JP2020088531A - Band pass filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a good reflection characteristic over a wide band including a band while maintaining a low loss in the band and a large attenuation characteristic outside the band in a band pass filter. SOLUTION: A parallel circuit of a first resistor 1 connected between an input terminal 9 and an output terminal 10 and a series resonant circuit 3, and two second resistors having one end connected to both ends of the parallel circuit. A band-passing filter including 2 and a parallel resonant circuit 6 having one end connected to the other end of the second resistor 2 and the other end grounded, and at a frequency on the low frequency side in the band, the above-mentioned first. One resistor 1 is short-circuited by the series resonance circuit 3, the parallel resonance circuit 6 can be regarded as an equivalent capacitor, and at the frequency on the high frequency side in the band, the series resonance circuit 3 is equivalent. Can be regarded as an inductor, and the other end of the second resistor 2 is open. [Selection diagram] Fig. 1

Description

The present invention relates to a bandpass filter that allows signals in a band to pass without being attenuated, significantly suppresses unnecessary waves outside the band, and has good reflection characteristics over a wide band.

In RF (Radio Frequency) equipment such as radar equipment, communication equipment, and observation equipment, unnecessary waves outside the desired signal are prevented from entering unwanted waves, or harmonics or intermodulation distortion components generated inside these equipment are prevented. A band pass filter is used to prevent unnecessary waves such as leaking out of the device.
As such a bandpass filter, there is one that is composed of a lumped constant element of an inductor and a capacitor, and is designed to maintain good reflection characteristics with low loss in the band and to have almost total reflection outside the band. Therefore, a signal in the band can be passed without being attenuated, and a large amount of attenuation can be obtained outside the band to remarkably suppress unnecessary waves (for example, refer to Patent Document 1).

JP, 2010-141859, A

In order to apply such a bandpass filter to an RF device, a module using an active device such as an FET (Field Effect Transistor) or a HEMT (High Electron Mobility Transistor) is connected to the input/output unit of the bandpass filter. There are many. Usually, the module is designed to have good reflection characteristics in the band, but it is not always good reflection characteristics outside the band, and often the reflection characteristics are poor (close to total reflection).

In such a case, there is a problem that large multiple reflection occurs between the band pass filter and the module outside the band, and the module oscillates or operates unstable. Further, the attenuation characteristic of the bandpass filter outside the band is significantly deteriorated, and the RF device is deteriorated due to a high level unwanted wave entering the RF device, or the unwanted wave generated inside the RF device is outside the RF device. There was also a problem to be leaked to.

The bandpass filter of the present invention is
A parallel circuit of a resistor X and a series resonance circuit connected between the input terminal and the output terminal,
A resistor Y having one end connected to the parallel circuit,
A band pass filter comprising: a parallel resonance circuit having one end connected to the other end of the resistor Y and the other end grounded,
At low frequencies in the band,
The resistor X is short-circuited by the series resonance circuit,
The parallel resonant circuit can be equivalently regarded as a capacitor,
At high frequencies in the band,
The series resonance circuit can be equivalently regarded as an inductor,
The other end of the resistor Y is open.

According to the present invention, it is possible to obtain a good reflection characteristic over a wide band including the band while maintaining a low loss in the band and a large attenuation property outside the band.

It is a figure which shows the structure of the band pass filter by Embodiment 1 of this invention. It is a figure which shows the simple equivalent circuit of the band pass filter by Embodiment 1 of this invention. FIG. 3 is a diagram showing loss characteristics of a parallel circuit of a first resistor and a series resonant circuit and a series circuit of a second resistor and a parallel resonant circuit which configure the bandpass filter according to the first embodiment of the present invention. It is a figure which shows the design example of the characteristic of the band pass filter by Embodiment 1 of this invention. It is a figure which shows the structure of the other Example of the band pass filter by Embodiment 1 of this invention. It is a figure which shows the structure of the band pass filter by Embodiment 2 of this invention. It is a figure which shows the simple equivalent circuit of the band pass filter by Embodiment 2 of this invention. It is a figure which shows the example of a design of the characteristic of the band pass filter by Embodiment 2 of this invention. It is a figure which shows the structure of the other Example of the band pass filter by Embodiment 2 of this invention. It is a figure which shows the structure of the band pass filter by Embodiment 3 of this invention. It is a figure which shows the design example of the characteristic of the band pass filter by Embodiment 3 of this invention. It is a figure which shows the structure of the band pass filter by Embodiment 4 of this invention. It is a figure which shows the design example of the characteristic of the band pass filter by Embodiment 4 of this invention.

