JP2019161399A - Filter and electronic apparatus - Google Patents

Abstract

To provide a filter, capable of obtaining big loss outside a pass band and low-loss characteristics inside the pass band even if the number of fitter stages is few.SOLUTION: A low-pass filter 10 includes 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, 16 are connected in parallel with the two first inductors 13, 15, respectively, and form two parallel resonant circuits 18, 19, respectively, which have resonant frequencies that are different from each other and out-of-band frequencies. The one second capacitor 17 is connected between ground and a connection between the two first inductors 13, 15.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a filter and an electronic device.

  For radar equipment, communication equipment, observation equipment, etc., prevent unwanted waves that are out of band of the desired signal from entering the equipment, or prevent unwanted waves and harmonics generated inside the equipment from leaking outside the equipment. For this purpose, a low-pass filter is used.

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

Japanese Patent Application Laid-Open No. 08-078916

  In a device in which leakage of unnecessary waves or harmonics generated inside the device to the outside of the device is significantly limited, a low-pass filter having a large loss outside the band is required.

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

  The loss outside the band greatly depends on the number of filter stages. That is, as the number of stages increases, the slope of loss outside the band increases, and a large loss characteristic can be obtained. However, loss in the band also tends to increase. There is also a problem that the low-pass filter is increased in size.

  An object of the present invention is to provide a filter capable of obtaining a large loss outside the band and a low loss characteristic within the band even if the number of stages is small.

According to one embodiment of the present invention, a filter that passes a signal having a frequency within a certain band is provided.
Two or more first inductors connected in series;
At least two first capacitors connected in parallel to at least two first inductors of the two or more first inductors to form at least two parallel resonant circuits having different resonance frequencies and having frequencies outside the band. When,
One of the two or more first inductors, and one or more second capacitors connected between a connection portion of any two first inductors and the ground.

According to another aspect of the present invention, in a filter that passes a signal having a frequency within a certain band,
Two or more second capacitors connected in parallel;
At least two second inductors connected in series to at least two second capacitors of the two or more second capacitors to form at least two series resonant circuits having different resonance frequencies and having frequencies outside the band. When,
And one or more first inductors connected between any two of the at least two series resonance circuits.

  According to the present invention, it is possible to provide a filter capable of obtaining large loss outside the band and low loss characteristics within the band even if the number of stages is small.

FIG. 3 shows a configuration of a low-pass filter according to the first embodiment. The figure which shows the T-shaped structure of the low-pass filter which concerns on the comparative example 1. FIG. FIG. 3 is a diagram showing a simple equivalent circuit of a parallel resonant circuit used for the low-pass filter according to the first embodiment. The graph which shows the loss characteristic example of the low-pass filter which concerns on Embodiment 1 and the comparative example 1. FIG. 3 is a graph showing a design example of a low-pass filter according to the first embodiment. FIG. 6 shows a configuration of a low-pass filter according to a modification of the first embodiment. FIG. 4 is a diagram illustrating a configuration of a low-pass filter according to a second embodiment. The figure which shows the pi-type structure of the low-pass filter which concerns on the comparative example 2. The figure which shows the simple equivalent circuit of the parallel resonant circuit used for the low-pass filter which concerns on Embodiment 2. FIG. 6 is a graph showing a design example of a low-pass filter according to Embodiment 2. FIG. 6 is a diagram illustrating a configuration of a low-pass filter according to a modification of the second embodiment. FIG. 6 is a diagram showing a configuration of a low-pass filter according to Embodiment 3. 10 is a graph showing an example of loss characteristics of a low-pass filter according to Embodiment 3. FIG. 10 shows a configuration of a low-pass filter according to a modification of the third embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals. In the description of the embodiments, the description of the same or corresponding parts will be omitted or simplified as appropriate. The present invention is not limited to the embodiments described below, and various modifications can be made as necessary. For example, two or more embodiments among the embodiments described below may be combined and executed. Alternatively, among the embodiments described below, one embodiment or a combination of two or more embodiments may be partially implemented.

