JP7183349B2 - Filters and electronics - Google Patents

Description

The present invention relates to filters and electronic devices.

Radar equipment, communication equipment, observation equipment, etc., prevent unwanted waves, which are radio waves outside the desired signal band, from entering from the outside of the equipment, or prevent unwanted waves and harmonics generated inside the equipment from leaking to the outside of the equipment A low-pass filter is used for

Patent Document 1 describes a low-pass filter having a configuration in which two inductors are connected in series between an input terminal and an output terminal, and a capacitor is connected between the connection point of these inductors and ground. ing.

JP-A-08-078916

A low-pass filter having a large out-of-band loss is required for a device in which the leakage of unwanted waves or harmonics generated inside the device to the outside of the device is significantly restricted.

In the low-pass filter described in Patent Document 1, as the frequency increases, the loss outside the band increases, but the slope of the increase is gentle. Therefore, the above requirements cannot be satisfied.

Out-of-band loss is highly dependent on the number of filter stages. That is, as the number of stages increases, the slope of the loss outside the band increases, and a large loss characteristic is obtained. However, in-band loss also tends to increase. Moreover, there is also a problem that the low-pass filter is enlarged.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a filter with a small number of stages that can achieve large out-of-band loss and low in-band loss characteristics.

According to one aspect of the present invention, a filter that passes signals of frequencies within a certain band includes
two or more second capacitors connected in parallel;
At least two second inductors connected in series to at least two second capacitors out of the two or more second capacitors to form at least two series resonant circuits having different resonant frequencies and being out of the band. When,
and one or more first inductors connected between any two of the at least two series resonant circuits.

According to the present invention, it is possible to provide a filter with a small number of stages, but with large out-of-band loss and low in-band loss characteristics.

2 is a diagram showing the configuration of a low-pass filter according to Embodiment 1; FIG. 4 is a diagram showing a T-shaped configuration of a low-pass filter according to Comparative Example 1; FIG. 4 is a diagram showing a simple equivalent circuit of a parallel resonant circuit used in the low-pass filter according to Embodiment 1; FIG. 5 is a graph showing an example of loss characteristics of low-pass filters according to Embodiment 1 and Comparative Example 1; 4 is a graph showing a design example of the low-pass filter according to Embodiment 1; FIG. 4 shows a configuration of a low-pass filter according to a modification of Embodiment 1; FIG. 4 shows a configuration of a low-pass filter according to Embodiment 2; 8 is a diagram showing a π-shaped configuration of a low-pass filter according to Comparative Example 2; FIG. FIG. 8 is a diagram showing a simple equivalent circuit of a parallel resonant circuit used in the low-pass filter according to Embodiment 2; 9 is a graph showing a design example of a low-pass filter according to Embodiment 2; FIG. 10 is a diagram showing the configuration of a low-pass filter according to a modification of the second embodiment; FIG. FIG. 10 is a diagram showing the configuration of a low-pass filter according to Embodiment 3; 10 is a graph showing an example of loss characteristics of the low-pass filter according to Embodiment 3; FIG. 12 is a diagram showing a configuration of a low-pass filter according to a modification of Embodiment 3; FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. In each figure, the same reference numerals are given to the same or corresponding parts. In the description of the embodiments, the description of the same or corresponding parts will be omitted or simplified as appropriate. It should be noted that the present invention is not limited to the embodiments described below, and various modifications can be made as necessary. For example, among the embodiments described below, two or more embodiments may be combined and implemented. Alternatively, one embodiment or a combination of two or more of the embodiments described below may be partially implemented.

Embodiment 1.
This embodiment will be described with reference to FIGS. 1 to 5. FIG.

*** Configuration description ***
The configuration of low-pass filter 10 according to the present embodiment will be described with reference to FIG.

The low-pass filter 10 is a filter that passes signals of frequencies within a certain band. Specifically, the low-pass filter 10 is a filter that passes signals in a band lower than a certain frequency without attenuation, and significantly attenuates unnecessary waves or harmonics outside the band. This operation corresponds to the filtering method according to this embodiment. A configuration similar to that of the low-pass filter 10 may be applied to other band-pass filters or matching circuits of amplifiers.