***Definition of terms***
Minimum frequency fmin=Minimum frequency of band of band pass filter Maximum frequency fmax=Maximum frequency of band of band pass filter Low frequency fl=Low frequency of band (fmin≦fl)
High frequency side frequency fh = High frequency side frequency of the band (fh ≤ fmax)
Out-of-band=frequency region lower than minimum frequency and frequency region higher than maximum frequency The relationship between frequencies is as follows.
Out of band <fmin≦fl<fh≦fmax<Out of band R1, R2, R3, R4=resistance values L1, L2, L1′=inductance C1, C2, C2′=capacitance

Embodiment 1.
The first embodiment according to the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing a configuration of a bandpass filter according to the first embodiment.
This band pass filter has a parallel circuit of a first resistor 1 and a series resonance circuit 3 loaded between an input terminal 9 and an output terminal 10, and one end of a second resistor 2 is provided at both ends of this parallel circuit. Each is connected.
Further, the other end of each second resistor 2 is terminated by the parallel resonant circuit 6.
The series resonance circuit 3 is composed of a series circuit of an inductor 4 and a capacitor 5, and the parallel resonance circuit 6 is composed of a parallel circuit of an inductor 7 and a capacitor 8. Such a bandpass filter is configured by using a chip-shaped first resistor 1, a second resistor 2, inductors 4 and 7, and capacitors 5 and 8.
Here, the first resistor 1 and the second resistor 2 are selected as R1 and R2, respectively, and the inductor 4, the capacitor 5, the inductor 7 and the capacitor 8 are selected as L1, C1, L2 and C2, respectively.

FIG. 2 is a simple equivalent circuit of the bandpass filter according to the first embodiment of the present invention.
When the resonance frequency of the series resonance circuit 3 is selected as the frequency fl on the low frequency side in the band, the first resistor 1 is short-circuited by the series resonance circuit 3 in fl.
Further, at the frequency fh on the high frequency side in the band, the series resonance circuit 3 can be equivalently regarded as an inductor (inductance L1′).
On the other hand, when the resonance frequency of the parallel resonance circuit 6 is selected to be fh, the other end of the second resistor 2 is opened at the parallel resonance circuit 6, and the parallel resonance circuit 6 is equivalent to a capacitor (capacitance C2′) at fl. Can be considered
Moreover, since the series resonance circuit 3 has a high impedance characteristic and the parallel resonance circuit 6 has a low impedance characteristic outside the band, the π-type attenuator composed of the first resistor 1 and the second resistor 2 outputs the input terminal 9 and the output. It can be regarded as loaded between the terminal 10 and the terminal 10.

Here, the relationship between R1 and R2 forming the π-type attenuator and the attenuation amount K can be obtained by Expression 1.

[Formula 1]
R1=2*R2*Z0*Z0/(R2*R2-Z0*Z0)
R2=Z0*(K+1)/(K-1)

In this equation, Z0 is a power source impedance or a load impedance, which is usually 50Ω. By selecting R1 and R2 in this way, it is possible to obtain a π-type attenuator having an attenuation amount K over a wide band and excellent reflection characteristics regardless of the frequency.

Further, the relationship between L1 and C1 of the series resonance circuit 3 and L2 and C2 of the parallel resonance circuit 6 can be obtained by Expression 2. Here, ωl and ωh are angular frequencies, and have a relationship of ωl=2πfl and ωh=2πfh.

[Formula 2]
L1*C1=1/(ωl*ωl)
L2*C2=1/(ωh*ωh)

FIG. 3 is a diagram showing the loss characteristics of the parallel circuit of the first resistor 1 and the series resonance circuit 3 and the series circuit of the second resistor 2 and the parallel resonance circuit 6 which constitute the band pass filter of this embodiment. Is.
FIG. 3 shows the case where the minimum frequency fmin of the band is the frequency fl on the low frequency side, and the maximum frequency fmax of the band is the frequency fh on the high frequency side.
In the figure, as shown in (a), the parallel circuit of the first resistor 1 and the series resonance circuit 3 exhibits a convex characteristic in which the loss is minimum at fl.
Further, as shown in (b), the series circuit of the second resistor 2 and the parallel resonant circuit 6 exhibits a convex characteristic in which the loss is minimum at fh.
Here, if fl and fh are close to each other, the influence of L1′ in fh of the series resonance circuit 3 and the influence of C2′ in fl of the parallel resonance circuit 6 can be ignored. Therefore, the parallel circuit of the first resistor 1 and the series resonant circuit 3 has a low impedance characteristic, and the series circuit of the second resistor 2 and the parallel resonant circuit 6 has a high impedance characteristic over the band fl or more and fh or less. Therefore, nothing is loaded between the input terminal 9 and the output terminal 10, and it can be approximately regarded as a through.

