JP2009201034A - Filter circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that, when the number of filter stages is reduced, in a conventional filter circuit, a loss can be reduced in a desired band but smoothness of the loss within the band and a great out-of-band attenuation characteristic can not be obtained and that, when the number of stages is increased oppositely, flat loss characteristics within the band and a great out-of-band attenuation amount can be obtained but the loss is increased within the band. <P>SOLUTION: In a filter circuit, a T-type circuit 5 and a π-type circuit 8 constituted of inductors and capacitors are connected in parallel in such a way that, input impedances of these T-type circuit and π-type circuit become a double of power supply impedance, respectively, and cut-off frequencies of the T-type circuit and the π-type circuit become approximately equal. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a microwave or millimeter wave band filter circuit used in a radar device, a communication device, or the like.

  In microwave equipment used for radar equipment and communication equipment, etc., in order to suppress unnecessary waves other than the desired signal incident from the antenna, or to prevent unnecessary waves generated in these equipment from being output to the outside A filter circuit is used. In general, the filter circuit includes a low-pass filter, a high-pass filter, a band-pass filter, and the like, and these filter circuits are selectively used according to the unnecessary wave to be suppressed.

  Among these filter circuits, a low-pass filter is known as one that passes a low-frequency band signal and suppresses an unnecessary wave in a higher frequency band than the signal (for example, a patent document). 1).

JP 2002-208808 JP

  A conventional low-pass filter basically includes a T-shaped circuit including two inductors connected in series and a capacitor provided between a connection portion of these inductors and the ground. In such a low-pass, by selecting a filter cut-off frequency fc (usually a frequency at which a pass loss is 3 dB) between a signal frequency band and an unnecessary wave frequency band, an unnecessary wave is not attenuated. Can only be suppressed.

  In general, it is desirable for such a low-pass filter to obtain a flat and low loss characteristic within a desired band and to obtain a large attenuation characteristic outside the band. For this purpose, it is necessary to obtain a steep attenuation characteristic near the cutoff frequency fc. Such characteristics depend on the number of elements of the inductor and the capacitor, that is, the number of stages. When the number of elements is small, the characteristic exhibits a gentle attenuation characteristic near the cutoff frequency fc, and as a result, the loss is low in the band. However, there is a problem that the flatness of loss within the band and a large amount of attenuation cannot be obtained outside the band.

On the other hand, when the number of elements is large, the attenuation characteristics are steep near the cutoff frequency, and a flat loss characteristic within the band and a large attenuation outside the band are obtained, but there is a problem that the loss increases within the band. .
For example, when a multi-stage low-pass filter is applied to a microwave device such as a high-power amplifier in order to give priority to unwanted wave suppression outside the band, the output power of the high-power amplifier is reduced and the efficiency is also lowered. It will be. In particular, when a filter circuit with a large number of stages is applied with priority given to unnecessary wave suppression of a 100 W class high-power amplifier using a wide band gap device, the efficiency of the high-power amplifier is reduced and high heat generation occurs. However, there is a problem that the reliability of the high-power amplifier is also lowered.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain a filter circuit that is flat in the band, has a low loss characteristic, and has a large attenuation characteristic outside the band.

  A filter circuit according to the present invention includes a T-type circuit including two first inductors connected in series, a first capacitor provided between a connection portion of the first inductor and a ground, and a second circuit. A π-type circuit including an inductor and a second capacitor provided between both ends of the second inductor and the ground is provided, and the T-type circuit and the π-type circuit are connected in parallel.

  A T-type circuit including two third capacitors connected in series; a third inductor provided between a connection portion of the third capacitor and the ground; a fourth capacitor; And a π-type circuit comprising a fourth inductor provided between both ends of the capacitor 4 and the ground, and the T-type circuit and the π-type circuit may be connected in parallel.