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

*** Explanation of configuration ***
With reference to FIG. 1, the structure of the low-pass filter 10 which concerns on this Embodiment is demonstrated.

  The low-pass filter 10 is a filter that passes a signal having a frequency within a certain band. Specifically, the low-pass filter 10 is a filter that performs an operation of allowing a signal in a band lower than a certain frequency to pass without being attenuated and significantly attenuating unnecessary waves or harmonics outside the band. This operation corresponds to the filtering method according to the present embodiment. Note that the same configuration as the low-pass filter 10 may be applied to other band-pass filters or amplifier matching circuits.

  The 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 an electronic device including 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 includes two first inductors 13 and 15, two first capacitors 14 and 16, and one second capacitor 17. The two first inductors 13 and 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 resonance frequencies and frequencies outside the band. One second capacitor 17 is connected between the connection portion of the two first inductors 13 and 15 and the ground.

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

  With reference to FIG. 2, the structure of the low-pass filter 90 which concerns on the comparative example 1 is demonstrated.

  The low-pass filter 90 loads two first inductors 20 in series between the input terminal 11 and the output terminal 12, and a second capacitor is connected between the connection portion of the first inductors 20 and the ground. 17-shaped configuration with 17 loaded. The number of filter stages is the same as that of the low-pass filter 10 of FIG.

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

  Here, the inductance value of the first inductor 20 and the capacitance value of the second capacitor 17 of the low-pass filter 90 of FIG. Moreover, 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 of FIG. Let L2 and C2 be the inductance value of the first inductor 15 and the capacitance value of the first capacitor 16, respectively. The capacitance value of the second capacitor 17 is set to C which is the same as that of the low-pass filter 90 of FIG.

In FIG. 1, the impedance Z <b> 1 of the parallel resonant circuit 18 is obtained by Equation 1. Here, ω is an angular frequency. When the frequency is f, ω = 2πf.
Formula 1: Z1 = jωL1 / (1-ω ² L1C1)

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

In addition, the impedance Z1 at the frequency ω0 within the band is expressed by Expression 3 from Expression 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, assuming that Z1 = Z2 in ω0 and rearranging using the relationship of Equation 2, the relationship between L1 and C1 constituting the parallel resonant circuit 18 and L is Equation 5.
Formula 5:
L1 = L (1-ω0 ² / ω1 ² )
C1 = 1 / L (ω1 ² −ω0 ² )

Similarly, when L is the same as that of the first inductor 20 constituting the low-pass filter 90 of FIG. 2 at the frequency ω2 outside the band and the frequency ω0 is within the band, L2 and C2 constituting the parallel resonance circuit 19 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 )

  By selecting the relationship between L1 and C1 constituting the parallel resonance circuit 18 and L as shown in Equation 5 and selecting the relationship between L2 and C2 and L constituting the parallel resonance circuit 19 as shown in Equation 6, ω0 , Ω1 and ω2, a simple equivalent circuit of the parallel resonant circuit 18 and the parallel resonant circuit 19 can be expressed as shown in FIG. For comparison, an equivalent circuit of the first inductor 20 constituting the low-pass filter 90 of FIG. 2 is also shown.

  As shown in FIG. 3, the parallel resonant circuit 18 is L equal to the first inductor 20 of the low-pass filter 90 in FIG. 2 at ω0, open at ω1, and equivalent capacitor C ′ obtained from Equation 1 at ω2. Further, the parallel resonant circuit 19 is L equal to the first inductor 20 of the low-pass filter 90 in FIG. 2 at ω0, and is opened at the equivalent inductors L ′ and ω2 obtained from Equation 1 at ω2.

  FIG. 4 shows an example of the loss characteristic of each low-pass filter. In the figure, (a) shows the characteristics of the low-pass filter 10 shown in FIG. 1, and (b) shows the characteristics of the low-pass filter 90 shown in FIG. Each of ω0 within the band exhibits a low loss characteristic, and outside the band, the low-pass filter 90 in FIG. 2 gradually increases in loss as the frequency increases, whereas the low-pass filter 10 in FIG. In addition, the loss increases sharply at ω2 and ω2, and a large loss characteristic is obtained from ω1 to ω2.

