JP2000341071A - Distributed constant filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress unnecessary coupling between resonators of a multiple- resonator type filter formed according to a transfer function composed of rational polynomials having a real root and an imaginary root of complex frequency. SOLUTION: Distributed constant circuit elements 18 to 24 are formed by forming a circular conductor pattern on a dielectric substrate. There are electric field maximum points at positions of 180 deg. of the outer periphery of the circular conductor pattern and electric fields are coupled at the parts; and there are magnetic field maximum points at positions of 90 deg. from them and magnetic fields are coupled at the parts. At respective unit coupling circuit parts, 1st coupling circuits 10 and 14 and 3rd coupling circuits 12 and 16 use magnetic field coupling and 2nd coupling circuits 11 and 15 use electric field coupling. At a unit coupling circuit part corresponding to a rational polynomial having a real root, a 4th coupling circuit 13 uses electric field coupling and at a unit coupling circuit part corresponding to a rational polynomial having an imaginary root, a 4th coupling circuit 17 uses magnetic field coupling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mobile communication device such as a mobile communication device.
Regarding a distributed constant filter used as a band-pass filter for removing an interfering signal or noise in an F stage or the like, specifically, the amplitude characteristic and the group delay characteristic of the pass band are simultaneously flat and the transmission zero point is in the stop band. The present invention relates to a distributed constant filter whose structure is simplified and loss is suppressed to improve performance.

[0002]

2. Description of the Related Art A high frequency circuit section such as an RF stage of a transmitting circuit and a receiving circuit of a mobile communication device such as an analog or digital portable telephone or a radio telephone has, for example, the same antenna as a transmitting circuit and a receiving circuit. Unwanted signals such as interfering waves and sidebands other than the desired signal wave to separate the transmission frequency band and the reception frequency band when sharing, or to attenuate harmonics generated based on the nonlinearity of the amplifier circuit A band-pass filter (bandpass filter: BPF) is often used to remove waves.

Such a band-pass filter as a filter for a communication device is generally realized as a filter circuit having a desired band characteristic by connecting a plurality of series resonance circuits or parallel resonance circuits constituted by various circuit elements. However, since the filter circuit section can be downsized and the electrical characteristics as a high-frequency circuit are good, the filter circuit section is configured by an unbalanced distributed constant line such as a microstrip line or a strip line. Often.

In general, in a filter having a band-pass characteristic, the amplitude characteristic and the group delay characteristic of the pass band are simultaneously flat as shown in the diagrams of FIGS. 9 (a) and 9 (b), and the transmission zero is located in the stop band. In order to make it, a complicated circuit configuration was required.

[0005] A method of directly constructing a bandpass filter having such characteristics using a clear design theory has not been known so far.
Various filters have been empirically constructed from filters.

For example, as shown in a block diagram of FIG.
First, focusing only on the amplitude characteristic, the filter having a known configuration has a flat pass band amplitude characteristic and a transmission zero point in the stop band, and has a desired amplitude characteristic but does not consider the group delay characteristic. In order to design the filter 1 having the characteristic and then to compensate the group delay characteristic of the filter 1 to obtain the desired group delay characteristic as a whole, the phase equalizer 2 having the all-pass characteristic for flattening the group delay characteristic in the pass band. Was added to this. According to this method, the phase or group delay characteristics are improved while adding the phase equalizer 2 to the filter 1.

[0007]

However, in general, there is a problem that such phase equalization and correction have little effect, and a sufficient correction effect cannot be obtained. In addition, since the configuration is made up of more circuit elements than originally required, there is much waste in the circuit configuration, and conversely, the amplitude characteristics due to the imperfect all-pass characteristics of the phase equalizer 2 are reduced. There has been a problem that adverse effects such as an adverse effect and an increase in loss due to circuit complexity are large.

On the other hand, conventionally, two methods have been known to realize a transmission zero in a filter stop band. One is to insert a parallel resonator or a series resonator inside the filter in series or in parallel, or to realize a transmission zero by a combination thereof. For example,
As shown in the circuit diagram of FIG. 11, a transmission zero is formed in the stop band on both outer sides of the pass band by the combination 5 of the parallel resonator and the series resonator for the filter having the band pass characteristic by the resonators 3 and 4. That is.

Another method is to realize a transmission zero point by branching a transmission path into two parts, making the amplitudes of the paths the same, and inverting the phases to combine them.