  Further, a cascaded series circuit formed by cascading two series circuits of a fifth inductor and a fifth capacitor, and a sixth inductor and a sixth capacitor provided between the connection part of the cascaded series circuit and the ground A T-type circuit composed of a parallel circuit with a capacitor, a seventh inductor and a seventh capacitor in series, an eighth inductor and an eighth inductor provided between both ends of the series circuit and the ground, respectively. A π-type circuit including a parallel circuit with a capacitor may be provided, and the T-type circuit and the π-type circuit may be connected in parallel.

  Furthermore, a cascaded series circuit formed by cascading two parallel circuits of a ninth inductor and a ninth capacitor, and a tenth inductor and a tenth provided between the connection part of the cascaded series circuit and the ground. A T-type circuit composed of a capacitor, a parallel circuit of an eleventh inductor and an eleventh capacitor, and a twelfth inductor and a twelfth provided between both ends of the parallel circuit and the ground, respectively. A π-type circuit composed of a series circuit with the capacitor may be provided, and the T-type circuit and the π-type circuit may be connected in parallel.

  According to the present invention, it is possible to obtain a flat loss characteristic over a required band up to the vicinity of the cutoff frequency and to obtain a large attenuation characteristic outside the band.

Embodiment 1 FIG.
Embodiment 1 according to the present invention will be described below with reference to the drawings. 1 is a diagram showing a configuration of a filter circuit according to Embodiment 1 of the present invention. This figure shows a case where a low-pass filter is configured as a filter circuit.

  In the figure, the filter circuit is composed of a T-type circuit 5 and a π-type circuit 8, and is connected between the input terminal 1 and the output terminal 2 so that these T-type circuit 5 and π-type circuit 8 are connected in parallel. Provided. The T-type circuit 5 includes two first inductors 3 connected in series, and a first capacitor 4 provided between a connection portion between the two first inductors 3 and the ground. The π-type circuit 8 includes one second inductor 6 and two second capacitors 7 respectively provided between both ends of the second inductor 6 and the ground.

These T-type circuit 5 and π-type circuit 8 both exhibit electrical characteristics that allow low-frequency band signals to pass through and attenuate high-frequency band unnecessary waves.
That is, the filter circuit of the present invention is characterized in that a low pass filter of a π-type circuit 8 is newly connected in parallel to the conventional low-pass filter of the T-type circuit 5.

  FIG. 2 illustrates a wiring structure in the case where the filter circuit according to the first embodiment is realized using the microwave integrated circuit technology. In the wiring structure example shown in FIG. 2A, for example, on an alumina ceramic substrate, two first inductors 3 made of microstrip lines that are sufficiently shorter than the wavelength and have high impedance, and one second An inductor 6 is formed. A multi-end of a chip-like first capacitor 4 whose one end is grounded through a through-hole 9 is connected to the connection portion of these two first inductors 3. Further, a chip-like second capacitor 7 having one end grounded through a through hole 9 is connected to both ends of the second inductor 6.

FIG. 2B shows another wiring structure in which a filter circuit is configured by connecting an open-ended line in place of the chip-like first capacitor 4 and second capacitor 7 shown in FIG. An example is shown. With this structure, although the shape is slightly larger than when the chip-like first capacitor 4 and second capacitor 7 are used, the element variation caused by the first capacitor 4 and the second capacitor 7 is increased. Therefore, a filter circuit with better reproducibility than that of FIG. 2A can be realized.
The first inductor 3 and the second inductor 6 can be easily realized by using coiled inductors instead of those formed by microstrip lines.

  Here, the first inductor 3 forming the T-type circuit 5 is L1, the first capacitor 4 is C1, the second inductor 6 forming the π-type circuit 8 is L2, and the second capacitor 7 is C2. By selecting L1 = L2 / 2 and C1 = 2 * C2, the cut-off frequencies fc of the low-pass filters composed of the T-type circuit 5 and the π-type circuit 8 can be made equal.

