JP2000068774A - Filter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To attain a function as a band filter having a rapid filter characteris tic by providing a configuration by which the miniaturization of a whole circuit is facilitated, an adjustment work concerning plural trap generating parts is reduced and a cost is reduced. SOLUTION: A filter circuit is constituted by including the plural trap generating parts 17, 18 and 19 being different in resonance frequency, capacitance elements 21, 23 and 25 for forming the plural trap generating parts 17, 18 and 19 are made to be a centralized constant number element and an inductance element 22 forming the trap generating part 18 among the plural ones 17, 18 and 19 is made to be the centralized constant number element. Together with it, the inductance elements 20 and 24 forming the trap generating parts 17 and 19 except the part 18 among the three are made to be a distribution constant number element.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention described in the claims of the present application relates to a filter circuit including a plurality of trap forming portions having different resonance frequencies and functioning as a bandpass filter.

[0002]

2. Description of the Related Art A filter circuit including a plurality of trap forming portions having different resonance frequencies and functioning as a bandpass filter having a low-pass filter characteristic or a high-pass filter characteristic, or a low-pass filter 2. Description of the Related Art A filter circuit functioning as a bandpass filter having both a portion having a filter characteristic and a portion having a high-pass filter characteristic is used for various purposes in various electronic devices. For example, a filter circuit functioning as a bandpass filter having both a portion having a low-pass filter characteristic and a portion having a high-pass filter characteristic includes a signal passing through a portion having a low-pass filter characteristic and a high-pass filter. The mixed output signal is obtained by deriving a signal passing through a portion having a band-pass filter characteristic to a common output terminal, and such a filter circuit is called a mixed filter circuit.

In such a filter circuit, each of the plurality of trap forming portions is a resonance circuit portion formed by a capacitance element and an inductance element. Conventionally, as a resonance circuit part forming a trap forming part, both a capacitance element and an inductance element forming the trap forming part are respectively lumped constant elements, or
It has been proposed that both the capacitance element and the inductance element forming the element are distributed constant elements.

When the resonance circuit section forming the trap forming section is formed by a capacitance element and an inductance element, each of which is a lumped constant element, an air-core coil or the like is used as the inductance element, for example, at 100 MHz. It is possible to obtain an extremely steep trap characteristic in which the value of Q is about 200 for the frequency of. Therefore, the filter circuit in which the resonance circuit part forming each of the plurality of trap forming parts is formed by the capacitance element and the inductance element each of which is a lumped constant element has a steep filter characteristic. It is assumed.

In the case where the resonance circuit portion forming the trap forming portion is formed by a capacitance element and an inductance element each of which is a distributed constant element, for example, a microstrip having an inductance element provided on a substrate By obtaining by a line, it can be provided on a substrate in a planar manner. therefore,
The overall thickness of the filter circuit, in which the resonance circuit section forming each of the plurality of trap forming sections is formed by a capacitance element and an inductance element each being a distributed constant element, is to be formed with a small thickness. Is possible.

[0006]

However, the resonance circuit portion forming each of the plurality of trap forming portions is provided in a filter circuit which is formed by a capacitance element and an inductance element each of which is a lumped constant element. Therefore, for example, the number of air-core coils used as the inductance element is required by the number corresponding to the number of trap forming portions, and adjustment of each air-core coil is relatively troublesome work. Long working time is required. Therefore, a filter circuit configured using such a capacitance element and an inductance element, each of which is a lumped constant element, has a disadvantage that appears mainly in terms of cost.

In a filter circuit in which a resonance circuit part forming each of a plurality of trap forming parts is formed by a capacitance element and an inductance element each of which is a distributed constant element, And the inductance element are formed on the substrate, requiring a relatively large substrate area. Further, in each trap forming portion, for example, 100 MHz
Only the trapping characteristic of which the value of Q is several tens with respect to the frequency is obtained. Therefore, a filter circuit formed using such a capacitance element and an inductance element each of which is a distributed constant element has difficulty in achieving downsizing and has difficulty in obtaining steep filter characteristics. It is assumed that.

[0008] In view of the above, the invention described in the claims of the present application includes a plurality of trap forming portions having different resonance frequencies from each other, so that the overall size can be easily reduced. And a filter circuit capable of functioning as a bandpass filter having a steep filter characteristic with a configuration in which adjustment work for each trap forming portion is reduced and cost is reduced.