For example, as shown in the block diagram of FIG.
The circuit is branched into two, and at a certain frequency, the output amplitudes are the same and the phases are different by 180 degrees.
By leading the output to the ports 6 and 7, the output obtained by combining the outputs becomes a transmission zero at that frequency.

In general, the latter method is easier to realize and can actually realize a filter with a circuit configuration with less loss.

As a modification of the latter, a method using a simple reactance feedback path is also known. However, in this method, an accurate design theory and a method for synthesizing a filter from a target network function are known. No, it is used approximately or empirically. For example, as shown in the circuit diagram of FIG.
And a coupling circuit 9 corresponding to a branch circuit or a feedback path, thereby forming a transmission zero point.

However, according to this method, there is an effect of reducing the loss by simplifying the circuit. However, since an accurate design method of filter synthesis is not known, the design is approximate, so that the approximate characteristic is obtained. However, there is a problem that the characteristics are insufficient.

[0014] Conventionally, there has been known a method of combining a ladder-type circuit and a method of generating these transmission zeros, and then correcting the group delay by a phase equalizer. According to such a configuration, it is possible to obtain a bandpass characteristic filter in which the amplitude characteristics and the group delay characteristics of the passband are simultaneously flat and the transmission zero point is in the stopband.

However, even with this method, accurate characteristics cannot be obtained because the design is approximate.
There is a problem that the circuit configuration is complicated. further,
Such a filter also has a problem that transmission loss increases or that only approximate and insufficient characteristics can be obtained. Particularly, loss when a distributed constant filter such as a microstrip circuit is used is remarkable. Was.

In view of the above problems, the present inventor has proposed in Japanese Patent Application No. Hei 10-337219 a method of directly configuring a bandpass filter having the above desired characteristics using a clear design theory. This band-pass filter is a network function consisting of a denominator rational polynomial that is an even function of complex frequency s and has at least one set of real roots and at least one set of imaginary roots, and a numerator rational polynomial that is a Hurwitz polynomial of complex frequency s. A distributed constant filter having a frequency band-pass characteristic, obtained by performing frequency conversion on a normalized low-pass filter whose transfer function is represented by, and realized by an unbalanced distributed constant circuit, wherein the denominator rational polynomial A circuit portion corresponding to a real root or an imaginary root of the first resonator includes a first resonator, a second resonator, a first coupling circuit that cascade-couples the first resonator and a circuit outside the first resonator, A second coupling circuit that cascade-couples the resonator, a third coupling circuit that cascade-couples the second resonator and a circuit outside the second resonator, and an outside of the first coupling circuit and the third coupling circuit The unit coupling circuit unit comprising a fourth coupling circuit for coupling the bridge coupling 2
The unit coupling circuit portion, which is realized by a multi-resonator filter having at least one real resonator, is characterized in that the second coupling circuit and the fourth coupling circuit each have the same sign of a reactance element or the same type of electric field coupling or magnetic field coupling. And the unit coupling circuit portion corresponding to the imaginary root is such that the second coupling circuit and the fourth coupling circuit are respectively composed of reactance elements having different signs or different coupling circuits of electric field coupling or magnetic field coupling. It is characterized by the following. According to this, the amplitude characteristic and the group delay characteristic in the passband characteristic are simultaneously flat characteristics, and the bandpass characteristic has a transmission zero point in the stop band. It is possible to provide a distributed constant filter having characteristics of low element sensitivity and low loss while being configured and realized.

However, in the proposal in Japanese Patent Application No. 10-337219, when each coupling circuit is realized by electric field coupling or magnetic field coupling in the unit coupling circuit section,
Intended first coupling circuit, second coupling circuit, third coupling circuit,
In addition to the fourth coupling circuit, there is another problem to be improved in that another weak coupling is likely to occur between the resonators and unintended parasitic characteristic deterioration is likely to occur. For example, when the band-pass filter shown in FIG. 14 is configured, the resonator 33 and the third resonator 35 jump over the first resonator 34 and are easily coupled with each other due to the generation of an electric or magnetic field coupling circuit. Resonator 33
And the maximum point of the magnetic field of the second resonator 35 are opposed to each other, and magnetic field coupling is easily generated. Further, there is a possibility that the electric field of the first coupling circuit 25 and the electric field of the second coupling circuit 26 are coupled. Further, these relationships may be related to resonators 34, 35, 36 or
The same was true for 37.38.39 or 36.37.38.