  FIG. 3 shows an example in which the input impedances of the T-type circuit 5 and the π-type circuit 8 under such conditions are represented on a Smith chart. In the figure, the solid line is the π-type circuit 8 and the broken line is the T-type circuit 5, both of which exist at the center of the Smith chart at the frequency f1 near the desired band, and as the frequency increases, the outer circumference of the Smith chart rotates clockwise. The reflection coefficient moves to a place close to 1 at the frequency f3 outside the band.

  The input impedance of the T-type circuit 5 and the input impedance of the π-type circuit 8 have the same absolute value of the reflection coefficient and a phase difference of 180 degrees, and both input impedances have an input reflection coefficient at f2 near the cutoff frequency. The real part of becomes zero.

  Further, when the T-type circuit 5 and the π-type circuit 8 are connected in parallel as in the filter circuit according to the first embodiment, the input impedance of the filter circuit is the T-type circuit 5 and the π-type circuit 8. Is the combined impedance. Accordingly, the input impedance of the T-type circuit 5 and the π-type circuit 8 in the required band is set to 100Ω, which is twice that of the conventional low-pass filter including only the T-type circuit 5, for example, that is, the first inductor 3 and the second By choosing the inductor 6 twice as much as the conventional one and the first capacitor 4 and the second capacitor 7 as half the conventional one, 50Ω equal to the input impedance of the conventional filter circuit can be obtained.

FIG. 4 is a diagram illustrating an example of loss characteristics of the filter circuit according to the first embodiment. In the figure, the solid line is an example of the loss characteristic of the low-pass filter according to the present invention, and the broken line is an example of the loss characteristic of the conventional low-pass filter composed of only the T-type circuit 5, which is shown for comparison.
As shown in FIG. 3, the input impedances of the T-type circuit 5 and the π-type circuit 8 constituting the low-pass filter according to the first embodiment have the same absolute value of the reflection coefficient and different phases by 180 degrees. In other words, at the frequency at which the input impedance of the T-type circuit 5 becomes high, the input impedance of the π-type circuit 8 becomes low. In this manner, the input impedances of the T-type circuit 5 and the π-type circuit 8 cancel each other, and the combined input impedance is substantially constant within the desired band near the frequency f1. For this reason, a flat loss characteristic can be obtained over the desired band as compared with the conventional low-pass filter.
In addition, by arranging the T-type circuit 5 and the π-type circuit 8 in parallel, the loss can be reduced by about 1/2 compared to a conventional low-pass filter constituted by one T-type circuit.

  At the frequency f2 near the cutoff frequency fc, the input reflection coefficients of the T-type circuit 5 and the π-type circuit 8 are in a conjugate relationship, and the real part of the input reflection coefficient is zero. Therefore, the combined input impedance of the T-type circuit 5 and the π-type circuit 8 becomes very high, and a large attenuation, that is, a notch is obtained in the vicinity of the frequency f2. Therefore, a steep attenuation characteristic near the cutoff frequency fc can be obtained as compared with the conventional low-pass filter.

  Furthermore, at the frequency f3 higher than the cutoff frequency fc, the input reflection coefficients of the T-type circuit 5 and the π-type circuit 8 are approximately 1. That is, since both the T-type circuit 5 and the π-type circuit 8 totally reflect the microwave incident from the input terminal 1, a large loss characteristic can be obtained as in the conventional low-pass filter.

  As described above, the filter circuit according to the first embodiment of the present invention can obtain a flat loss characteristic over a required band up to the vicinity of the cutoff frequency fc, and can obtain a large attenuation characteristic outside the band. In the conventional low-pass filter including only the T-type circuit 5, it is necessary to increase the number of stages in order to obtain such characteristics, and there is a problem that the loss increases. However, the low-pass filter of the present invention is desired without increasing the number of stages. In addition, there is an advantage that the loss can be reduced as compared with the conventional low-pass filter with the same number of stages.

Therefore, by applying the filter circuit of the present invention to microwave devices such as amplifiers, oscillators, converters, etc., unnecessary out-of-band waves can be significantly suppressed without degrading the performance of the microwave device due to increased loss of the filter circuit. be able to.
In particular, when applied to a hundred watt class amplifier using a wide band gap device that has been attracting attention in recent years, there is an advantage that it is possible to significantly reduce output and efficiency reduction and to sufficiently suppress unnecessary waves outside the band.