[0009]

According to a first aspect of the present invention, there is provided a filter circuit comprising a plurality of trap forming portions having resonance frequencies different from each other. Function as a bandpass filter, the capacitance element forming each of the plurality of trap forming portions is a lumped constant element, and
An inductance element forming at least one of the plurality of trap forming portions is a lumped constant element, and an inductance element forming other than the at least one of the plurality of trap forming portions is a distributed constant element. It is assumed to be.

[0010] In particular, in the filter circuit according to the invention described in claim 2 of the present application, at least one of the plurality of trap forming portions in which the inductance element is a lumped element is substantially used. It includes a trap forming unit that determines a cutoff frequency.

Further, in the filter circuit according to any one of claims 4 to 6 in the claims of the present application, the plurality of trap forming portions constitute the first low-pass filter. And a second group forming a high-pass filter.
In each of the groups, the capacitance element forming each of the divided trap forming portions is a lumped constant element, and the inductance element forming at least one of the divided trap forming portions is While being a lumped constant element, the inductance element forming at least one other than the above-described one of the plurality of divided trap forming portions is configured as a distributed constant element, In the filter circuit according to the present invention, at least one of the divided trap forming portions, in which the inductance element is a lumped constant element, is substantially a low-pass filter. Alternatively, it includes a trap forming unit that determines the cutoff frequency of the high-pass filter.

In the filter circuit according to any one of the first to sixth aspects of the present invention configured as described above, a plurality of resonance circuits having different resonance frequencies are provided. In the trap forming portion, the capacitance element forming each of them is a lumped constant element, and the inductance element forming at least one of them is a lumped constant element, An inductance element forming at least one other than the one is a distributed constant element. That is,
A plurality of trap forming parts having different resonance frequencies,
Some of them are formed by a capacitance element that is a lumped constant element and an inductance element that is a lumped constant element, and others are a capacitance element that is a lumped constant element and an inductance element that is a distributed constant element. It is formed by

Therefore, the filter circuit according to any one of claims 1 to 6 in the claims of the present application provides a filter circuit including a plurality of trap forming portions having different resonance frequencies. Is formed by a capacitance element that is a lumped constant element and an inductance element that is a lumped constant element, thereby facilitating miniaturization as a whole, and in particular, claims in the claims of the present application. As in the filter circuit according to the invention described in item 2, at least one of the plurality of trap forming portions, in which the inductance element is a lumped element, includes a trap forming portion that substantially determines a cutoff frequency. As a result, the filter functions as a bandpass filter having steep filter characteristics, and the resonance frequencies are different from each other. The other one of the plurality of trap forming portions is formed by a capacitance element serving as a lumped constant element and an inductance element serving as a distributed constant element, so that the adjustment work for each trap forming portion is reduced. Thus, the cost can be reduced.

Further, in the filter circuit according to any one of claims 4 to 6 in the claims of the present application, the plurality of trap forming sections constitute a low-pass filter. And a second group forming a high-pass filter,
In each of the first and second groups, a capacitance element forming each of the plurality of divided trap forming sections is a lumped element, and at least one of the plurality of divided trap forming sections is replaced with a lumped element. Since the formed inductance element is a lumped constant element and the inductance element forming at least one of the divided trap forming portions other than at least one of the above-described trap forming portions is a distributed constant element, the first group In each of the low-pass filter composed of the second group and the high-pass filter composed of the second group, the overall size can be easily reduced, and in particular, claim 5 in the claims of the present application. Like the filter circuit according to the invention,
At least one of the divided plurality of trap forming sections, wherein the inductance element is a lumped element, includes a trap forming section that substantially determines a cutoff frequency of a low-pass filter or a high-pass filter. As a result, the filter functions as a bandpass filter having steep filter characteristics, and the adjustment work for each trap forming portion is reduced, so that the cost is reduced.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of a filter circuit according to any one of claims 1 to 3 in the claims of the present application. It functions as a bandpass filter having filter characteristics.