Therefore, in order to further improve such a distributed constant filter, it is desirable to suppress unintended coupling between these resonators and to suppress parasitic characteristic deterioration.

The present invention has been devised in view of the above problems, and an object of the present invention is to provide a passband characteristic in which the amplitude characteristic and the group delay characteristic are simultaneously flat, and the transmission zero is defined in the stopband. It has a bandpass characteristic that can be realized and can be realized by designing with an accurate design method and configuring with a simple circuit, and also suppresses unintended coupling between resonators to suppress parasitic characteristic deterioration. An object of the present invention is to provide a distributed constant filter having low element sensitivity and low loss characteristics.

[0020]

SUMMARY OF THE INVENTION A distributed constant filter according to the present invention comprises a numerator rational polynomial which is an even function of a complex frequency s and has at least one set of real and at least one imaginary root, and a Hurwitz polynomial of a complex frequency s. Is obtained by frequency-transforming a normalized low-pass filter whose transfer function is represented by a network function consisting of a denominator rational polynomial
A distributed constant filter having a frequency band-pass characteristic realized by an unbalanced distributed constant circuit, wherein a circuit portion corresponding to a real root or an imaginary root of the molecular rational polynomial includes a first and a second resonator; A first coupling circuit that cascade-couples one resonator and a circuit outside the first resonator;
A second coupling circuit for cascading the resonator and a third coupling circuit for cascading the second resonator and a circuit outside the second coupling circuit;
A multi-resonator filter having two or more unit coupling circuit units each including a coupling circuit and a fourth coupling circuit that couples the outside of the first coupling circuit and the third coupling circuit by bridge coupling. The first coupling circuit and the third coupling circuit are the same kind of combination of electric field coupling or magnetic field coupling, and the first coupling circuit, the third coupling circuit, and the second coupling circuit are respectively coupled by electric field coupling or magnetic field coupling. The unit coupling circuit portion, which is a combination of different types, and corresponds to the real root, wherein the second coupling circuit and the fourth coupling circuit each include the same type of coupling circuit of electric field coupling or magnetic field coupling, and correspond to the imaginary root. The unit coupling circuit section is characterized in that the second coupling circuit and the fourth coupling circuit are respectively composed of different coupling circuits of electric field coupling or magnetic field coupling. .

[0021]

According to the distributed constant filter of the present invention, there is provided a multi-resonator having two or more unit-coupled circuit portions having a circuit portion corresponding to a real root or an imaginary root of a molecular rational polynomial of a network function. Since the filter is realized, a circuit can be configured and realized theoretically accurately, the structure of the filter is simplified, the loss is suppressed, and the performance is improved.

Here, the degree of the numerator rational polynomial is the fourth order or higher having at least one set of real roots and imaginary roots, and each set of roots is assigned to the formation of each coupling circuit unit. The degree of the Hurwitz polynomial of the denominator rational polynomial is an order that is at least three orders greater than the order of the numerator rational polynomial (degree of numerator + 3 ≤ order of denominator). Are assigned. A set of imaginary roots of a molecular rational polynomial is assigned to the formation of the unit coupling circuit portion in which the second coupling circuit and the fourth coupling circuit are heterogeneous couplings of electric field coupling or magnetic field coupling. The real connection of the molecular rational polynomial is assigned to the formation of the unit connection circuit section in which the fourth connection circuit and the fourth connection circuit are the same kind of connection of electric field coupling or magnetic field coupling.

Although the distributed constant filter of the present invention is realized by an unbalanced distributed constant circuit, each coupling circuit of each unit coupling circuit unit has a charge on each resonator of the unit coupling circuit unit. And electric field coupling, or current and magnetic field coupling.

Further, the fourth coupling circuit of each unit coupling circuit section is formed by, for example, a lumped-constant reactance element, or a coupling between electric field and electric field or a coupling between current and magnetic field on resonators at both ends of the unit coupling circuit section. Can also be realized.

