. A graph indicated by a two-dot chain line is a dB display of the attenuation amount of the forward transmission coefficient of the port 61 of the Chebychev bandpass filter 60. The graph indicated by the broken line is a graph showing the attenuation amount of the input reflection coefficient (the value obtained by dividing the signal output from the port 111 by the signal input to the port 111) in dB in the pole-type bandpass filter 110. is there. The graph shown by the solid line is the amount of attenuation of the forward transfer coefficient of the port 111 of the polarized bandpass filter 110 (the value obtained by dividing the signal output from the port 112 by the signal input to the port 111) in dB. It is.

  As shown in this graph, in the polarized bandpass filter 110, an attenuation pole due to the series resonance action of the resonator 40E is formed at a point having a cutoff frequency of 0.8 GHz, and further, an attenuation pole due to the series resonance action of the resonator 30 is provided. Since it is formed at a point with a cutoff frequency of 1.2 GHz, the attenuation gradient in the transition region from the passband to the attenuation zone can be increased. In addition, the polarized bandpass filter 110 includes a degree of coupling between the pair of interdigitally coupled quarter wavelength resonators 41 and 42 and a pair of interdigitally coupled quarter wavelength resonators 31 and 32. By increasing the degree of coupling, the sizes of the resonators 40E and 30 can be further reduced. In other words, the polarized bandpass filter 110 can obtain a steep skirt characteristic with a small size and low loss.

  Although not shown in the graph, the simulation of the present inventor shows that practically sufficient filter characteristics can be obtained even if there is an error of ± 10% in the calculation results of the equations (35) to (38). It has been confirmed.

It is a circuit block diagram of the polarized bandpass filter concerning this embodiment. It is explanatory drawing of the 1st resonance mode in a pair of 1/4 wavelength resonator. It is explanatory drawing of the 2nd resonance mode in a pair of 1/4 wavelength resonator. 4A is an explanatory diagram illustrating the distribution of the electric field E in the odd mode of the coupling transmission line, and FIG. 4B is an explanatory diagram illustrating the distribution of the electric field E in the even mode of the coupling transmission line. is there. 5A is an explanatory diagram showing a transmission line equivalent to FIG. 4A, and FIG. 5B is an explanatory diagram showing a transmission line equivalent to FIG. 4B. It is explanatory drawing which shows the distribution state of the resonant frequency in a pair of 1/4 wavelength resonator by which the interdigital coupling was carried out. It is explanatory drawing of a resonator provided with two or more pairs of a quarter wavelength resonator. FIG. 3 is an equivalent circuit diagram of a polarized bandpass filter at a cutoff frequency. It is an equivalent circuit diagram of a polarized bandpass filter at the center frequency. 1 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 1. FIG. It is a circuit block diagram of the conventional Chebychev bandpass filter. 1 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 1. FIG. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 1 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 1 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 1 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 1 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 1 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 1 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 1 with the filter characteristics of a conventional Chebychev bandpass filter. 5 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 2. FIG. 5 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 2. FIG. 6 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 2 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 3. FIG. 6 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 3. FIG. 10 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 3 with the filter characteristics of a conventional Chebychev bandpass filter. 6 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 4. FIG. 6 is a circuit configuration diagram of a polarized bandpass filter according to Embodiment 4. FIG. It is a graph for comparing the filter characteristic of the polarized bandpass filter concerning Example 4 with the filter characteristic of the conventional Chebychev bandpass filter. FIG. 10 is a circuit configuration diagram of a polarized bandpass filter according to a fifth embodiment. FIG. 10 is a circuit configuration diagram of a polarized bandpass filter according to a fifth embodiment. 10 is a graph for comparing the filter characteristics of a polarized bandpass filter according to Example 5 with the filter characteristics of a conventional Chebychev bandpass filter.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Polarized band pass filter 20 ... Line 21 ... Input terminal 22 ... Output terminal 23 ... Connection point 30 ... Resonator 31, 32 ... 1/4 wavelength resonator 40 ... Reactance element

Claims (9)

A filter comprising a line connecting an input terminal and an output terminal, a resonator connected to the line, and a reactance element connected between the line and the ground,
The resonator includes a pair of interdigitally coupled first resonators, and a short-circuit end of one of the pair of first resonators is connected to the line. The filter of claim 1,
At the pass frequency of the filter, the resonator and the reactance element equivalently form a parallel resonant circuit between the line and the ground,
The filter, wherein at the cut-off frequency of the filter, the resonator equivalently forms a series resonant circuit between the line and the ground.   The filter according to claim 1 or 2, wherein the reactance element is a capacitor.   4. The filter according to claim 3, wherein the capacitor is an open stub shorter than a quarter wavelength.   The filter according to claim 1 or 2, wherein the reactance element is an inductance element.   6. The filter according to claim 5, wherein the inductance element is a short stub shorter than a quarter wavelength.   3. The filter according to claim 1, wherein the reactance element includes a pair of second resonators that are interdigitally coupled, and one of the pair of second resonators. The short-circuit end is a filter connected to the line.   3. The filter according to claim 1, wherein the resonator further includes at least one pair of interdigitally coupled third resonators, and the pair of first resonators and the one pair. A pair of third resonators that are greater than or equal to the set are interdigitally coupled, a filter. A filter according to any one of claims 2 to 8,
The angular frequency is ω, the angular frequency corresponding to the pass frequency is ω ₀ , the electrical angle of the reactance element is θ ₁ , the electrical angle of the reactance element corresponding to the pass frequency is θ ₁₀ , and the electrical angle of the resonator is θ ₂ , the electrical angle of the resonator corresponding to the passing frequency is θ ₂₀ , the susceptance of the filter is B (θ ₁ , θ ₂ ), and an equivalent circuit of the parallel resonant circuit is a capacitor C ₀ and an inductance L ₀ . When consisting of parallel connection circuits of
The circuit constants of the reactance element and the resonator are B (θ ₁ , θ ₂ ) = ω ₀ C ₀ −1 / ω ₀ L when ω = ω ₀ , θ ₁ = θ ₁₀ , θ ₂ = θ _{20. 0} = 0, and ω∂B / ∂ω = 2ω in ₀ C ₀ that satisfies the range of ± 10% of the value, the filter.

Images