By selecting the relationship between R1 and R2 and K as shown in Equation 1, and the relationship between L1, C1 and fl and the relationship between L2, C2 and fh as shown in Equation 2, low loss is achieved in the band from fl to fh. A band pass filter having an attenuation amount K outside the band and having a good reflection characteristic over a wide band including the band can be obtained.
In this way, by selecting the resonance frequencies of the series resonance circuit 3 and the parallel resonance circuit 6 within the band and by selecting these resonance frequencies at different frequencies, the band pass filter can be broadened.

FIG. 4 is a design example of the characteristics of the bandpass filter according to the first embodiment.
Here, the series resonance circuit 3 is selected to have L1=4.7 nH and C1=1.5 pF so that series resonance occurs at the frequency fl=1.9 GHz on the low frequency side in the band. Further, the parallel resonance circuit 6 is selected to have L2=7.2 nH and C2=0.8 pF so that parallel resonance occurs at the frequency fh=2.1 GHz on the high frequency side in the band. Further, R1=71.2Ω and R2=96.2Ω are selected so that the attenuation amount becomes 10 dB.
As shown in FIG. 4, the bandpass filter of this embodiment has an attenuation of approximately 0 dB over a band of 1.9 GHz or more and 2.1 GHz or less, and an attenuation of approximately 10 dB in other frequency bands. Good reflection characteristics of about 20 dB or more can be obtained over a wide band of frequencies from 0 to 10 GHz.

As described above, the bandpass filter according to the present embodiment selects the resonance frequencies of the series resonance circuit 3 and the parallel resonance circuit 6 within the band, and selects these resonance frequencies at different frequencies. It is possible to achieve a wide band and low loss. Further, a desired amount of attenuation can be obtained outside the band, and good reflection characteristics can be obtained over a wide band including the band.
By applying this bandpass filter to an RF device, it is possible to significantly suppress large multiple reflections between the module and the bandpass filter that occur outside the band, and prevent unstable operation of the module and significant deterioration of the attenuation of the bandpass filter. You can For this reason, it is possible to prevent characteristic deterioration of the RF device due to entry of a high-level unwanted wave outside the band and leakage of the unwanted wave generated in the RF device to the outside of the RF device.

In FIG. 4, the case where the resonance frequency of the series resonance circuit 3 is selected to be 1.9 GHz on the low band side and the resonance frequency of the parallel resonance circuit 6 is selected to 2.1 GHz on the high band side is shown. Stated. However, the resonance frequency of the series resonance circuit 3 may be 2.1 GHz, the resonance frequency of the parallel resonance circuit 6 may be 1.9 GHz, and these resonance frequencies may be other frequencies.
Further, in order to widen the band of the band pass filter, it can be achieved by satisfying the expression 2 and making L1 small and C1 large, or making L2 large and C2 small.
In addition, widening the band of the band pass filter can be achieved by widening the detuning width of the resonance frequencies of the series resonant circuit 3 and the parallel resonant circuit 6. In this case, although the loss in the band slightly increases, the desired attenuation characteristic can be obtained in the band and the good reflection characteristic can be obtained in the wide band.
Further, R1 and R2 are shown so that the attenuation amount becomes 10 dB, but R1 and R2 can be arbitrarily set from the mathematical expression 1 according to the request.

*** Other examples ***
FIG. 5 is a diagram showing the configuration of another example of the bandpass filter according to the first embodiment.
The same or corresponding parts as in FIG. 1 are designated by the same reference numerals.
In this bandpass filter, one end of a second resistor 2 is connected to both ends of a parallel circuit of a first resistor 1 and a series resonance circuit 3, and the other ends of these second resistors 2 are combined into one. It is terminated by the parallel resonant circuit 6.
Even in the case of such a configuration, the equivalent circuit inside and outside the band is the same as that of FIG. Therefore, even with this bandpass filter, low loss can be achieved within the band, and good reflection characteristics can be obtained over a wide band.

Therefore, by applying this bandpass filter to an RF device, it is possible to prevent characteristic deterioration of the RF device and leakage of unnecessary waves generated in the RF device to the outside of the RF device as in the case of FIG.
In FIG. 1, the other ends of the two second resistors 2 are terminated by the parallel resonant circuit 6, respectively, whereas in FIG. 5, the other ends of the two second resistors 2 are combined into one parallel resonant circuit. It ends at 6. As a result, the number of parallel resonance circuits 6 can be reduced, and the band pass filter and the RF device can be downsized and the price can be reduced.