Further, according to the distributed constant filter of the present invention, the network function representing the transfer function of the scaled low-pass filter is an even function of the complex frequency s, at least one set of real roots and at least one set of imaginary functions. Because the numerator rational polynomial with roots and the denominator rational polynomial that is a Hurwitz polynomial of complex frequency s are used, the passband characteristic of the amplitude is flattened with a set of real roots of the numerator rational polynomial. And an attenuation pole, which is a transmission zero point whose frequency is given by a set of imaginary roots, can be generated in the vicinity of the pass band. Band-pass characteristics that ensure desired simultaneous flatness characteristics for amplitude characteristics and group delay characteristics by imposing individual conditions, while ensuring sufficient attenuation by transmission zeros in the stop band. It can be obtained a filter having.

By using an unbalanced distributed constant circuit such as a microstrip circuit, an ideal transformer and a gyrator can be easily realized, and a series resonance circuit and a parallel resonance circuit can be easily realized. A distributed constant filter having a bandpass characteristic and having a simplified circuit configuration constituted by an unbalanced distributed constant circuit can be obtained.

In order to realize the distributed constant filter of the present invention, the phase characteristics of the normalized low-pass filter are first determined by a Hurwitz polynomial having a phase linear characteristic, which is a denominator rational polynomial, and then by a numerator rational polynomial. The imaginary root of the even function of a certain complex frequency s is specified so that the transmission zero is arranged at a desired frequency, and the real root of the numerator rational polynomial is determined so that the amplitude characteristic becomes flat in the pass band.

Next, a normalized low-pass filter using this as a transfer function is synthesized from a network function composed of the numerator rational polynomial and the denominator rational polynomial.

Next, the element having a negative value is converted into an element having a real positive value by equivalent conversion, the frequency is converted into a band-pass characteristic, and then equivalently converted into an unbalanced distributed constant circuit, thereby forming a distributed constant filter. Realize.

Hereinafter, the distributed constant filter of the present invention will be described in detail.

As an example of realizing the minimum order of the distributed constant filter of the present invention, a numerator rational polynomial is a fourth-order polynomial f (s) having a set of real and imaginary roots, and a denominator rational polynomial is a 7th-order Hurwitz polynomial. Let g (s) be the network function as a function of complex frequency s = jω,

[0032]

(Equation 1)

## EQU1 ## Where the denominator rational polynomial g
(S) is a polynomial having a flat group delay characteristic, such as a Bessel polynomial.

Next, the amplitude characteristic of this denominator rational polynomial is
Along with correcting with a numerator rational polynomial without adversely affecting the group delay characteristic, a set of imaginary roots of the numerator rational polynomial gives
A transmission zero is provided in the stop band. Further, the amplitude characteristic is corrected using a set of real roots of the numerator rational polynomial so that the amplitude characteristic is as flat as possible in the pass band. In this way,
The numerator rational polynomial is determined corresponding to the denominator rational polynomial and the target filter characteristic.

Then, from the polynomial thus determined, a standardized low-pass filter as shown in an example of a circuit diagram in FIG. 3 is determined. In this scaled low-pass filter, the number of parallel or series ladder-type connections corresponds to the order of the Hurwitz polynomial, and in this example, seven. The two series resonant circuits connected in parallel are circuit units corresponding to a set of real roots and imaginary roots of the numerator rational polynomial, respectively.

Of the circuit units corresponding to the set of real roots and imaginary roots, the circuit elements of the series resonance circuit corresponding to the set of imaginary roots are both positive values, and can be realized by an actual circuit. On the other hand, one of the circuit elements of the series resonance circuit corresponding to the set of real roots has a negative value, and cannot be realized as an actual circuit as it is.

Then, equivalent conversion to the unit coupling circuit section is performed next. That is, the circuit shown in FIG. 3 is equivalently converted into a circuit as shown in FIG. 4 using an imaginary gyrator. The circuit shown in FIG. 4 is divided into two parts before and after the middle ideal transformer.

First, paying attention to the circuit on the left side,
A circuit as shown in FIG. Considering this circuit, as shown in FIG. 2B, a circuit including two imaginary gyrators, the circuits shown in FIGS. 2A and 2B can be obtained by appropriately replacing each other's parameters. It can be seen that both are equivalent. L and C in FIG. 5B indicate that the circuit element is an inductance and a capacitance, respectively, and their values are not shown. Similarly, j indicates a gyrator, and its value is not indicated. The sign of the gyrator j is assumed to be + unless otherwise indicated. These also apply to FIGS. 6 to 8 described below.

By focusing on the right portion of the circuit shown in FIG. 4 and performing the same processing, an equivalent circuit of this portion can be obtained. As a result, the scaled low-pass filter shown in FIG. 3 passes through the equivalent transformations of FIGS.
It is converted to an equivalent scaled low-pass filter as shown in FIG.