  FIG. 5 shows another example of the filter circuit according to Embodiment 1 of the present invention. In FIG. 1, the case where one T-type circuit 5 and one π-type circuit 8 are connected in parallel has been described. However, a plurality of T-type circuits 5 and π-type circuits 8 connected in cascade as shown in FIG. Even if both circuits are connected in parallel, the same effect as in FIG. 1 can be obtained. Furthermore, in this embodiment, although a plurality of T-type circuits 5 and π-type circuits 8 are connected in cascade, the loss in the band is slightly increased as compared with the case where each is constituted by one, A significantly large attenuation is obtained outside the band as compared with FIG. Therefore, the filter circuit having this configuration is effective when applied to a microwave device in which out-of-band attenuation is more problematic than loss.

Embodiment 2. FIG.
In the first embodiment, an example in which a low-pass filter is configured as a filter circuit has been described. In the second embodiment, a case in which a high-pass filter is configured will be described.
6A and 6B are diagrams showing an example of the configuration and structure of a filter circuit according to Embodiment 2 of the present invention. FIG. 6A is a configuration diagram and FIG. 6B is a structural diagram. Here, a case where a high-pass filter is used as the filter circuit will be described.

  As shown in FIG. 6A, this filter circuit is configured such that a T-type circuit 5 and a π-type circuit 8 are connected in parallel between an input terminal 1 and an output terminal 2. . The T-type circuit 5 includes one third inductor 10 and two third capacitors 11. The π-type circuit 8 includes two fourth inductors 12 and one fourth capacitor 13.

  These T-type circuit 5 and π-type circuit 8 both exhibit electrical characteristics as high-pass filters that block low frequencies and allow high frequencies to pass.

  FIG. 6B is an example of a wiring structure in the case where the filter circuit shown in FIG. 6A is realized using a microwave integrated circuit technology. In this high pass filter, two third capacitors 11 connected in series and one fourth capacitor 13 are arranged in parallel on an alumina ceramic substrate, for example. A third inductor 10 having one end grounded through a through hole 9 is connected to the connection portion. A fourth inductor 12 having one end grounded through a through hole 9 is connected to both ends of one fourth capacitor 13. The third inductor 10 and the fourth inductor 12 are formed of microstrip lines that are sufficiently shorter than the wavelength and have a high impedance.

Here, the third inductor 10 forming the T-type circuit 5 is L1, the third capacitor 11 is C1, the fourth inductor 12 and L2 forming the π-type circuit 8, and the fourth capacitor 13 is C2. By selecting L1 = L2 / 2 and C1 = 2 * C2, the cut-off frequencies fc of the high-pass filters composed of the T-type circuit 5 and the π-type circuit 8 can be made equal.
Under such conditions, similar to the low-pass filter shown in the first embodiment, the input impedance of the T-type circuit 5 and the input impedance of the π-type circuit 8 have the same absolute value of the reflection coefficient and a phase difference of 180 degrees. In both input impedances, the real part of the input reflection coefficient is zero in the vicinity of the cutoff frequency fc.

  Further, when the T-type circuit 5 and the π-type circuit 8 are connected in parallel as in the filter circuit according to the second embodiment, the input impedance of the filter circuit is the T-type circuit 5 and the π-type circuit 8. And the combined impedance. Therefore, the input impedance of the T-type circuit 5 and the π-type circuit 8 in the required band is set to 100Ω, which is twice that of the conventional high-pass filter, for example, that is, the third inductor 10 and the fourth inductor 12 are twice that of the conventional circuit. By selecting the third capacitor 11 and the fourth capacitor 13 to be 1/2 of the conventional one, 50Ω which is equal to the input impedance of the conventional filter circuit can be obtained.