***Features of Embodiment 1***
The bandpass filter of this embodiment has a parallel circuit of a first resistor 1 (resistor X) and a series resonance circuit 3 between an input terminal 9 and an output terminal 10, and a parallel circuit is loaded at both ends of the parallel circuit. One end of the second resistor 2 (resistor Y) is connected. Further, in the band-pass filter of this embodiment, the other end of each second resistor 2 (resistor Y) is grounded via the parallel resonance circuit 6, or each second resistor 2 (resistor Y) is connected. The other ends of the above are collectively grounded via one parallel resonance circuit 6.
The series resonance circuit 3 and the parallel resonance circuit 6 are composed of an inductor and a capacitor. The resonance frequencies of these resonance circuits are different from each other, and the respective resonance frequencies are selected within the band.

***Effect of Embodiment 1***
According to the bandpass filter of this embodiment, the first resistor 1 is kept in the band by the series resonance circuit 3 at a low impedance close to a short circuit, and the second resistor 2 is close to the open state in the parallel resonance circuit 6. It is kept at high impedance. Therefore, the input terminal 9 and the output terminal 10 can be approximately regarded as a through.
On the other hand, outside the band, the series resonance circuit 3 has a high impedance characteristic and the parallel resonance circuit 6 has a low impedance characteristic. Therefore, a π-type attenuator composed of the first resistor 1 and the second resistor 2 is used. It can be regarded as loaded between the input terminal 9 and the output terminal 10.
Therefore, it is possible to obtain a good reflection characteristic over a wide band including the band while maintaining a low loss in the band and a large attenuation characteristic outside the band.
As a result, a low-loss bandpass filter is achieved within the band, a wide band is realized, and a desired large attenuation characteristic is obtained outside the band, and a bandpass filter having good reflection characteristics over the wideband including the band is provided. Obtainable.
When such a bandpass filter is applied to an RF device, a large multiple reflection that occurs between the bandpass filter and the module outside the band can be significantly suppressed, and the unstable operation of the module outside the band, It is possible to prevent deterioration of the attenuation amount of the bandpass filter.
As a result, it is possible to prevent the characteristics of the RF device from deteriorating due to the intrusion of unnecessary waves outside the band and the leakage of unnecessary waves generated in the RF device to the outside of the RF device.

Embodiment 2.
In the second embodiment, points different from the above-described embodiments will be described.
FIG. 6 is a diagram showing the configuration of the bandpass filter according to the second embodiment.
The same or corresponding parts as in FIG. 1 are designated by the same reference numerals.
This bandpass filter has two third resistors 11 connected in series between the input terminal 9 and the output terminal 10, and the series resonance circuit 3 is connected in parallel to each of these third resistors 11. Person.
Further, one end of the fourth resistor 12 is connected to the connecting portion of these two third resistors 11, and the other end of the fourth resistor 12 is terminated by the parallel resonant circuit 6.
Further, the series resonant circuit 3 and the parallel resonant circuit 6 are the same as those in the first embodiment, but the third resistor 11 and the fourth resistor 12 are different from those in the first embodiment and are selected as R3 and R4, respectively. Has been.

FIG. 7 is a simple equivalent circuit of the bandpass filter according to the second embodiment of the present invention. Similar to the first embodiment, when the resonance frequency of the series resonance circuit 3 is selected as the frequency fl on the low frequency side in the band, the third resistor 11 is short-circuited by the series resonance circuit 3 in fl.
Further, at the frequency fh on the high frequency side in the band, the series resonance circuit 3 can be equivalently regarded as an inductor (inductance L1′). On the other hand, when the resonance frequency of the parallel resonant circuit 6 is selected to be fh, the other end of the fourth resistor 12 is opened at the parallel resonant circuit 6, and the parallel resonant circuit 6 is equivalent to a capacitor (capacitance C2′) at fl. Can be considered
Further, since the series resonance circuit 3 has a high impedance characteristic and the parallel resonance circuit 6 has a low impedance characteristic outside the band, a T-type attenuator including the third resistor 11 and the fourth resistor 12 is used as the input terminal 9 and the output terminal. It can be regarded as loaded between 10 and.

Here, the relationship between the attenuation amount K and R3 and R4 forming the T-type attenuator can be obtained by the mathematical formula 3.