  FIG. 5 shows a design example of the low-pass filter 10 of FIG. The element values L1, C1, L2, and C2 shown here are ω0 is f0 = 1.25 GHz, ω1 is f1 = 2.3 GHz, ω2 is f2 = 2.7 GHz, and the low-pass filter 90 of FIG. The inductance value of the first inductor 20 is L = 4.84 nH, and is a value obtained by Expression 5 and Expression 6. In this example, the loss at 1.25 GHz is approximately 0 dB, the loss at 2.3 GHz and 2.7 GHz out of band is approximately 70 dB, respectively, and the loss from 2.3 GHz to 2.7 GHz is 30 dB out of band. Large loss characteristics are obtained. On the other hand, in the case of the low-pass filter 90 of FIG. 2 in which L = 4.84 nH and C = 1.74 pF are selected, the loss at 1.25 GHz is almost 0 dB, but 2.3 GHz and 2.2. At 7 GHz, large loss characteristics of 0.9 dB and 3.4 dB cannot be obtained, respectively.

  As described above, the low-pass filter 10 of the present embodiment loads the parallel resonant circuit 18 and the parallel resonant circuit 19 in series between the input terminal 11 and the output terminal 12, and is parallel to the parallel resonant circuit 18. The 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 within the band, respectively, and the parallel resonant circuit 18 has a resonance frequency ω1 outside the band and the parallel resonant circuit 19. The element value is selected such that the resonance frequency of ω2 is ω2 outside the band. As a result, a low loss characteristic can be obtained within the band, and a large loss characteristic can be 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 as described above, the present embodiment has an effect of obtaining a low loss characteristic with the small low-pass filter 10.

*** Explanation of the effect of the embodiment ***
According to the present embodiment, it is possible to provide a filter that can obtain a large loss outside the band and a low loss characteristic within the band even if the number of stages is small.

  According to the present embodiment, a plurality of parallel resonant circuits maintain their respective inductances required for the low-pass filter 10 within the band, while utilizing the characteristics of the parallel resonant circuit outside the band, A large loss can be obtained outside the band without increasing the frequency, and a low loss characteristic can be obtained within the band. Also, by setting the resonance frequency of each parallel resonance circuit to a different frequency, there is an effect of expanding the bandwidth outside the band where a large loss can be obtained. In particular, it is effective for application to a low-pass filter 10 having a T-shaped configuration.

*** Other configurations ***
The low-pass filter 10 may include two or more second capacitors connected between the connection portions of the two first inductors 13 and 15 and the ground instead of the one second capacitor 17. Alternatively, the low-pass filter 10 may include two or more second capacitors connected between a connection portion of any two first inductors and three or more first inductors described later and the ground. .

  The low-pass filter 10 may include three or more first inductors connected in series instead of the two first inductors 13 and 15. 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. Similarly to the present embodiment, the first capacitors may be connected in parallel to all the first inductors to form a parallel resonant circuit for each first inductor. A first capacitor may be connected in parallel to only the first inductor, and a number of parallel resonant circuits smaller than the total number of first inductors may be formed. In any of the configurations described here, the low-pass filter 10 is connected in parallel to at least two first inductors out of three or more first inductors, and the resonance frequencies are different from each other. And at least two first capacitors forming at least two parallel resonant circuits having a frequency of.

  With reference to FIG. 6, the structure of the low-pass filter 10 which concerns on the modification of this Embodiment is demonstrated.

  In this modification, instead of the first inductor 20 on the output terminal 12 side in FIG. 2, a parallel resonant circuit 23 including a first inductor 21 and a first capacitor 22, a first inductor 24 and a first capacitor 25 are used. A series circuit with a parallel resonant circuit 26 is used.