In the example shown in FIG. 1, between the signal input terminal 11 and the signal output terminal 12, there are provided inductance elements 13, 14, 15 and 1 each of which is a distributed constant element.
6 are connected in series. A trap forming portion 17 is connected between a connection point between the inductance elements 13 and 14 and the ground potential point, and a trap is formed between a connection point between the inductance elements 14 and 15 and the ground potential point. The trap forming portion 1 is connected between the connection point between the inductance element 15 and the inductance element 16 and the ground potential point.
9 is connected.

The trap forming section 17 adjusts the resonance frequency formed by the inductance element 20 as a distributed constant element and the capacitance element 21 as a lumped constant element within a band of, for example, several hundreds to several hundreds of MHz. a series resonance circuit to f ₁ that is intended. Trap forming part 1
8, and concentration constant element has been the inductance element 22 is formed by a lumped constant element has been the capacitance element 23, a series resonant circuit for the resonant frequency f ₂ lower than f _1. Trap forming unit 19, which is formed by the capacitance element 25 is an inductance element 24, which is a distributed constant element with lumped elements, the series resonance circuit for a resonance frequency higher f ₃ than f ₂ lower than f ₁ It is.

The inductance elements 13 to 16 and the trap forming portions 17, 18 and 19 are all shown in FIG.
(Horizontal axis: frequency, vertical axis: response) constitutes a filter circuit functioning as a bandpass filter having a low-pass filter characteristic. In the low-pass filter characteristics shown in FIG. 2, fca indicates a cutoff frequency.

The trap forming section 18 is steeper than each of the trap forming section 17 and the trap forming section 19 because both the inductance element 22 and the capacitance element 23 forming the same are lumped constant elements. It is assumed to have the a trap characteristic, since the resonance frequency f ₂ is selected lower than any of the resonant frequency f ₃ of the resonance frequency f ₁ and the trap forming part 19 of the trap forming portion 17, substantially the inductance Element 13
Cut-off frequency fca of a filter circuit functioning as a bandpass filter having low-pass filter characteristics and including trap forming sections 17, 18, and 19 and
Plays a role in determining Further, as shown in FIG. 2, the inductance elements 13 to 16 and the trap forming portion 1 are formed.
7, 18 and 19, the low-pass filter characteristic is 20 dB.
The above attenuation ratio is obtained.

The distributed constant elements, which are the inductance elements 20 to 24 forming the inductance elements 13 to 16 and the trap forming portions 17 and 19, respectively, are provided, for example, on an inexpensive substrate having a relatively high dielectric constant. Formed by the formed microstrip line. Also,
Inductance element 22 forming trap forming portion 18
Is formed by, for example, an air-core coil.
, 16, 20 and 24 are attached to the substrate on which the microstrip lines forming the distributed constant elements, which are to be each of .about.16, 20, and 24, are arranged. Further, the trap forming portions 17, 18 and 1
9 respectively forming capacitance elements 21, 23 and 2
5 are formed by capacitors, and such capacitors are also formed by inductance elements 13.
, 16, 20 and 24 are attached to the substrate on which the microstrip lines forming the distributed constant elements, which are to be each of .about.16, 20, and 24, are arranged.

In the example shown in FIG. 1,
Inductance elements 22 to form a trap formation portion 18 of the resonance frequency f _1, f ₂ and f ₃ respectively have traps forming parts 17, 18 and 19, by being a lumped elements, miniaturization of the whole And the trap forming portion 18 substantially reduces the cutoff frequency f
By setting ca, it functions as a bandpass filter having a steep low-pass filter characteristic with which a sufficient attenuation ratio can be obtained. Forming parts 17 and 1
9 are formed by inductance elements 20 and 24, respectively.
By being a distributed constant element together with the inductance elements 13 to 16, the adjustment work for the trap forming portions 17 and 19 can be reduced, and the cost can be reduced.

Next, FIG. 3 shows another example of the filter circuit according to any one of claims 1 to 3 in the claims of the present application. It functions as a bandpass filter having a pass filter characteristic.

In the example shown in FIG. 3, between the signal input terminal 31 and the signal output terminal 32, capacitance elements 33, 34, 35 and 3 each of which is a lumped constant element are provided.
6 are connected in series. A trap forming portion 37 is connected between a connection point between the capacitance elements 33 and 34 and the ground potential point, and a trap is formed between the connection point between the capacitance elements 34 and 35 and the ground potential point. Is connected between the capacitance element 35 and the capacitance element 36 and the ground potential point.
9 is connected.