Low-pass filter 10 is applied to any electronic device. Although not shown, as a specific example, the low-pass filter 10 is applied to electronic equipment having a signal processing device that processes a signal that has passed through the low-pass filter 10 . Such electronic equipment includes radar equipment, communication equipment, observation equipment, and the like.

The low-pass filter 10 comprises two first inductors 13, 15, two first capacitors 14, 16 and one second capacitor 17. The two first inductors 13, 15 are connected in series. The two first capacitors 14 and 16 are connected in parallel to the two first inductors 13 and 15 to form two parallel resonant circuits 18 and 19 having different resonant frequencies and out-of-band frequencies. A second capacitor 17 is connected between the junction of the two first inductors 13, 15 and ground.

Specifically, the low-pass filter 10 includes a parallel resonance circuit 18 of a first inductor 13 and a first capacitor 14, a first inductor 15 and a first capacitor 16, and a parallel resonance circuit 18 of a first inductor 13 and a first capacitor 14, between an input terminal 11 and an output terminal 12. and a parallel resonant circuit 19 are mounted in series, and a second capacitor 17 is loaded between the connecting portion of the parallel resonant circuit 18 and the parallel resonant circuit 19 and the ground.

The configuration of the low-pass filter 90 according to Comparative Example 1 will be described with reference to FIG.

The low-pass filter 90 has two first inductors 20 loaded in series between the input terminal 11 and the output terminal 12, and a second capacitor between the junction of these first inductors 20 and ground. It has a T-shaped configuration loaded with 17. The number of filter stages is three, which is the same as the low-pass filter 10 in FIG.

A method of obtaining the element values of the low-pass filter 10 of FIG. 1 for maintaining the characteristics of the low-pass filter 90 of FIG. 2 within the band and obtaining a large loss characteristic outside the band will be described.

Let L be the inductance value of the first inductor 20 of the low-pass filter 90 in FIG. 2, and C be the capacitance value of the second capacitor 17 . Let L1 and C1 be the inductance value of the first inductor 13 and the capacitance value of the first capacitor 14 of the low-pass filter 10 in FIG. 1, respectively. Let the inductance value of the first inductor 15 and the capacitance value of the first capacitor 16 be L2 and C2, respectively. Let the capacitance value of the second capacitor 17 be C, which is the same as that of the low-pass filter 90 in FIG.

In FIG. 1, the impedance Z1 of the parallel resonant circuit 18 is obtained by Equation (1). where ω is the angular frequency. When the frequency is f, ω=2πf.
Equation 1: Z1=jωL1/(1− ^ω2L1C1 )

In this equation, the condition for parallel resonance at the out-of-band frequency ω1 is Equation 2.
Equation 2: ω1 ² = 1/L1C1

Also, the impedance Z1 at the frequency ω0 within the band is given by Equation 3 from Equation 1.
Formula 3: Z1=jω0L1/(1−ω0 ² L1C1)

On the other hand, the impedance Z2 at ω0 of the first inductor 20 constituting the low-pass filter 90 of FIG.
Formula 4: Z2=jω0L

Here, when Z1=Z2 at ω0 and arrangement is made using the relationship of Equation 2, the relationship between L1 and C1 constituting the parallel resonant circuit 18 and L is Equation 5.
Equation 5:
L1=L(1−ω0 ² /ω1 ² )
C1=1/L(ω1 ² −ω0 ² )

Similarly, if the frequency ω2 outside the band is parallel resonance, and the frequency ω0 inside the band is the same as that of the first inductor 20 forming the low-pass filter 90 of FIG. The relationship with L is given by Equation 6.
Formula 6:
L2=L(1−ω0 ² /ω2 ² )
C2=1/L(ω2 ² −ω0 ² )

ω0 , .omega.1 and .omega.2. For comparison, an equivalent circuit of the first inductor 20 that constitutes the low-pass filter 90 of FIG. 2 is also shown.

As shown in FIG. 3, the parallel resonant circuit 18 has L equal to the first inductor 20 of the low-pass filter 90 of FIG. The parallel resonant circuit 19 has an L equal to that of the first inductor 20 of the low-pass filter 90 of FIG.