Here, the inductance value of the first inductor 21 and the capacitance value of the first capacitor 22 are L3 and C3, respectively. Let L4 and C4 be the inductance value of the first inductor 24 and the capacitance value of the first capacitor 25, respectively. Further, the resonance frequencies of the parallel resonance circuit 23 and the parallel resonance circuit 26 are ω1 and ω2, respectively, and the inductance values of the inductors equivalent to the parallel resonance circuit 23 and the parallel resonance circuit 26 at ω0 within the band are shown in FIG. L3, C3, L4, and C4 are obtained by Expression 7 similarly to Expression 5 and Expression 6, assuming that L / 2 of the first inductor 20 constituting the low-pass filter 90 of FIG.
Formula 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 modification, 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 in the band. The sum of the inductance values of the various inductors is made equal to the inductance value L required for the low-pass filter 10. Further, the resonance frequencies outside the bands of the parallel resonance circuit 23 and the parallel resonance circuit 26 are set to different values. As a result, a low-pass filter 10 having a low loss within the band and a large loss characteristic outside the band as in FIG.

  By replacing one first inductor 20 with a plurality of parallel resonance circuits as in this configuration, it is possible to provide more parallel resonance points without increasing the number of stages of the filter and to obtain a large loss. This has the effect of increasing the bandwidth outside.

  In Equation 7, the inductance values of the equivalent inductors of the parallel resonance circuit 23 and the parallel resonance circuit 26 are each L / 2. However, if the total inductance value is L, the effect is the same even if other ratios are used. It is.

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

Embodiment 2. FIG.
In this embodiment, differences from the first embodiment will be mainly described with reference to FIGS.

*** Explanation of configuration ***
With reference to FIG. 7, the structure of the low-pass filter 10 which concerns on this Embodiment is demonstrated.

  In the present embodiment, the 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 and 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 resonance frequencies and having frequencies outside the band. One first inductor 20 is connected between the two series resonant circuits 29 and 32.

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

  With reference to FIG. 8, the structure of the low-pass filter 90 which concerns on the comparative example 2 is demonstrated.

  The low-pass filter 90 loads one first inductor 20 between the input terminal 11 and the output terminal 12, between one terminal of the first inductor 20 and the ground, and between the other terminals and the ground. Π-type configuration in which the second capacitor 17 is loaded therebetween. The number of filter stages is the same as that of the low-pass filter 10 of FIG.

  A method of obtaining the element value of the low-pass filter 10 in FIG. 7 in order to maintain the characteristics of the low-pass filter 90 in FIG. 8 within the band and obtain a large loss characteristic outside the band will be described.

  Here, the inductance value of the first inductor 20 and the capacitance value of the second capacitor 17 of the low-pass filter 90 of FIG. Further, 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 are L5 and C5, respectively. The inductance value of the second inductor 30 and the capacitance value of the second capacitor 31 are L6 and C6, respectively. The inductance value of the first inductor 20 is set to L which is the same as that of the low-pass filter 90 of FIG.

In the low-pass filter 10 of FIG. 7 as well, each element value is obtained as shown in Expression 8 based on the element value of FIG. However, ω0 is a frequency within the band, and ω1 and ω2 are resonance frequencies outside the band of the series resonance circuit 29 and the series resonance circuit 32, respectively.
Formula 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 L5 and C5 and C constituting the series resonance circuit 29 and the relationship between L6 and C6 and C constituting the series resonance circuit 32 as shown in Expression 8, the series resonance circuit at ω0, ω1 and ω2 is selected. A simple equivalent circuit of 29 and the series resonance circuit 32 can be expressed as shown in FIG. For comparison, an equivalent circuit of the second capacitor 17 constituting the low-pass filter 90 of FIG. 8 is also shown.

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

  In this way, by maintaining the capacitance value C of the second capacitor 17 required for the low-pass filter 10 within the band, and by providing short-circuit points at two locations between the input terminal 11 and the output terminal 12, FIG. As with the solid line 4, low loss characteristics can be obtained within the band, and large loss characteristics can be obtained from ω1 to ω2 outside the band.