[Formula 3]
R3=Z0*(K-1)/(K+1)
R4=2*K*Z0/((K*K)-1)

In this formula 3, Z0 is a power source impedance or a load impedance, which is usually 50Ω, like the formula 1. By selecting R3 and R4 in this way, it is possible to obtain a T-type attenuator having an attenuation amount K over a wide band regardless of the frequency and having a good reflection characteristic.

FIG. 8 is a design example of the characteristics of the bandpass filter according to the second embodiment of the present invention. Here, as in the first embodiment, L1=2.3 nH, C1=3.0 pF, L2=3 so that the resonance frequencies of the series resonance circuit 3 and the parallel resonance circuit 6 are 1.9 GHz and 2.1 GHz, respectively. .4 nH, C2=1.7 pF, and R3=26.0 Ω and R2=31.5 Ω so that the attenuation amount is 10 dB. Here, L1 and L2 that configure the series resonant circuit 3 and the parallel resonant circuit 6 are selected to be 1/2 of FIG. 4 and C1 and C2 are doubled, but the resonant frequency does not change.
As shown in FIG. 8, the bandpass filter according to the present embodiment provides a large attenuation amount of about 0 dB at 1.9 GHz or more and 2.1 GHz or less, and about 10 dB in other frequency bands. Moreover, the bandpass filter of this embodiment can obtain a good reflection characteristic of about 20 dB or more over a wide band of frequencies 0 to 10 GHz. This characteristic is equivalent to that of the first embodiment shown in FIG.

In this design example as well, the case where the resonance frequency of the series resonance circuit 3 is selected to 1.9 GHz on the low band side and the resonance frequency of the parallel resonance circuit 6 is selected to 2.1 GHz on the high band side has been described. The resonance frequency of the series resonance circuit 3 may be 2.1 GHz and the resonance frequency of the parallel resonance circuit 6 may be 1.9 GHz, or may be another frequency.
Further, in order to widen the band of the band pass filter, it can be achieved by satisfying the expression 2 and making L1 small and C1 large, or making L2 large and C2 small.
In addition, widening the band of the band pass filter can be achieved by selecting the resonant frequencies of the series resonant circuit 3 and the parallel resonant circuit 6 in accordance with the bandwidth. In this case, although the loss in the band slightly increases, the desired attenuation characteristic can be obtained in the band and the good reflection characteristic can be obtained in the wide band.

Moreover, although R3 and R4 are set so that the attenuation amount is 10 dB, R3 and R4 can be arbitrarily set from the mathematical expression 3 in accordance with a request.
Further, by adopting a T-type attenuator like the band-pass filter of this embodiment, R3 and R4 for obtaining the same amount of attenuation are larger than R1 and R2 of the π-type attenuator shown in FIG. , A resistance value of about 1/3 is sufficient. There is an effect that the choices of the first embodiment or the second embodiment are increased depending on the required attenuation amount outside the band.

As described above, even in the bandpass filter according to the second embodiment of the present invention, the attenuation amount in the band is about 0 dB, the large attenuation amount is out of the band, and the good reflection characteristic is obtained over the wide band including the band. By applying this bandpass filter to an RF device, it is possible to significantly suppress large multiple reflections between the module and the bandpass filter that occur outside the band, and prevent unstable operation of the module and significant deterioration of the attenuation of the bandpass filter. You can For this reason, it is possible to prevent characteristic deterioration of the RF device due to entry of a high-level unwanted wave outside the band and leakage of the unwanted wave generated in the RF device to the outside of the RF device.

*** Other examples ***
FIG. 9 is a diagram showing the configuration of another example of the bandpass filter according to the second embodiment.
The same or corresponding parts as in FIG. 6 are designated by the same reference numerals.
In this bandpass filter, one series resonance circuit 3 is connected in parallel to both ends of two third resistors 11 connected in series, and one end of a fourth resistor 12 is connected to a connection portion of these third resistors 11. And the other end of the fourth resistor 12 is terminated by the parallel resonant circuit 6.
Even in the case of such a configuration, the equivalent circuit inside and outside the band is almost the same as that of FIG. Therefore, even with this bandpass filter, the amount of attenuation in the band can be suppressed low, and good reflection characteristics can be obtained over a wide band.

Therefore, by applying this bandpass filter to an RF device, it is possible to prevent characteristic deterioration of the RF device and leakage of unnecessary waves generated in the RF device to the outside of the RF device as in the case of FIG. While the series resonance circuit 3 is connected in parallel to each of the two third resistors 11 in FIG. 6, one series resonance circuit 3 is connected in parallel by combining the two third resistors 11 as shown in FIG. Connected. As a result, the number of series resonance circuits 3 can be reduced, and the band pass filter and the RF device can be downsized and the price can be reduced.