FIG. 7 is an example showing loss characteristics of the filter circuit according to the second embodiment. In the figure, the solid line is an example of the loss characteristic of the filter circuit according to the second embodiment, and the broken line is an example of the loss characteristic of a conventional high-pass filter constituted by only a general T-type circuit, and is shown for comparison.
The input impedances of the T-type circuit 5 and the π-type circuit 8 constituting the high-pass filter of the second embodiment have the same absolute value of the reflection coefficient and a phase of 180 as in the low-pass filter shown in FIG. Varies. In other words, at the frequency at which the input impedance of the T-type circuit 5 becomes high, the input impedance of the π-type circuit 8 becomes low. As described above, the input impedances of the T-type circuit 5 and the π-type circuit 8 cancel each other, and the combined input impedance is substantially constant within the desired band. For this reason, a flatter loss characteristic than the conventional high-pass filter can be obtained over a desired band.
In addition, by arranging the T-type circuit 5 and the π-type circuit 8 in parallel, the loss can be reduced by about ½ compared to a conventional high-pass filter composed of one T-type circuit.

  At frequencies near the cutoff frequency fc, the reflection coefficients of the input impedances of the T-type circuit 5 and the π-type circuit 8 are in a conjugate relationship, and the real part of the reflection coefficient is zero. Therefore, the combined input impedance of the T-type circuit 5 and the π-type circuit 8 becomes very high, and a large attenuation, that is, a notch is obtained in the vicinity of the frequency f2. Therefore, a steep attenuation characteristic can be obtained in the vicinity of the cutoff frequency fc as compared with the conventional high-pass filter.

  Further, at a frequency lower than the cut-off frequency fc, the reflection coefficient of the T-type circuit 5 and the π-type circuit 8 is approximately 1. That is, since both the T-type circuit 5 and the π-type circuit 8 totally reflect the microwave incident from the input terminal 1, a large loss characteristic can be obtained as in the conventional high-pass filter.

  As described above, the filter circuit according to the second embodiment can obtain a flat loss characteristic over the band up to a frequency near the cutoff frequency fc, and can obtain a large attenuation characteristic outside the band. In the conventional high-pass filter only with the T-type circuit, it is necessary to increase the number of stages in order to obtain such characteristics, and there is a problem that the loss increases. However, in the high-pass filter of the second embodiment, the number of stages is increased. Therefore, desired characteristics can be obtained, and the loss can be reduced as compared with a high-pass filter having the same number of stages.

Therefore, by applying the filter circuit of the present invention to microwave devices such as amplifiers, oscillators, converters, etc., it is possible to remarkably suppress out-of-band unnecessary waves without incurring performance degradation of the microwave devices due to increased loss of the high-pass filter. be able to.
In particular, when applied to a hundred watt class amplifier using a wide band gap device that has been attracting attention in recent years, there is an advantage that it is possible to significantly reduce output and efficiency reduction and to sufficiently suppress unnecessary waves outside the band.

FIG. 8 shows another example of the filter circuit according to the second embodiment. In FIG. 6, the case where one T-type circuit 5 and one π-type circuit 8 are connected in parallel is described, but a plurality of T-type circuits 5 and π-type connected in cascade as shown in FIG. This is the same even if the circuit 8 is connected in parallel. By cascading a plurality of T-type circuits 5 and π-type circuits 8 in this way, the loss in the band is slightly increased as compared with the case of one each, but a significantly large attenuation can be obtained outside the band. .
This filter circuit is effective when applied to microwave equipment in which out-of-band attenuation is more problematic than loss.

Embodiment 3 FIG.
In the first and second embodiments, the example in which the low-pass filter and the high-pass filter are configured as the filter circuit has been described, but in the third embodiment according to the present invention, the case where the band-pass filter is configured as the filter circuit will be described.
FIG. 9 is a diagram showing an example of (a) configuration and (b) characteristics of a filter circuit according to Embodiment 3 of the present invention. Here, a case of a bandpass filter as the filter circuit is shown.