FIG. 4 shows an example of loss characteristics of each low-pass filter. In the figure, (a) is the characteristic of the low-pass filter 10 in FIG. 1, and (b) is the characteristic of the low-pass filter 90 in FIG. At ω0 within the band, each exhibits a low loss characteristic. Outside the band, the loss increases gradually as the frequency increases with the low-pass filter 90 of FIG. and .omega.2, and a large loss characteristic is obtained from .omega.1 to .omega.2.

FIG. 5 shows a design example of the low-pass filter 10 of FIG. Element values L1, C1, L2, and C2 shown here are set to f0=1.25 GHz for ω0, f1=2.3 GHz for ω1, and f2=2.7 GHz for ω2. This value is obtained by equations 5 and 6, where the inductance value of the first inductor 20 is L=4.84 nH. In this example, the loss at 1.25 GHz is almost 0 dB, the out-of-band loss is about 70 dB each at 2.3 GHz and 2.7 GHz, and the loss from 2.3 GHz to 2.7 GHz is 30 dB. A large loss characteristic is obtained. In contrast, for the low-pass filter 90 of FIG. 2 with L=4.84 nH and C=1.74 pF, the loss at 1.25 GHz is nearly 0 dB, but the out-of-band losses at 2.3 GHz and 2.3 GHz are almost zero. At 7 GHz, they are 0.9 dB and 3.4 dB, respectively, and large loss characteristics are not obtained.

As described above, in the low-pass filter 10 of the present embodiment, the parallel resonance circuit 18 and the parallel resonance circuit 19 are connected in series between the input terminal 11 and the output terminal 12, and the parallel resonance circuit 18 is connected in parallel. A second capacitor 17 is loaded between the connection with the resonance circuit 19 and the ground. The parallel resonant circuit 18 and the parallel resonant circuit 19 maintain the inductance L required for the low-pass filter 10 at ω0 in the band, respectively, and the resonant frequency of the parallel resonant circuit 18 is ω1 outside the band, the parallel resonant circuit 19 The element values are selected such that the resonance frequency of is ω2 which is out of the band. As a result, a low loss characteristic is obtained within the band, and a large loss characteristic is obtained over a wide band from ω1 to ω2 outside the band.

Since the number of filter stages is not increased in order to obtain a large loss outside the band, the present embodiment has the effect of obtaining low loss characteristics with a small low-pass filter 10 .

***Description of the effect of the embodiment***
According to the present embodiment, it is possible to provide a filter that has a large out-of-band loss and a low in-band loss characteristic even with a small number of stages.

According to the present embodiment, a plurality of parallel resonant circuits maintains the respective inductances required for the low-pass filter 10 within the band, and utilizes the characteristics of the parallel resonant circuits outside the band, thereby reducing the number of stages. A large out-of-band loss and a low in-band loss characteristic can be obtained without increasing . In addition, by setting the resonance frequencies of the respective parallel resonance circuits to different frequencies, there is also the effect of expanding the bandwidth outside the band where a large loss is obtained. In particular, it is effective for application to the low-pass filter 10 having a T-shaped configuration.

***Other Configurations***
Instead of one second capacitor 17, the low-pass filter 10 may comprise two or more second capacitors connected between the junction of the two first inductors 13, 15 and ground. Alternatively, the low-pass filter 10 may include two or more second capacitors connected between the connecting portion of any two of the three or more first inductors described later and the ground. .

Instead of the two first inductors 13 and 15, the low-pass filter 10 may have three or more first inductors connected in series. A first capacitor is connected in parallel to each of at least two of the three or more first inductors to form a parallel resonant circuit. As in the present embodiment, the first capacitor may be connected in parallel to all the first inductors to form a parallel resonant circuit for each first inductor. The first capacitors may be connected in parallel only to the first inductors of to form parallel resonant circuits whose number is less than the total number of the first inductors. In any configuration described here, the low-pass filter 10 is connected in parallel to at least two first inductors among the three or more first inductors, and has different out-of-band resonance frequencies. will have at least two first capacitors forming at least two parallel resonant circuits with a frequency of .

A configuration of low-pass filter 10 according to a modification of the present embodiment will be described with reference to FIG.

In this modification, instead of the first inductor 20 on the output terminal 12 side of FIG. A series circuit with a parallel resonant circuit 26 is used.