By introducing an imaginary gyrator and an ideal transformer into the equivalent scaled low-pass filter shown in FIG. 6 and equivalently converting all the circuit elements into capacitors having the same value in parallel, as shown in FIG. Thus, an equivalent scaled low-pass filter can be obtained. The equivalent normalized low-pass filter shown in FIG. 7 is exactly exactly equivalent to the normalized low-pass filter shown in FIG. The sign of the imaginary gyrator at the stage of performing this equivalent transformation is as shown in FIG.

Next, the standardized low-pass filter is converted into a band-pass filter having a desired band-pass characteristic by performing frequency conversion and impedance conversion. At this time, the capacitance in the circuit of FIG. 7 becomes a parallel resonance circuit by frequency conversion, but the imaginary gyrator remains unchanged. Then, when the imaginary gyrator is realized by connecting π-type constant reactance elements, the target band-pass filter becomes as shown in FIG.
The circuit configuration shown in FIG. In this band pass filter, the constant reactance element of the coupling circuit can be realized by electric field coupling or magnetic field coupling by narrow band approximation near the pass band.

Next, in the circuit shown in FIG. 8, attention is focused on the circuit on the left side from the central ideal transformer. Here, the constant reactance elements of four coupling circuits 10, 11, 12, and 13 in the figure and four resonance circuits (resonators) of 18, 19, 20, and 21
Are one unit coupling circuit unit. Reference numeral 10 denotes a first coupling circuit that cascade-connects the first resonator 19 and a circuit outside the first resonator 19, 11 denotes a second coupling circuit that cascade-connects the first resonator 19 and the second resonator 20, and 12 denotes a second coupling circuit. A third coupling circuit cascade-couples the two resonators 20 and circuits outside the second resonator 20, and a fourth coupling circuit 13 which couples the outside of the first coupling circuit 10 and the third coupling circuit 12 by bridge coupling.

Next, in order to suppress unnecessary jump coupling between the coupling circuits, the coupling reactance elements 10 and 11 shown in FIG.
In the cascade connection by 12 or 14, 15 and 16, the signs of the adjacent coupling circuits are different signs, that is, the first coupling circuits 10 and 14 and the third coupling circuits 12 and 16
Have the same sign, and the first coupling circuits 10 and 14 or the third coupling circuits 12 and 16 and the second coupling circuits 11 and 15 have different signs,
The combination of the first coupling circuits 10 and 14, the second coupling circuits 11 and 15, and the third coupling circuits 12 and 16 is repeated, that is, electric field coupling and magnetic field coupling are repeated. First coupling circuit 10/14
And the second coupling circuits 11 and 15 are coupling circuits of different signs from each other, that is, one is an electric field coupling and the other is a magnetic field coupling. Undesirable jump-to-links to the substrate are suppressed. This effect is the same for the third coupling circuits 12 and 16, the second coupling circuits 11 and 15, and the resonator 21 and the first resonator 19.

The condition between the sign of the second coupling circuit 11 and the sign of the fourth coupling circuit 13 depends on the state of the root of the numerator rational polynomial.

That is, when the signs of the circuit elements of the series resonance circuit connected in parallel in the normalized low-pass filter of FIG. 3 are both positive values, that is, the numerator rational polynomial f
In the case of a circuit element corresponding to the set of imaginary roots of (s), the sign of the second coupling circuit 11 and the sign of the fourth coupling circuit 13 are opposite, that is, opposite signs, and one of them becomes electric field coupling, The other is a magnetic field coupling and a heterogeneous coupling. Further, a transmission zero is formed at this portion.

On the other hand, in the standardized low-pass filter of FIG. 3, the signs of the circuit elements of the series resonant circuit connected in parallel are different, and when one is positive and the other is negative, that is, the numerator rational polynomial f ( If the circuit element corresponds to the set of real roots of s), the sign of the second coupling circuit 11 and the fourth coupling circuit 13
Is the same as the reference symbol, that is, the same reference symbol, and both are electric field coupling or magnetic field coupling, resulting in the same kind of coupling. Further, in this portion, the amplitude characteristic of the pass band is corrected to be flat.