  FIG. 10 shows a design example of the low-pass filter 10 of FIG. The element values L5, C5, L6 and C6 shown here are ω0 is f0 = 1.25 GHz, ω1 is f1 = 2.3 GHz, ω2 is f2 = 2.7 GHz, and the low-pass filter 90 of FIG. The capacitance value of the second capacitor 17 is C = 1.74 pF, and is a value obtained by Expression 8. In this example, similar to the example of FIG. 5, the loss at 1.25 GHz is approximately 0 dB, the loss at 2.3 GHz and 2.7 GHz out of band is approximately 70 dB, and from 2.3 GHz to 2.7 GHz. The loss of 30 dB is a large loss characteristic outside the band. On the other hand, in the case of the low-pass filter 90 of FIG. 8 in which L = 4.84 nH and C = 1.74 pF are selected, the loss at 1.25 GHz is approximately 0 dB, but 2.3 GHz and 2.2 outside the band. At 7 GHz, large loss characteristics of 0.9 dB and 3.4 dB cannot be obtained, respectively.

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

  According to the present embodiment, a plurality of series resonant circuits maintain the respective capacitances required for the low-pass filter 10 within the band, and use the characteristics of the series resonant circuit outside the band, thereby providing the number of stages. A large loss can be obtained outside the band without increasing the frequency, and a low loss characteristic can be obtained within the band. Further, by setting the resonance frequency of each series resonance circuit to a different frequency, there is an effect that the bandwidth outside the band where a large loss can be obtained is expanded. In particular, it is effective for application to a low-pass filter 10 having a π-type configuration.

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

  The low-pass filter 10 may include three or more second capacitors connected in parallel instead of the two second capacitors 28 and 31. A second inductor is connected in series to each of at least two of the three or more second capacitors to form a series resonant circuit. Similarly to the present embodiment, the second inductor may be connected in series to all the second capacitors to form a series resonance circuit for each second capacitor, that is, three or more series resonance circuits. As in a modification described later, the second inductors may be connected in series to only some of the second capacitors, and a series resonant circuit having a number smaller than the total number of second capacitors may be formed. In any of the configurations described here, the low-pass filter 10 is connected in series to at least two second capacitors out of three or more second capacitors, and the resonance frequencies are different from each other. And at least two second inductors forming at least two series resonant circuits having a frequency of.

  The low-pass filter 10 may further include one first capacitor. This one first capacitor is connected in parallel to one first inductor 20 between two series resonance circuits 29, 32, and the resonance frequency is different from any resonance frequency of the two series resonance circuits 29, 32. One parallel resonant circuit having an external frequency is formed. As described above, three or more series resonance circuits may be formed. In that case, the resonance frequency of the parallel resonance circuit is a frequency outside the band different from any of the resonance frequencies of the three or more series resonance circuits. .

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

  With reference to FIG. 11, the structure of the low-pass filter 10 which concerns on the modification of this Embodiment is demonstrated.

  In this modification, instead of the second capacitor 17 on the output terminal 12 side in FIG. 8, a series resonance circuit 29 including a second inductor 27 and a second capacitor 28, a second inductor 30, and a second capacitor 31 are used. A parallel circuit with the series resonant circuit 32 is used.

Here, let L7 and C7 be the inductance value of the second inductor 27 and the capacitance value of the second capacitor 28, respectively. The inductance value of the second inductor 30 and the capacitance value of the second capacitor 31 are L8 and C8, respectively. Further, the resonance frequencies of the series resonance circuit 29 and the series resonance circuit 32 are ω1 and ω2, respectively, and the capacitance values of the capacitors equivalent to the series resonance circuit 29 and the series resonance circuit 32 at ω0 in the band are shown in FIG. Assuming that 1/2 of the second capacitor 17 constituting the low-pass filter 90, that is, C / 2, L 7, C 7, L 8, and C 4 are obtained by Equation 9 as in Equation 8.
Formula 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 modified example, a parallel circuit of two series resonance circuits 29 and a series resonance circuit 32 is employed instead of one second capacitor 17, and the series resonance circuit 29 and the series resonance are within the band. The sum of the capacitance values of the capacitors equivalent to the circuit 32 is made equal to the capacitance value C required for the low-pass filter 10. Further, the resonance frequencies outside the band 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 a low loss within the band and a large loss characteristic outside the band as in FIG.