The trap forming section 37 adjusts the resonance frequency formed by the inductance element 40, which is a distributed constant element, and the capacitance element 41, which is a lumped constant element, within a band of several hundreds to several hundreds of MHz, for example. a series resonance circuit to f ₄ that is intended. Trap forming part 3
8, formed by a lumped constant element has been the inductance element 42 and the lumped elements have been the capacitance element 43, a series resonant circuit for a resonance frequency higher f ₅ than f _4. The trap forming section 39 is a series resonance circuit section having a resonance frequency of f ₆ higher than f ₄ and lower than f ₅ formed by an inductance element 44 serving as a distributed constant element and a capacitance element 45 serving as a lumped constant element. It is.

The capacitance elements 33 to 36 and the trap forming portions 37, 38 and 39 are all shown in FIG.
(Horizontal axis: frequency, vertical axis: response), a filter circuit functioning as a bandpass filter having high-pass filter characteristics as shown in FIG. In the high-pass filter characteristics shown in FIG. 4, fcb indicates a cutoff frequency.

The trap forming section 38 is steeper than each of the trap forming section 37 and the trap forming section 39 because the inductance element 42 and the capacitance element 43 forming the trap forming section are both lumped constant elements. It is assumed to have the a trap characteristic, since the resonance frequency f ₅ is selected higher than any of the resonant frequency f ₆ of the resonance frequency f ₄ and the trap forming part 39 of the trap forming portion 37, substantially, the capacitance Element 33
Cut-off frequency fcb of a filter circuit functioning as a bandpass filter having high-pass filter characteristics and including the trap forming portions 37, 38 and 39.
Plays a role in determining Also, as shown in FIG.
7, 38 and 39, the high-pass filter characteristic is 20 dB.
The above attenuation ratio is obtained.

The distributed constant elements which are the inductance elements 40 and 44 forming the trap forming portions 37 and 39, respectively, are formed, for example, by microstrip lines provided on an inexpensive substrate having a relatively high dielectric constant. It is formed. Further, the lumped constant element serving as the inductance element 42 forming the trap forming portion 38 is formed of, for example, an air-core coil, and the air-core coil is a distributed constant element serving as each of the inductance elements 40 and 44. The microstrip line to be formed is attached to the substrate provided. Further, the capacitance elements 33 to 36
The lumped-constant elements, which are each of the capacitance elements 41, 43, and 45, which form the trap forming portions 37, 38, and 39, respectively, are formed by capacitors, and the capacitors are also each of the inductance elements 40, 44. A microstrip line forming a distributed constant element is attached to a substrate provided thereon.

In the example shown in FIG. 3,
Since the inductance element 42 forming the trap forming part 38 of the trap forming parts 37, 38 and 39 having the resonance frequencies f ₄ , f ₅ and f _{6 respectively} is a lumped element, the overall size is reduced. And the trap forming portion 38 substantially reduces the cutoff frequency f
By setting cb, it functions as a bandpass filter having a steep high-pass filter characteristic with which a sufficient attenuation ratio can be obtained. Forming parts 37 and 3
9 are formed by inductance elements 40 and 44, respectively.
By forming a distributed constant element, the trap forming portion 37
And 39 are reduced, and the cost is reduced.

FIG. 5 shows an example of a filter circuit according to any one of the first to sixth aspects of the present invention. This example has a low-pass filter characteristic. The filter circuit functions as a band-pass filter having both the above-mentioned portion and a portion having a high-pass filter characteristic.

In the example shown in FIG. 5, a portion FL having a low-pass filter characteristic and a portion FH having a high-pass filter characteristic are provided. Then, the portion FL having the low-pass filter characteristic corresponds to the example shown in FIG. 1 described above, and the portion FH having the high-pass filter characteristic is shown in FIG. It is supposed to correspond to the example.

In FIG. 5, a portion FL having a low-pass filter characteristic is shown with the same reference numeral as that of the corresponding example shown in FIG. FIG. 3 shows the corresponding portion FH.
Are given the same reference numerals as in the example shown in FIG. 1, and a detailed description of the part FL itself having a low-pass filter characteristic and the part FH itself having a high-pass filter characteristic is shown in FIG. And the detailed description of the example shown in FIG.