Next, attention is paid to the circuit on the right side of the central ideal transformer in FIG. As shown in the figure, if 14 is a first coupling circuit, 15 is a second coupling circuit, 16 is a third coupling circuit, and 17 is a fourth coupling circuit, it can be handled in exactly the same manner as the circuit on the left side of the ideal transformer described above. . Here, 22 to 24 are resonators, 22 is a first resonator, and 23 is a second resonator.

For example, assume that the circuit on the left side of the ideal transformer corresponds to the set of real roots of the numerator rational polynomial f (s), and the circuit on the right side corresponds to the set of imaginary roots of the numerator rational polynomial f (s).
Then, of the respective coupling circuits, 10, 12, 14, 16, 17 are magnetic field coupling, and 11, 13, 15 are electric field coupling. Alternatively, it is a combination in which the electric field coupling and the magnetic field coupling are interchanged.

Next, FIG. 1 shows a circuit diagram of an embodiment of the circuit of the band-pass filter obtained as a result of performing narrow-band approximation. FIG. 1 shows a distributed constant filter according to the present invention.
FIG. 2 is a plan view showing a configuration example in which the embodiment of the narrow band approximation shown in FIG.

The configuration example of the distributed constant filter of the present invention shown in FIG. 2 is formed by a conductor pattern as a distributed constant circuit element on a dielectric substrate. In this example, seven circular resonators 50 to 50 are provided. 56 is used in _E110 mode.

In such a distributed constant filter according to the present invention, the coupling sections 42 to 45 in FIG. 2 are connected to the coupling circuits 10 to 13 of the first unit coupling circuit section in FIG. 53 correspond to the resonance circuits (resonators) 18 to 21 in FIG. Similarly, the coupling portions 46 to 49 in FIG. 2 correspond to the coupling circuits 14 to 17 of the second unit coupling circuit portion in FIG. 8, and the resonators 54 to 56 in FIG. (Resonators) 22 to 24, respectively. When the resonance mode of each resonator 50-56 is E ₁₁₀ mode, as shown in FIG. 2, there is an electric field maximum point 180 degree intervals around the circumference of the respective resonators, electric coupling in this part Can be. The magnetic field maximum point is located at the midpoint of the electric field maximum point, that is, at a position shifted by 90 degrees from the electric field maximum point, and magnetic field coupling can be performed at this portion. Utilizing such an arrangement, as shown in FIG. 2, a distributed constant filter as a target band-pass filter in which seven resonators 50 to 56 are coupled can be formed.

In FIG. 2, 43, 45 and 47 indicate electric field coupling and 42
44, 46, 48, and 49 are magnetic field couplings, so that undesired jumping couplings are suppressed.

According to such a distributed constant filter of the present invention, a bandpass filter of an accurate equivalent circuit as shown in FIG. 1 can be accurately realized for each element as the conductor pattern shown in FIG. It can be realized by designing it with an accurate design method and configuring it with a simple circuit, and because it can configure a filter with the minimum number of elements and patterns for a given characteristic, it has low element sensitivity and low loss. It becomes a characteristic distribution constant filter.

It should be noted that the above is only an example of the embodiment of the present invention, and the present invention is not limited to the embodiment. Various modifications and improvements can be made without departing from the spirit of the present invention. No problem. For example, a resonator pattern having another shape may be used. Further, all of the electric field coupling and the magnetic field coupling in the embodiment shown in FIG. 2 may be exchanged, or the electric field coupling and the magnetic field coupling may be exchanged for each unit coupling circuit unit in the embodiment shown in FIG. Good.

[0055]