***Features of the embodiment***
The bandpass filter of this embodiment is loaded with two third resistors 11 (resistors X) connected in series between the input terminal 9 and the output terminal 10, and each of the third resistors 11 (resistors X) is loaded. Alternatively, the series resonance circuit 3 is connected in parallel to the two third resistors 11 (resistors X) connected in series.
Further, one end of the fourth resistor 12 (resistor Y) is connected to the connecting portion of the two third resistors 11 (resistor X), and the other end of the fourth resistor 12 (resistor Y) is connected to the parallel resonance circuit 6 Grounded through.
The series resonance circuit 3 and the parallel resonance circuit 6 are composed of an inductor and a capacitor. The resonance frequencies of these resonance circuits are different from each other, and the respective resonance frequencies are selected within the band.

According to the bandpass filter of this embodiment, the third resistor 11 is kept in the band by the series resonance circuit at a low impedance close to a short circuit, and the fourth resistor 12 is the parallel resonance circuit in a high impedance close to an open circuit. Kept in. Therefore, the input terminal 9 and the output terminal 10 can be approximately regarded as a through.
On the other hand, outside the band, the series resonant circuit has a high impedance and the parallel resonant circuit has a low impedance characteristic. Therefore, the T-type attenuator including the third resistor 11 and the fourth resistor 12 serves as the input terminal 9. It can be regarded as being loaded between the output terminal 10. Therefore, it is possible to obtain a bandpass filter having good reflection characteristics over a wide band including the band while maintaining low loss within the band.
When such a bandpass filter is applied to an RF device, large multiple reflections that occur outside the band can be significantly suppressed, and unstable operation of the module outside the band and deterioration of the attenuation of the bandpass filter can be prevented. be able to.
Further, the values of the third resistor 11 and the fourth resistor 12 for obtaining the same amount of attenuation outside the band are about 1/3 of the values of the first resistor and the second resistor constituting the π-type attenuator. Then, by selecting these and configuring the band pass filter, there is an effect that the degree of freedom in design is increased.

Embodiment 3.
In the third embodiment, points different from the above-described embodiments will be described.
FIG. 10 is a diagram showing the configuration of the bandpass filter according to the third embodiment.
The same or corresponding parts as in FIG. 1 are designated by the same reference numerals.
In general, a transmission line whose length is sufficiently shorter than a wavelength (for example, 1/10 or less) and has a high characteristic impedance can be approximately regarded as an inductor, and a transmission line having a low characteristic impedance can be regarded as a capacitor. ..
FIG. 10 shows an example in which the inductor 7 constituting the parallel resonant circuit 6 of the first embodiment shown in FIG. 1 is replaced with a first transmission line 13 having a characteristic impedance Z1 and one end having a length a1 short-circuited. Is replaced with a second transmission line 14 having one end having a characteristic impedance Z2 and a length a2.

FIG. 11 is a design example of the characteristics of the bandpass filter according to the third embodiment of the present invention. Here, the characteristic impedance of the first transmission line 13 and the length in the free space are selected as Z1=80Ω and a1=20 mm, and the characteristic impedance and length of the second transmission line 14 are Z2=25Ω and a2=6. The value is selected to be 0.5 mm, and the other values are the same as those shown in FIG.
In the bandpass filter of this embodiment, a large resonance is observed near 8 GHz outside the band. On the other hand, the amount of attenuation is almost 0 dB in the band of 1.9 GHz or more and 2.1 GHz or less, and about 10 dB outside the band, and good reflection characteristics of about 20 dB or more are obtained over 0 to 7 GHz. Performance equivalent to that of the first embodiment shown can be obtained.
The resonance near 8 GHz is because the inductor component of the first transmission line 13 and the capacitor component of the second transmission line 14 have frequency characteristics, and the parallel resonant circuit 6 resonates in parallel at 8 GHz in addition to 2 GHz. ..

In the band-pass filter according to the third embodiment of the present invention, although the attenuation amount and the reflection characteristic are deteriorated at a specific frequency outside the band, there are many cases in which it does not substantially cause a problem. If resonance at 8 GHz is a problem, it is possible to shift the resonance frequency to a non-problematic frequency by further increasing Z1 and shortening a1, or further lowering Z2 and shortening a2.
Therefore, by applying this bandpass filter to an RF device, it is possible to remarkably suppress large multiple reflections between the module and the bandpass filter that occur outside the band, and cause unstable operation of the module and significant deterioration of the attenuation amount of the bandpass filter. Can be prevented. For this reason, it is possible to prevent characteristic deterioration of the RF device due to entry of a high-level unwanted wave outside the band and leakage of the unwanted wave generated in the RF device to the outside of the RF device.