Here, let the inductance value of the first inductor 21 and the capacitance value of the first capacitor 22 be L3 and C3, respectively. Let the inductance value of the first inductor 24 and the capacitance value of the first capacitor 25 be L4 and C4, respectively. Further, let the resonant frequencies of the parallel resonant circuit 23 and the parallel resonant circuit 26 be ω1 and ω2, respectively, and the inductance values of the equivalent inductors of the parallel resonant circuit 23 and the parallel resonant circuit 26 at ω0 within the band, respectively, as shown in FIG. L3, C3, L4, and C4 are obtained by Equation 7, as well as Equations 5 and 6, when L/2, that is, L/2, of the first inductor 20 constituting the low-pass filter 90 of .
Equation 7:
L3=L(1−ω0 ² /ω1 ² )/2
C3=2/L(ω1 ² −ω0 ² )
L4=L(1−ω0 ² /ω2 ² )/2
C4=2/L(ω2 ² −ω0 ² )

As described above, in this modified example, the parallel resonant circuit 23 and the parallel resonant circuit 26 are employed instead of the single first inductor 20, and the parallel resonant circuit 23 and the parallel resonant circuit 26 are equivalent to each other within the band. The sum of the inductance values of the inductors is equal to the inductance value L required for the low-pass filter 10 . Also, the out-of-band resonance frequencies of the parallel resonance circuit 23 and the parallel resonance circuit 26 are set to different values. As a result, the low-pass filter 10 with low loss in the band and large loss out of the band can be obtained as in FIG.

By replacing one first inductor 20 with a plurality of parallel resonance circuits as in this configuration, more parallel resonance points can be provided without increasing the number of filter stages, and a band in which a large loss can be obtained. There is an effect that the bandwidth can be widened outside.

In Equation 7, the inductance values of the equivalent inductors of the parallel resonant circuit 23 and the parallel resonant circuit 26 are each set to L/2, but if the total inductance value is L, other ratios have the same effect. is.

In the above embodiment, as shown in FIGS. 1 and 6, a three-stage configuration is adopted as the number of stages of the filter, but a configuration with more stages may be adopted. In that case, more parallel resonance points can be provided, so that the bandwidth outside the band where a large loss can be obtained can be further widened.

Embodiment 2.
This embodiment will be described mainly with reference to FIGS. 7 to 10 for differences from the first embodiment.

*** Configuration description ***
The configuration of low-pass filter 10 according to the present embodiment will be described with reference to FIG.

In this embodiment, low-pass filter 10 includes two second capacitors 28 and 31 , two second inductors 27 and 30 and one first inductor 20 . The two second capacitors 28, 31 are connected in parallel. The two second inductors 27 and 30 are connected in series to the two second capacitors 28 and 31 to form two series resonant circuits 29 and 32 having different resonant frequencies and out-of-band frequencies. One first inductor 20 is connected between two series resonant circuits 29 and 32 .

Specifically, the low-pass filter 10 includes a series resonance circuit 29 including a second inductor 27 and a second capacitor 28 between one terminal of the first inductor 20 and the ground, A series resonance circuit 32 consisting of a second inductor 30 and a second capacitor 31 is mounted between the other terminal of and the ground.

A configuration of a low-pass filter 90 according to Comparative Example 2 will be described with reference to FIG.

The low-pass filter 90 loads one first inductor 20 between the input terminal 11 and the output terminal 12, and between one terminal of the first inductor 20 and ground, and between the other terminal and ground. and a second capacitor 17 are loaded between them, respectively. The number of stages of the filter is three, which is the same as the low-pass filter 10 in FIG.

A method of obtaining the element values of the low-pass filter 10 of FIG. 7 for maintaining the characteristics of the low-pass filter 90 of FIG. 8 in the band and obtaining a large loss characteristic out of the band will be described.

Let L be the inductance value of the first inductor 20 of the low-pass filter 90 of FIG. 8, and C be the capacitance value of the second capacitor 17 . Let L5 and C5 be the inductance value of the second inductor 27 and the capacitance value of the second capacitor 28 of the low-pass filter 10 of FIG. 7, respectively. Let the inductance value of the second inductor 30 and the capacitance value of the second capacitor 31 be L6 and C6, respectively. Assume that the inductance value of the first inductor 20 is L, which is the same as that of the low-pass filter 90 in FIG.