In this filter circuit, as shown in FIG. 9 (a), a T-type circuit 5 and a π-type circuit 8 are arranged in parallel between an input terminal 1 and an output terminal 2 in the same manner as the low-pass filter and the high-pass filter described above. Configured to be connected. The T-type circuit 5 includes two series circuits in which a fifth inductor 14 and a fifth capacitor 15 are connected in series, and is connected between the series circuits, and a sixth inductor 16 and a sixth capacitor 17. And a parallel circuit. The π-type circuit 8 includes a series circuit including a seventh inductor 18 and a seventh capacitor 19, and a parallel circuit of an eighth inductor 20 and an eighth capacitor 21 respectively connected to both ends of the series circuit. Consists of.
Each of the T-type circuit 5 and the π-type circuit 8 has a characteristic of passing a signal in a desired band and blocking the others.

Here, the fifth inductor 14 and the sixth inductor 16 forming the T-type circuit 5 are L1, L1 ′, the fifth capacitor 15, and the sixth capacitor 17 are C1, C1 ′, respectively, and π The seventh inductor 18 and the eighth inductor 20 forming the circuit 8 are denoted by L2 and L2 ′, respectively, the seventh capacitor 19 and the eighth capacitor 21 are denoted by C2 and C2 ′, respectively, and L1 = L2 / 2, By selecting C1 = 2 * C2, L1 ′ = L2 ′ / 2, and C1 ′ = 2 * C2 ′, the cut-off frequencies fc of the respective bandpass filters including the T-type circuit 5 and the π-type circuit 8 are made equal. be able to.
Under such conditions, similar to the low-pass filter shown in the first embodiment, the input impedance of the T-type circuit 5 and the input impedance of the π-type circuit 8 have the same absolute value of the reflection coefficient and a phase difference of 180 degrees. In both input impedances, the real part of the input reflection coefficient is zero near the cutoff frequency fc.

  Further, when the T-type circuit 5 and the π-type circuit 8 are connected in parallel as in the filter circuit according to the third embodiment, the input impedance of the filter circuit is the T-type circuit 5 and the π-type circuit 8. And the combined impedance. Therefore, by selecting the input impedance of the T-type circuit 5 and the π-type circuit 8 in the required band to be 100Ω, which is twice that of the conventional bandpass filter, for example, 50Ω equal to the input impedance of the conventional filter circuit can be obtained.

FIG. 9B is an example of the loss characteristic of the filter circuit according to the third embodiment. In the figure, the solid line is an example of the loss characteristic of the filter circuit of the present invention, and the broken line is an example of the loss characteristic of a general conventional bandpass filter, which is shown for comparison.
The input impedances of the T-type circuit 5 and the π-type circuit 8 constituting the band-pass filter of the third embodiment are the same in the absolute value of the reflection coefficient and the phase is 180, as in the low-pass filter shown in FIG. Varies. In other words, at the frequency at which the input impedance of the T-type circuit 5 becomes high, the input impedance of the π-type circuit 8 becomes low. As described above, the input impedances of the T-type circuit 5 and the π-type circuit 8 cancel each other, and the combined input impedance is substantially constant within the desired band.
For this reason, a flatter loss characteristic than the conventional band pass filter can be obtained over the desired band.
Also in the filter circuit of this configuration, by arranging the T-type circuit 5 and the π-type circuit 8 in parallel, about 1 as in the low-pass filter and the high-pass filter configured by one conventional T-type circuit. The loss of / 2 can be reduced.

  At frequencies near the cutoff frequency fc, the reflection coefficients of the input impedances of the T-type circuit 5 and the π-type circuit 8 are in a conjugate relationship, and the real part of the reflection coefficient is zero. Therefore, the input impedance of the combination of the T-type circuit 5 and the π-type circuit 8 becomes very high, and a large attenuation amount, that is, a notch is obtained locally. For this reason, compared with the conventional band pass filter, a large amount of attenuation can be obtained outside the band near the cutoff frequency fc.