Thus, by replacing the inductor 7 forming the parallel resonant circuit 6 with the first transmission line 13 and the capacitor 8 with the second transmission line 14, the transmission lines 13 and 14 are integrated into the microwave integrated circuit. It can be integrally formed on the dielectric substrate by using a technique. By using the transmission lines 13 and 14, it is not necessary to use the chip-shaped inductor 7 and the capacitor 8, so that an inexpensive band pass filter can be realized.
In the embodiment shown here, the case where the inductor 7 is replaced with the first transmission line 13 and the capacitor 8 is replaced with the second transmission line 14 has been described, but when either one is replaced with the transmission line. However, the effect is the same. Further, even when the inductor 4 forming the series resonance circuit 3 is replaced with a transmission line, the effect remains the same.
Furthermore, the effect is the same even when these transmission lines 13 and 14 are applied to the second embodiment.

***Features of Embodiment 3***
In the bandpass filter of this embodiment, at least one of the inductor and the capacitor forming the series resonance circuit 3 or the parallel resonance circuit 6 is formed by a transmission line that is sufficiently shorter than the wavelength.

***Effects of Embodiment 3***
According to the bandpass filter of this embodiment, by configuring at least one of the inductor and the capacitor forming the series resonance circuit 3 or the parallel resonance circuit 6 by a transmission line that is sufficiently shorter than the wavelength, It is possible to maintain good reflection characteristics over a wide range with low loss. Therefore, it is possible to prevent the characteristics of the RF device from deteriorating due to the intrusion of unnecessary waves outside the band and the leakage of the unnecessary waves generated in the RF device to the outside of the RF device.
This transmission line can be easily realized on a dielectric substrate by using microwave integrated circuit technology, and it is not necessary to use chip parts such as an inductor and a capacitor, so that an inexpensive band pass filter and RF equipment can be obtained.

Fourth Embodiment
In the fourth embodiment, points different from the above-described embodiments will be described.
FIG. 12 is a diagram showing the configuration of the bandpass filter according to the fourth embodiment.
The same or corresponding parts as in FIG. 10 are designated by the same reference numerals.
In the bandpass filter of this embodiment, a fifth resistor 15 is loaded at one end of the second transmission line 14 constituting the parallel resonant circuit 6 shown in the third embodiment of FIG. 10 on the second resistor 2 side. It is a thing. By loading the fifth resistor 15 in this way, it is possible to reduce the selectivity Q of the parallel resonant circuit 6 by loading a slight loss on the second transmission line 14 outside the band.

FIG. 13 is a design example of the characteristics of the bandpass filter according to the fourth embodiment of the present invention. Here, the fifth resistor 15 is selected to be 3Ω, and the others are the same as those in the third embodiment shown in FIG.
Also in the bandpass filter of this embodiment, the amount of attenuation is approximately 0 dB within the band and approximately 10 dB outside the band, and a good reflection characteristic of approximately 18 dB is obtained over 0 to 10 GHz. In the case of the present invention, although the attenuation and the reflection characteristics are slightly deteriorated in the vicinity of 8 GHz, a large deterioration in the attenuation and the reflection characteristics as shown in FIG. 11 can be significantly suppressed.

As described above, by loading the fifth resistor 15 on one end of the second transmission line 14, it is possible to remarkably suppress the large deterioration of the attenuation amount and the reflection characteristic as seen at 8 GHz, and the first embodiment and the second embodiment. The characteristics equivalent to are obtained. Similarly to the first transmission line 13 and the second transmission line 14, the fifth resistor 15 can be integrally formed on the dielectric substrate by using the microwave integrated circuit technology, and the fifth chip-shaped fifth resistor 15 can be formed. It is not necessary to use the resistor 15, and a high-performance and inexpensive band pass filter can be realized.
In addition, in this embodiment, the case where the fifth resistor 15 is loaded on the second transmission line 14 is shown. However, even if the fifth resistor 15 is loaded on the first transmission line 13 or both of them are effective. There is no change.

***Features of Embodiment 4***
The bandpass filter according to this embodiment is one in which a fifth resistor 15 is loaded on a transmission line forming the parallel resonant circuit 6.