In the low-pass filter 10 of FIG. 7 as well, each element value is obtained by Equation 8 based on the element values of FIG. 8, as in the first embodiment. where ω0 is the in-band frequency, and ω1 and ω2 are the out-of-band resonant frequencies of the series resonant circuit 29 and the series resonant circuit 32, respectively.
Equation 8:
L5=1/C(ω1 ² −ω0 ² )
C5=C(1−ω0 ² /ω1 ² )
L6=1/C(ω2 ² −ω0 ² )
C6=C(1−ω0 ² /ω2 ² )

By selecting the relationship between C and L5 and C5 that configure the series resonant circuit 29 and the relationship between L6 and C6 that configure the series resonant circuit 32 and C as shown in Equation 8, the series resonant circuit at ω0, ω1 and ω2 is A simple equivalent circuit of 29 and series resonance circuit 32 can be expressed as shown in FIG. For comparison, an equivalent circuit of the second capacitor 17 forming the low-pass filter 90 of FIG. 8 is also shown.

As shown in FIG. 9, the series resonant circuit 29 has C equal to the second capacitor 17 of the low-pass filter 90 of FIG. 8 at ω0, a short circuit at ω1, and an equivalent inductor L' at ω2. The series resonance circuit 32 has C equal to the second capacitor 17 of the low-pass filter 90 of FIG. 8 at ω0, an equivalent capacitor C' at ω2, and a short circuit at ω2.

Thus, by maintaining the capacitance value C of the second capacitor 17 required for the low-pass filter 10 within the band and providing two short-circuit points between the input terminal 11 and the output terminal 12, 4, a low loss characteristic is obtained within the band and a large loss characteristic is obtained over ω1 to ω2 outside the band.

FIG. 10 shows a design example of the low-pass filter 10 of FIG. Element values L5, C5, L6, and C6 shown here are set to ω0 at f0=1.25 GHz, ω1 at f1=2.3 GHz, and ω2 at f2=2.7 GHz. This value is obtained by Equation 8, where the capacitance value of the second capacitor 17 is C=1.74 pF. In this example, similar to the example in FIG. 5, the loss at 1.25 GHz is nearly 0 dB, the out-of-band loss at 2.3 GHz and 2.7 GHz is approximately 70 dB each, and the loss from 2.3 GHz to 2.7 GHz is approximately 0 dB. loss is 30 dB, which is a large loss characteristic outside the band. In contrast, for the low-pass filter 90 of FIG. 8 with L=4.84 nH and C=1.74 pF, the loss at 1.25 GHz is nearly 0 dB, but the out-of-band 2.3 GHz and 2.3 GHz losses are almost zero. At 7 GHz, they are 0.9 dB and 3.4 dB, respectively, and large loss characteristics are not obtained.

***Description of the effect of the embodiment***
According to the present embodiment, as in the case of the first embodiment, it is possible to provide a filter that has a large loss outside the band and a low loss within the band even if the number of stages is small.

According to the present embodiment, a plurality of series resonance circuits maintain the respective capacitances required for the low-pass filter 10 within the band, and utilize the characteristics of the series resonance circuits outside the band, thereby reducing the number of stages. A large out-of-band loss and a low in-band loss characteristic can be obtained without increasing . Also, by setting the resonance frequencies of the respective series resonance circuits to different frequencies, there is an effect of expanding the bandwidth outside the band where a large loss is obtained. In particular, it is effective for application to the low-pass filter 10 of π-type configuration.

***Other Configurations***
The low-pass filter 10 may have two or more first inductors connected between the two series resonant circuits 29 and 32 instead of one first inductor 20 . Alternatively, low-pass filter 10 may include two or more first inductors connected between any two of three or more series resonance circuits to be described later.

Instead of the two second capacitors 28, 31, the low-pass filter 10 may have three or more second capacitors connected in parallel. A second inductor is connected in series to each of at least two of the three or more second capacitors to form a series resonance circuit. As in the present embodiment, the second inductors may be connected in series to all the second capacitors to form a series resonant circuit for each second capacitor, that is, three or more series resonant circuits. As in a modification described later, the second inductors may be connected in series only to some of the second capacitors to form series resonance circuits whose number is less than the total number of the second capacitors. In any configuration described here, the low-pass filter 10 is connected in series to at least two second capacitors among the three or more second capacitors, and has different out-of-band resonance frequencies. at least two second inductors forming at least two series resonant circuits with a frequency of .