As described above, according to the present invention, a polynomial that is an even function of a complex frequency s and has at least one set of real roots and at least one set of imaginary roots is defined as a numerator rational polynomial,
A distributed constant filter having a frequency band-pass characteristic whose characteristic is determined by a transfer function of a network function having a Hurwitz polynomial of a complex frequency s as a denominator rational polynomial, forming a circuit portion corresponding to the root of the numerator rational polynomial Means for cascading the first and second resonators, a first coupling circuit for cascading the first resonator and a circuit outside the first resonator, and a second coupling for cascading the first resonator and the second resonator. A unit coupling circuit including a circuit, a third coupling circuit for cascading the second resonator and a circuit outside the second resonator, and a fourth coupling circuit for coupling the outside of the first coupling circuit and the third coupling circuit by bridge coupling And a multi-resonator filter having two or more parts, wherein the first and third coupling circuits are of the same kind of combination of electric field coupling or magnetic field coupling.
The coupling circuit, the third coupling circuit, and the second coupling circuit are each a different combination of electric field coupling or magnetic field coupling,
The unit coupling circuit unit corresponding to the real root is configured such that the second coupling circuit and the fourth coupling circuit are respectively composed of the same type of coupling circuit of electric field coupling or magnetic field coupling, and the unit coupling circuit unit corresponding to the imaginary root is coupled to the second coupling circuit. Since the circuit and the fourth coupling circuit are respectively composed of different types of coupling circuits of electric field coupling or magnetic field coupling, the amplitude characteristic and the group delay characteristic of the pass band are simultaneously flat, and the frequency having the transmission zero point in the stop band is obtained. It has band-pass characteristics, can be implemented by designing with an accurate design method, and can be realized with a simple circuit, and suppresses unintended coupling between resonators to suppress parasitic characteristic degradation. It is possible to provide a distributed constant filter having low sensitivity and low element sensitivity.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an embodiment of a bandpass filter according to the present invention.

FIG. 2 is a plan view showing a configuration example in which the embodiment of the band-pass filter shown in FIG. 1 is realized by a distributed constant filter.

FIG. 3 is a circuit diagram showing an example of a normalized low-pass filter according to the present invention.

FIG. 4 is a circuit diagram showing an example of a normalized low-pass filter subjected to equivalent conversion in the present invention.

FIGS. 5A and 5B are circuit diagrams showing examples of equivalent conversion for a normalized low-pass filter according to the present invention.

FIG. 6 is a circuit diagram showing an example of a normalized low-pass filter subjected to equivalent conversion in the present invention.

FIG. 7 is a circuit diagram showing an example of a normalized low-pass filter subjected to equivalent conversion in the present invention.

FIG. 8 is a circuit diagram showing a configuration example of a bandpass filter according to the present invention.

FIGS. 9A and 9B are diagrams showing an amplitude characteristic and a group delay characteristic in a pass band of a band-pass filter, respectively.

FIG. 10 is a block diagram illustrating a configuration example of a conventional band-pass filter.

FIG. 11 is a circuit diagram showing a configuration example for realizing a transmission zero in a stop band of a conventional filter.

FIG. 12 is a block diagram showing a configuration example for realizing a transmission zero in a stop band of a conventional filter.

FIG. 13 is a circuit diagram showing a configuration example for realizing a transmission zero in a stop band of a conventional filter.

FIG. 14 is a plan view showing an example in which the embodiment of the band-pass filter is realized by a distributed constant filter.

[Explanation of symbols]

 10, 14, 42, 46 ... first coupling circuit 11, 15, 43, 47 ... second coupling circuit 12, 16, 44, 48 ... third coupling circuit 13, 17, 45, 49... Fourth coupling circuit 19, 22, 51, 54... First resonator 20, 23, 52, 55... Second resonator

Claims (1)

[Claims] 1. A network function comprising a numerator rational polynomial that is an even function of a complex frequency s and has at least one set of real roots and at least one imaginary root, and a denominator rational polynomial that is a Hurwitz polynomial of a complex frequency s. A distributed constant filter having a frequency band-pass characteristic, obtained by performing frequency conversion on a normalized low-pass filter whose transfer function is represented, and realized by an unbalanced distributed constant circuit, wherein the numerator rational polynomial A circuit portion corresponding to a real root or an imaginary root includes a first and a second resonator, a first coupling circuit for cascading the first resonator and a circuit outside the first resonator, A second coupling circuit for cascading the second resonator, a third coupling circuit for cascading the second resonator and a circuit outside the second resonator, and a third coupling circuit for coupling the first resonator to the third resonator.
A multi-resonator filter having two or more unit coupling circuit sections each including a fourth coupling circuit that couples the outside of the coupling circuit by bridge coupling; and realizing an electric field between the first coupling circuit and the third coupling circuit. The first coupling circuit and the third coupling circuit and the second coupling circuit are different kinds of combinations of electric field coupling and magnetic field coupling, respectively. The unit coupling circuit section may be configured such that the second coupling circuit and the fourth coupling circuit are respectively composed of the same kind of coupling circuits of electric field coupling or magnetic field coupling, and the unit coupling circuit section corresponding to the imaginary root is the second coupling circuit. And a fourth coupling circuit comprising a different coupling circuit of electric field coupling or magnetic field coupling.