  Further, in the frequency band lower and higher than the desired frequency band, the reflection coefficient of the T-type circuit 5 and the π-type circuit 8 is approximately 1. That is, since both the T-type circuit 5 and the π-type circuit 8 totally reflect the microwave incident from the input terminal 1, a large loss characteristic can be obtained as in the conventional high-pass filter.

  As described above, the filter circuit according to the third embodiment can obtain a flat loss characteristic over the band up to a frequency near the cutoff frequency fc, and can obtain a steep attenuation characteristic outside the band. In order to obtain such characteristics in the conventional band pass filter, it is necessary to increase the number of stages, and there is a problem that the loss increases, but in the band pass filter of the present invention, a desired characteristic can be obtained without increasing the number of stages, In addition, the loss can be reduced as compared with the band-pass filter having the same number of stages.

Therefore, by applying the bandpass filter of the third embodiment to microwave devices such as amplifiers, oscillators, converters, etc., out of band without incurring performance degradation of the microwave devices due to increased loss of the bandpass filter. Unwanted waves can be significantly suppressed.
Also, in the bandpass filter of the third embodiment, as in the case of the lowpass filter and the highpass filter, when applied to a hundred watt class amplifier using a wide bandgap device, the output and efficiency reduction can be remarkably reduced. In addition, there is an advantage that unnecessary waves outside the band can be sufficiently suppressed.
In the above, the case where one T-type circuit 5 and one π-type circuit 8 constituting the band-pass filter are used has been described. However, in the same manner as in FIG. 5 and FIG. There is no change.

Embodiment 4 FIG.
In the first and second embodiments, the example in which the low-pass filter and the high-pass filter are configured as the filter circuit has been described. In the fourth embodiment according to the present invention, a case in which the band rejection filter is configured as the filter circuit will be described.
FIG. 10 is a diagram showing the configuration of the filter circuit according to the fourth embodiment of the present invention. Here, a case of a band rejection filter as the filter circuit is shown.

As shown in FIG. 10, this filter circuit is provided between an input terminal 1 and an output terminal 2 so that a T-type circuit 5 and a π-type circuit 8 are connected in parallel, similarly to a band-pass filter.
The T-type circuit 5 is connected to two parallel circuits in which a ninth inductor 22 and a ninth capacitor 23 are connected in parallel, and to a connection part of these parallel circuits, and the tenth inductor 24 and the tenth circuit It consists of a series circuit with a capacitor 25. The π-type circuit 8 includes a parallel circuit of an eleventh inductor 26 and an eleventh capacitor 27, and a series circuit of a twelfth inductor 28 and a twelfth capacitor 29 connected to both ends of the parallel circuit, respectively. Consists of. Each of these T-type circuit 5 and π-type circuit 8 has a characteristic of passing a signal in a desired band and blocking the others.

Also in the filter circuit according to the fourth embodiment having such a configuration, as described in the first to third embodiments, the input impedances of the T-type circuit 5 and the π-type circuit 8 at frequencies near the cutoff frequency fc. Are in a conjugate relationship, and the real part of the reflection coefficient is zero. Therefore, the input impedance of the combination of the T-type circuit 5 and the π-type circuit 8 becomes very high, and a large attenuation amount, that is, a notch is obtained locally.
For this reason, an attenuation is obtained over a band near the cutoff frequency fc.
Further, by arranging the T-type circuit 5 and the π-type circuit 8 in parallel, it is flat outside the band and has a low loss as compared with the conventional one constituted by the T-type circuit 5 or the π-type circuit 8. Characteristics can be obtained.
By applying such a filter circuit to a microwave device, only an unnecessary wave having a specific frequency component can be attenuated.