Low-pass filter 10 may further comprise one first capacitor. This one first capacitor is connected in parallel to one first inductor 20 between the two series resonance circuits 29 and 32, and has a resonance frequency different from that of both of the two series resonance circuits 29 and 32. form a parallel resonant circuit at an external frequency. As described above, three or more series resonance circuits may be formed, in which case the resonance frequency of the parallel resonance circuit is a frequency outside the band different from the resonance frequencies of any of the three or more series resonance circuits. .

Instead of one first inductor 20, the low-pass filter 10 may include two or more first inductors connected in series and two or more first capacitors. These two or more first capacitors are connected in parallel with two or more first inductors between the two series resonant circuits 29, 32 to form a parallel resonant circuit for each first inductor. The resonance frequencies of the parallel resonance circuits are different from each other and out of the band from the resonance frequencies of the two series resonance circuits 29 and 32 . As described above, three or more series resonant circuits may be formed, in which case the resonant frequency of each parallel resonant circuit is different from each other and from any resonant frequency of the three or more series resonant circuits. out-of-band frequencies.

A configuration of low-pass filter 10 according to a modification of the present embodiment will be described with reference to FIG.

In this modification, instead of the second capacitor 17 on the output terminal 12 side in FIG. A parallel circuit with a series resonant circuit 32 is used.

Here, let the inductance value of the second inductor 27 and the capacitance value of the second capacitor 28 be L7 and C7, respectively. Let the inductance value of the second inductor 30 and the capacitance value of the second capacitor 31 be L8 and C8, respectively. Further, let ω1 and ω2 be the resonant frequencies of the series resonant circuit 29 and the series resonant circuit 32, respectively, and the equivalent capacitance values of the series resonant circuit 29 and the series resonant circuit 32 at ω0 within the band are shown in FIG. 1/2 of the second capacitor 17 constituting the low-pass filter 90, that is, C/2, L7, C7, L8 and C4 are obtained by the equation 9 as well as the equation 8.
Equation 9:
L7=2/C(ω1 ² −ω0 ² )
C7=C(1−ω0 ² /ω1 ² )/2
L8=2/C(ω2 ² −ω0 ² )
C8=C(1−ω0 ² /ω2 ² )/2

As described above, in this modification, instead of one second capacitor 17, a parallel circuit of two series resonance circuits 29 and 32 is adopted, and within the band, the series resonance circuit 29 and the series resonance The sum of the capacitance values of the equivalent capacitors with circuit 32 is made equal to the capacitance value C required for low pass filter 10 . Also, the out-of-band resonance frequencies of the series resonance circuit 29 and the series resonance circuit 32 are set to different values. As a result, the low-pass filter 10 having low loss characteristics in the band and large loss characteristics out of the band can be obtained as in FIG.

By replacing one second capacitor 17 with a plurality of series resonance circuits as in this configuration, more series resonance points can be provided without increasing the number of filter stages, and a band in which a large loss can be obtained. There is an effect that the bandwidth can be widened outside.

In the above embodiment, as shown in FIGS. 7 and 11, a three-stage configuration is adopted as the number of stages of the filter, but a configuration with more stages may be adopted. In that case, more series resonance points can be provided, so that the bandwidth outside the band where large loss can be obtained can be further widened.

Embodiment 3.
This embodiment will be described mainly with reference to FIGS. 12 and 13 for differences from the first embodiment.

*** Configuration description ***
The configuration of low-pass filter 10 according to the present embodiment will be described with reference to FIG.

In this embodiment, instead of the second capacitor 17 of the first embodiment shown in FIG. 1, a series resonance circuit 35 composed of a second inductor 33 and a second capacitor 34 is provided. That is, the low-pass filter 10 includes one second capacitor 34 and one second inductor 33 instead of one second capacitor 17 . The single second inductor 33 is connected in series to the single second capacitor 34 between the connecting portion of the two first inductors 13 and 15 and the ground, and the resonant frequency of the two parallel resonant circuits 18 and 19 A single series resonance circuit 35 is formed which is different from the resonance frequency of the other and has a frequency outside the band.