  By replacing one second capacitor 17 with a plurality of series resonance circuits as in this configuration, it is possible to provide more series resonance points without increasing the number of filter stages and to obtain a large loss. This has the effect of increasing the bandwidth outside.

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

Embodiment 3 FIG.
In this embodiment, differences from the first embodiment will be mainly described with reference to FIGS.

*** Explanation of configuration ***
With reference to FIG. 12, the structure of the low-pass filter 10 which concerns on this Embodiment is demonstrated.

  In the present embodiment, a series resonance circuit 35 including a second inductor 33 and a second capacitor 34 is provided instead of the second capacitor 17 of the first embodiment shown in FIG. That is, the low-pass filter 10 includes one second capacitor 34 instead of one second capacitor 17 and one second inductor 33. The one second inductor 33 is connected in series to one second capacitor 34 between the connection portion of the two first inductors 13 and 15 and the ground, and the resonance frequency is any of the two parallel resonance circuits 18 and 19. Unlike the resonance frequency, one series resonance circuit 35 having a frequency outside the band is formed.

Here, the inductance value of the second inductor 33 and the capacitance value of the second capacitor 34 are L9 and C9, respectively. The relationship between L9 and C and the relationship between C9 and C to maintain the same capacitance value C as the second capacitor 17 at ω0 in the band and to resonate at ω3 outside the band are the same as those in the second embodiment. Similarly, it can be obtained by Expression 10.
Formula 10:
L9 = 1 / C (ω3 ² −ω0 ² )
C9 = C (1-ω0 ² / ω3 ² )

  As described above, in this embodiment, the series resonance circuit 35 that resonates in parallel at ω3 is added to the parallel resonance circuit 18 and the parallel resonance circuit 19 that resonate in parallel at ω1 and ω2, respectively, in the first embodiment. .

  FIG. 13 shows an example of loss characteristics of the low-pass filter 10. A low loss is maintained within the band, and a large loss is obtained from ω1 to ω3 outside the band.

  As described above, in this embodiment, the parallel resonant circuit 18, the parallel resonant circuit 19, and the series resonant circuit 35 are simultaneously used, so that the loss is low within the band and the bandwidth is wide outside the band without increasing the number of filter stages. A small low-pass filter 10 that can obtain a large loss over a wide range can be realized.

*** Explanation of the effect of the embodiment ***
According to the present embodiment, a plurality of parallel resonance circuits and series resonance circuits maintain the inductance and capacitance required for the low-pass filter 10 within the band, respectively, while the parallel resonance circuit and the series resonance circuit are out of the band. By utilizing this characteristic, a large loss can be obtained outside the band without increasing the number of stages, and a low loss characteristic can be obtained within the band. Further, by simultaneously using the parallel resonance characteristic and the series resonance characteristic, the bandwidth outside the band where a large loss can be obtained can be further expanded, and the application to the T-type and π-type low-pass filter 10 is also effective. Thus, there is an effect of increasing the degree of freedom of design.

*** Other configurations ***
The low-pass filter 10 may include two or more second capacitors connected in parallel instead of one second capacitor 34. The low-pass filter 10 is connected in series to two or more second capacitors between the connection of the two first inductors 13 and 15 and the ground instead of the one second inductor 33, and the second Two or more second inductors forming a series resonant circuit for each capacitor may be provided. The resonance frequencies of the series resonance circuits are different from each other and are out of the band which is different from any of 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 frequency of each series resonant circuit is different from each other and is different from any of the resonant frequencies of the three or more parallel resonant circuits. Become.

  With reference to FIG. 14, the structure of the low-pass filter 10 which concerns on the modification of this Embodiment is demonstrated.