As described above, by forming a filter circuit such as a low-pass filter, a high-pass filter, or a band-pass filter by connecting a T-type circuit and a π-type circuit in parallel, the desired band is flat and low loss characteristics can be obtained. At the same time, a large attenuation can be obtained outside the band.
In addition, the band rejection filter can obtain a large attenuation characteristic in a desired band and a flat and low loss characteristic outside the band.
Furthermore, by applying these filter circuits to microwave devices and millimeter-wave circuits, it is possible to prevent unnecessary waves generated in the microwave devices and millimeter-wave circuits while minimizing the deterioration of the characteristics of the microwave devices and millimeter-wave circuits. Can be remarkably suppressed. Moreover, the influence on the microwave apparatus and millimeter wave circuit by the unnecessary wave input via an antenna etc. can be suppressed small.

It is a figure which shows the structure of the filter circuit by Embodiment 1 of this invention. It is a figure which shows an example of the structure in the case of implement | achieving the filter circuit by Embodiment 1 of this invention using a microwave integrated circuit technique. It is a figure which shows an example of the input impedance of the T-type circuit and pi-type circuit which comprise the filter circuit by Embodiment 1 of this invention. 6 is a diagram illustrating an example of loss characteristics of the filter circuit according to Embodiment 1. FIG. FIG. 6 is a diagram showing another example of the filter circuit according to the first embodiment of the present invention. FIG. 6 is a diagram showing the configuration and structure of a filter circuit according to a second embodiment of the present invention. FIG. 6 is a diagram showing an example of loss characteristics of the filter circuit according to the second embodiment of the present invention. FIG. 10 is a diagram showing another example of the filter circuit according to the second embodiment of the present invention. FIG. 6 is a diagram showing an example of the configuration and loss characteristics of a filter circuit according to a third embodiment of the present invention. FIG. 6 is a diagram showing a configuration of a filter circuit according to a fourth embodiment of the present invention.

Explanation of symbols

  1 input terminal, 2 output terminal, 3 first inductor, 4 first capacitor, 5 T-type circuit, 6 second inductor, 7 second capacitor, 8 π-type circuit, 9 through-hole, 10 third Inductor, 11 3rd capacitor, 12 4th inductor, 13 4th capacitor, 14 5th inductor, 15 5th capacitor, 16 6th inductor, 17 6th capacitor, 18 7th inductor, 19 7th capacitor, 20 8th inductor, 21 8th capacitor, 22 9th inductor, 23 9th capacitor, 24 10th inductor, 25 10th capacitor, 26 11th inductor, 27 8th inductor 11 capacitors, 28 12th inductor, 29 12th capacitor.

Claims (4)

A T-shaped circuit including two first inductors connected in series, and a first capacitor provided between a connection portion of the first inductor and the ground;
A π-type circuit comprising a second inductor, and a second capacitor provided between both ends of the second inductor and the ground,
With
A filter circuit in which the T-type circuit and the π-type circuit are connected in parallel. A T-shaped circuit comprising two third capacitors connected in series, and a third inductor provided between the connection portion of the third capacitor and the ground;
A π-type circuit comprising a fourth capacitor, and a fourth inductor provided between both ends of the fourth capacitor and the ground,
With
A filter circuit in which the T-type circuit and the π-type circuit are connected in parallel. A cascaded series circuit formed by cascading two series circuits of a fifth inductor and a fifth capacitor, and a sixth inductor and a sixth capacitor provided between a connection part of the cascaded series circuit and the ground; A T-shaped circuit comprising:
A π-type circuit comprising a seventh inductor and a seventh capacitor series circuit, and a parallel circuit of an eighth inductor and an eighth capacitor, each provided between both ends of the series circuit and the ground;
With
Filter circuit in which the T-type circuit and the π-type circuit are connected in parallel A cascaded series circuit formed by cascading two parallel circuits of a ninth inductor and a ninth capacitor, and a tenth inductor and a tenth capacitor provided between a connection part of the cascaded series circuit and the ground; A T-shaped circuit comprising:
A π-type circuit comprising a parallel circuit of an eleventh inductor and an eleventh capacitor, and a series circuit of a twelfth inductor and a twelfth capacitor, each provided between both ends of the parallel circuit and the ground;
With
A filter circuit in which the T-type circuit and the π-type circuit are connected in parallel.

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