Here, let the inductance value of the second inductor 33 and the capacitance value of the second capacitor 34 be L9 and C9, respectively. The relationship between L9 and C and the relationship between C9 and C for maintaining equivalently the same capacitance value C as the second capacitor 17 at ω0 in the band and resonating at ω3 outside the band are the same as those in the second embodiment. Similarly, it is obtained by Equation 10.
Equation 10:
L9=1/C(ω3 ² −ω0 ² )
C9=C(1−ω0 ² /ω3 ² )

Thus, in the present embodiment, the series resonant circuit 35 that series-resonates at ω3 is added to the parallel resonant circuits 18 and 19 that resonate in parallel at ω1 and ω2 in the first embodiment. .

FIG. 13 shows an example of loss characteristics of the low-pass filter 10. FIG. Low loss is maintained in-band and high loss is obtained over ω1 to ω3 out-of-band.

As described above, in the present embodiment, the parallel resonance circuit 18, the parallel resonance circuit 19 and the series resonance circuit 35 are used at the same time. A small low-pass filter 10 with a large loss over the entire range can be realized.

***Description of the effect of the embodiment***
According to the present embodiment, the plurality of parallel resonant circuits and series resonant circuits maintain the inductance and capacitance required for the low-pass filter 10 within the band, while the parallel resonant circuits and series resonant circuits are maintained outside the band. By using the characteristics of , a large out-of-band loss and a low in-band loss can be obtained without increasing the number of stages. In addition, by simultaneously using parallel resonance characteristics and series resonance characteristics, it is possible to further expand the bandwidth outside the band where a large loss is obtained, and it is also effective to apply to the low-pass filter 10 with T-type and π-type configurations. As a result, there is also an effect of increasing the degree of freedom in design.

***Other Configurations***
Instead of one second capacitor 34, the low-pass filter 10 may have two or more second capacitors connected in parallel. Then, instead of one second inductor 33, the low-pass filter 10 is connected in series with two or more second capacitors between the connecting portion of the two first inductors 13 and 15 and the ground. There may be more than one second inductor forming a series resonant circuit for each capacitor. The resonance frequencies of the series resonance circuits are different from each other and out of the band from the resonance frequencies of the two parallel resonance circuits 18 and 19 . Three or more parallel resonant circuits may be formed, in which case the resonant frequencies of the series resonant circuits are different from each other and out of band frequencies different from the resonant frequencies of any of the three or more parallel resonant circuits. Become.

A configuration of low-pass filter 10 according to a modification of the present embodiment will be described with reference to FIG.

In this modified example, instead of the single series resonance circuit 35 of FIG. A parallel circuit with the resonance circuit 41 is used.

Here, let the inductance value of the second inductor 36 and the capacitance value of the second capacitor 37 be L10 and C10, respectively. Let L11 and C11 be the inductance value of the second inductor 39 and the capacitance value of the second capacitor 40, respectively. Further, let ω3 and ω4 be the resonant frequencies of the series resonant circuit 38 and the series resonant circuit 41, respectively, and the equivalent capacitance values of the series resonant circuit 38 and the series resonant circuit 41 at ω0 within the band are shown in FIG. 1/2 of the second capacitor 17 constituting the low-pass filter 90, that is, C/2, L10, C10, L11 and C11 are obtained by the equation (11) as well as the equation (10).
Formula 11:
L10=2/C(ω3 ² −ω0 ² )
C10=C(1−ω0 ² /ω3 ² )/2
L11=2/C(ω4 ² −ω0 ² )
C11=C(1−ω0 ² /ω4 ² )/2

As described above, in this modification, by replacing the series resonant circuit 35 with the series resonant circuit 38 that resonates at ω3 and the series resonant circuit 41 that resonates at ω4, the out-of-band ω1 to .omega.4, the bandwidth can be further expanded.