***Effects of Embodiment 4***
According to the bandpass filter of this embodiment, at least one of the inductors or capacitors forming the parallel resonant circuit 6 is formed by a transmission line, and the transmission line is loaded with a fifth resistor 15. .. As a result, it is possible to suppress the significant deterioration of the attenuation characteristic and the reflection characteristic due to the resonance outside the band of the parallel resonant circuit, a low loss is obtained within the band, a desired attenuation amount is obtained outside the band, and a good reflection characteristic is obtained over the wide band. can get. Therefore, it is possible to prevent the characteristics of the RF device from deteriorating due to the intrusion of unnecessary waves and the leakage of unnecessary waves generated in the RF device to the outside of the RF device.
This fifth resistor can also be easily realized on the dielectric substrate by using the microwave integrated circuit technology together with the transmission line, it is not necessary to use chip parts such as an inductor and a capacitor, and a band pass filter with an inexpensive and good characteristic and RF equipment is obtained.

***Other Embodiments***
All or part of the above-described embodiments may be combined with other embodiments.
The bandpass filter according to the above-described embodiments may be applied to radar equipment, communication equipment, observation equipment, and the like, and may also be applied to microwave equipment such as telemetry transmitters and beacon transmitters.

1 1st resistance, 2 2nd resistance, 3 series resonance circuit, 4 inductor, 5 capacitor, 6 parallel resonance circuit, 7 inductor, 8 capacitor, 9 input terminal, 10 output terminal, 11 3rd resistance, 12th 4 resistance, 13 1st transmission line, 14 2nd transmission line, 15 5th resistance.

Claims (11)

A parallel circuit of a resistor X and a series resonance circuit connected between the input terminal and the output terminal,
A resistor Y having one end connected to the parallel circuit,
A band pass filter comprising: a parallel resonance circuit having one end connected to the other end of the resistor Y and the other end grounded,
At low frequencies in the band,
The resistor X is short-circuited by the series resonance circuit,
The parallel resonant circuit can be equivalently regarded as a capacitor,
At high frequencies in the band,
The series resonance circuit can be equivalently regarded as an inductor,
A band pass filter in which the other end of the resistor Y is open. A parallel circuit of a first resistor and a series resonance circuit connected between the input terminal and the output terminal,
Two second resistors, one end of which is connected to both ends of the parallel circuit,
A bandpass filter having a parallel resonant circuit, one end of which is connected to the other end of the second resistor and the other end of which is grounded,
At low frequencies in the band,
The first resistor is short-circuited by the series resonant circuit,
The parallel resonant circuit can be equivalently regarded as a capacitor,
At high frequencies in the band,
The series resonance circuit can be equivalently regarded as an inductor,
A bandpass filter in which the other end of the second resistor is opened. The band pass according to claim 2, wherein at a frequency out of the band, the π-type attenuator composed of the first resistor and the second resistor can be regarded as being connected between the input terminal and the output terminal. filter. The parallel resonance circuit includes two parallel resonance circuits that ground the other ends of the two second resistors, or one parallel resonance circuit that grounds the other ends of the two second resistors. The band pass filter according to 2 or 3. A parallel circuit of two serially connected third resistors connected in series between the input terminal and the output terminal, and a series resonant circuit;
A fourth resistor whose one end is connected to the connecting portion of the two third resistors,
A bandpass filter comprising a parallel resonant circuit, one end of which is connected to the other end of the fourth resistor and the other end of which is grounded,
At low frequencies in the band,
The third resistor is short-circuited by the series resonant circuit,
The parallel resonant circuit can be equivalently regarded as a capacitor,
At high frequencies in the band,
The series resonance circuit can be equivalently regarded as an inductor,
A band pass filter in which the other end of the fourth resistor is opened. The band-pass filter according to claim 5, wherein a T-type attenuator including a third resistor and a fourth resistor can be regarded as being connected between the input terminal and the output terminal at a frequency outside the band. The series resonance circuit has two series resonance circuits connected in parallel to the two third resistors respectively, or one series resonance circuit connected in parallel to the two third resistors. 6. The bandpass filter according to item 6. The resonance frequencies of the series resonance circuit and the parallel resonance circuit are different from each other,
8. The bandpass filter according to claim 1, wherein the resonance frequencies of the series resonance circuit and the parallel resonance circuit are within a band. 9. The band pass filter according to claim 1, wherein the series resonant circuit and the parallel resonant circuit each include an inductor and a capacitor. The band pass filter according to claim 1, wherein the series resonant circuit and the parallel resonant circuit have transmission lines that are sufficiently shorter than a wavelength. The bandpass filter according to claim 10, further comprising a fifth resistor connected in series with the transmission line.

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