  In this modification, instead of the single series resonance circuit 35 of FIG. 12, a series resonance circuit 38 composed of a second inductor 36 and a second capacitor 37, and a series composed of a second inductor 39 and a second capacitor 40. A parallel circuit with the resonance circuit 41 is used.

Here, let L10 and C10 be the inductance value of the second inductor 36 and the capacitance value of the second capacitor 37, 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, the resonance frequencies of the series resonance circuit 38 and the series resonance circuit 41 are ω3 and ω4, respectively, and the capacitance values of the capacitors equivalent to the series resonance circuit 38 and the series resonance circuit 41 at ω0 in the band are shown in FIG. L10, C10, L11, and C11 are obtained by Expression 11 similarly to Expression 10, assuming that the second capacitor 17 constituting the low-pass filter 90 is 1/2, that is, C / 2.
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, the series resonance circuit 35 is replaced with the series resonance circuit 38 that resonates at ω3 and the series resonance circuit 41 that resonates at ω4, so that ω1 outside the band is not increased without increasing the number of filter stages. To ω4, it is possible to further expand the bandwidth for obtaining a large loss.

  DESCRIPTION OF SYMBOLS 10 Low pass filter, 11 Input terminal, 12 Output terminal, 13 1st inductor, 14 1st capacitor, 15 1st inductor, 16 1st capacitor, 17 2nd capacitor, 18 Parallel resonant circuit, 19 Parallel resonant circuit, 20 1st inductor, 21 1st inductor, 22 1st capacitor, 23 parallel resonant circuit, 24 1st inductor, 25 1st capacitor, 26 parallel resonant circuit, 27 2nd inductor, 28 2nd 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 Second capacitor, 41 series resonant circuit, 90 low-pass Filter.

Claims (9)

In a filter that passes a signal of a frequency within a certain band,
Two or more first inductors connected in series;
At least two first capacitors connected in parallel to at least two first inductors of the two or more first inductors to form at least two parallel resonant circuits having different resonance frequencies and having frequencies outside the band. When,
A filter comprising one of the two or more first inductors and one or more second capacitors connected between a connection portion of any two first inductors and a ground.   The at least two first capacitors are connected in parallel to all the first inductors of the two or more first inductors to form a parallel resonant circuit for each first inductor as the at least two parallel resonant circuits. The filter according to claim 1.   Connected in series with the one or more second capacitors between the connection portion of the two first inductors and the ground, and the resonance frequency is different from any resonance frequency of the at least two parallel resonance circuits. The filter according to claim 1, further comprising one or more second inductors forming one or more series resonant circuits having a frequency of. Two or more second capacitors connected in parallel are provided as the one or more second capacitors,
A series resonance circuit for each second capacitor is formed as the one or more series resonance circuits by being connected in series to the two or more second capacitors between the connection portion of the two first inductors and the ground. The filter according to claim 3, further comprising two or more second inductors as the one or more second inductors. In a filter that passes a signal of a frequency within a certain band,
Two or more second capacitors connected in parallel;
At least two second inductors connected in series to at least two second capacitors of the two or more second capacitors to form at least two series resonant circuits having different resonance frequencies and having frequencies outside the band. When,
A filter comprising one or more first inductors connected between any two of the at least two series resonant circuits.   The at least two second inductors are connected in series to all the second capacitors of the two or more second capacitors to form a series resonance circuit for each second capacitor as the at least two series resonance circuits. The filter according to claim 5.   The two or more series resonant circuits are connected in parallel to the one or more first inductors, and the resonance frequency is different from any of the resonance frequencies of the at least two series resonance circuits. The filter according to claim 5 or 6, further comprising one or more first capacitors forming one or more parallel resonant circuits. Two or more first inductors connected in series as the one or more first inductors;
Two or more parallel resonance circuits connected to the two or more first inductors between any two series resonance circuits to form a parallel resonance circuit for each first inductor as the one or more parallel resonance circuits. The filter according to claim 7, comprising a first capacitor as the one or more first capacitors. A filter according to any one of claims 1 to 8;
An electronic device comprising: a signal processing device that processes a signal that has passed through the filter.

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