10 low-pass filter 11 input terminal 12 output terminal 13 first inductor 14 first capacitor 15 first inductor 16 first capacitor 17 second capacitor 18 parallel resonance circuit 19 parallel resonance circuit 20 first inductor 21 first inductor 22 first capacitor 23 parallel resonant circuit 24 first inductor 25 first capacitor 26 parallel resonant circuit 27 second inductor 28 second capacitor 29 series resonant circuit 30 second inductor 31 second capacitor 32 series resonant circuit 33 second inductor 34 second capacitor 35 series resonant circuit 36 second inductor 37 second capacitor 38 series resonant circuit 39 second inductor 40 2nd capacitor, 41 series resonant circuit, 90 low pass filter.

Claims (5)

In a filter that passes signals of frequencies within a certain band,
a second capacitor A and a second capacitor B connected in parallel;
A second capacitor is connected in series to the second capacitor A and the second capacitor B to form a series resonant circuit A and a series resonant circuit B having different resonant frequencies and out of the band , respectively . an inductor A and a second inductor B ;
A first inductor D connected between the series resonant circuit A and the series resonant circuit B ;
A first inductor X is loaded between the input terminal and the output terminal, between one terminal of the first inductor X and ground, and between the other terminal of the first inductor X and ground is loaded with the second capacitor Y, and the inductance value of the first inductor X is L, and the capacitance value of the second capacitor Y is C,
The inductance value of the second inductor A is L5,
the capacitance value of the second capacitor A is C5;
The inductance value of the second inductor B is L6,
the capacitance value of the second capacitor B is C6;
L is the inductance value of the first inductor D;
ω is the in-band frequency,
ω1 is the resonance frequency outside the band of the series resonance circuit A of the second inductor A and the second capacitor A;
Assuming that ω2 is the resonance frequency outside the band of the series resonance circuit B of the second inductor B and the second capacitor B,
L5=1/C(ω1 ² −ω0 ² )
C5=C(1−ω0 ² /ω1 ² )
L6=1/C(ω2 ² −ω0 ² )
C6=C(1−ω0 ² /ω2 ² )
ω0<ω1<ω2
while maintaining the characteristics of the filter with the π-configuration at the pass frequency ω0, and the loss is greater than that of the filter with the π-configuration over the resonance frequency ω1 to the resonance frequency ω2. A filter whose properties can be obtained . In a filter that passes signals of frequencies within a certain band,
loading one first inductor X between the input terminal and the output terminal;
loading a second capacitor Y between one terminal of the first inductor X and ground;
Between the other terminal of the first inductor X and the ground,
a second capacitor A and a second capacitor B connected in parallel;
A second inductor A and a second capacitor B are connected in series with the second capacitor A and the second capacitor B to respectively form a series resonant circuit A and a series resonant circuit B having different resonant frequencies and out of the band. 2 inductor B and
with
The inductance value of the second inductor A is L7,
the capacitance value of the second capacitor A is C7;
The inductance value of the second inductor B is L8,
the capacitance value of the second capacitor B is C8;
L is the inductance value of the first inductor X;
C is the capacitance value of the second capacitor Y;
ω is the in-band frequency,
ω1 is the resonance frequency outside the band of the series resonance circuit A of the second inductor A and the second capacitor A;
ω2 is the resonance frequency outside the band of the series resonance circuit B of the second inductor B and the second capacitor B;
C/2 the capacitance value of the equivalent capacitor of the series resonant circuit A at the frequency ω0 in the band,
Assuming that the capacitance value of the equivalent capacitor of the series resonant circuit B at the frequency ω0 in the band is C/2,
L7=2/C(ω1 ² -ω0 ² )
C7=C(1-ω0 ² /ω1 ² )/2
L8=2/C(ω2 ² -ω0 ² )
C8=C(1-ω0 ² /ω2 ² )/2
ω0<ω1<ω2
A filter that has a relationship of is connected in parallel to the first inductor D between the series resonance circuit A and the series resonance circuit B , and has a resonance frequency different from that of either the series resonance circuit A or the series resonance circuit B; 2. The filter of claim 1 , further comprising a first capacitor forming a parallel resonant circuit at frequencies outside said band. Two or more first inductors connected in series are provided as the first inductors D ,
Two or more first inductors each connected in parallel to the two or more first inductors between the series resonant circuit A and the series resonant circuit B to form the parallel resonant circuit for each first inductor. 4. The filter of claim 3, comprising a capacitor. A filter according to any one of claims 1 to 4;
and a signal processing device that processes a signal that has passed through the filter.

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