In the part FL having the low-pass filter characteristic and the part FH having the high-pass filter characteristic in the example shown in FIG. 5, the signal output terminal of the part FL having the low-pass filter characteristic is provided. 12 and the signal output terminal 32 in the portion FH having the high-pass filter characteristic are connected in common, and a common output terminal 50 is provided.

Part FL having low-pass filter characteristics
Has a low-pass filter characteristic as shown in FIG. 2 and a portion FH having a high-pass filter characteristic.
Has a high-pass filter characteristic as shown in FIG. Therefore, the filter circuit shown in FIG. 5 has a low-pass filter characteristic and a high-pass filter characteristic as shown in FIG. 6 (horizontal axis: frequency, vertical axis: response). The cutoff frequency fca in the low-pass filter characteristic is, for example, 1335 MHz.
z, and the cut-off frequency fcb in the high-pass filter characteristic is selected to be, for example, 1395 MHz. Even under such circumstances, in each of the low-pass filter characteristics and the high-pass filter characteristics,
An attenuation ratio of 0 dB or more is obtained.

In the example shown in FIG. 5,
The resonance frequency f _1, f ₂ and f ₃ of the inductance element 22 and the resonance frequency f ₄ which form a trap formation portion 18 of each have traps forming parts 17, 18 and 19,
the inductance element 42 to form a trap formation portion 38 of the trap forming portion 37, 38 and 39 the f ₅ and f ₆ had each is, by being a lumped element,
The overall size is reduced, and the trap forming portion 18 is formed.
However, it is assumed that the cut-off frequency fca is substantially determined, and the trap forming section 38 substantially determines the cut-off frequency fcb. It will function as a bandpass filter having a filter characteristic and a high-pass filter characteristic.
9 and the inductance elements 20 and 24 forming the trap forming parts 17 and 19, respectively, and the inductance elements 40 and 44 forming the trap forming parts 37 and 39 of the trap forming parts 37, 38 and 39, respectively.
Is a distributed constant element, the adjustment work for the trap forming parts 17, 19, 37 and 39 is reduced, and the cost is reduced.

Under the circumstances, the signal S1 is supplied to the signal input terminal 11 of the portion FL having the low-pass filter characteristic,
When the 31 signal S2 is supplied to the signal input terminal of the portion FH having the high-pass filter characteristic, the cutoff frequency fca in the low-pass filter characteristic based on the signal S1 is passed through the portion FL having the low-pass filter characteristic. Is 1
A signal S1 'having a frequency component of 335 MHz or less is led out to the common output terminal 50, and is a cutoff frequency fcb in the high-pass filter characteristic based on the signal S2 through the portion FH having the high-pass filter characteristic. A signal S 2 ′ having a frequency component of 1395 MHz or more is led out to the common output terminal 50. As a result, both the signal S1 'and the signal S2' are obtained at the common output terminal 50, and the example shown in FIG. 5 forms a mixing filter circuit.

[0036]

As is apparent from the above description, the filter circuit according to any one of the first to sixth aspects of the present invention has the following features.
Some of the plurality of trap forming portions having different resonance frequencies are formed of a lumped-constant capacitance element and a lumped-constant inductance element, so that the overall size can be easily reduced. In particular, as in the filter circuit according to the invention described in claim 2 of the present application, at least one of the plurality of trap forming portions in which the inductance element is a lumped element is provided. In addition, by including the trap forming portion that substantially determines the cutoff frequency, it functions as a bandpass filter having a steep filter characteristic, and a plurality of trap forming portions having different resonance frequencies are formed. The other part is a lumped-constant capacitance element and a distributed-constant inductance element. By being formed by a, it is adjusted work relief for each trap forming part, so that the cost reduction can be achieved.

Further, in the filter circuit according to any one of claims 4 to 6 in the claims of the present application, the plurality of trap forming portions constitute a low-pass filter. And a second group forming a high-pass filter,
In each of the low-pass filter constituted by the first group and the high-pass filter constituted by the second group, some of the divided trap forming portions are lumped elements. The filter is formed by the capacitance element and the inductance element which is a lumped constant element, so that the whole can be easily reduced in size, and in particular, the filter according to the invention described in claim 5 in the claims of the present application. As in the circuit, at least one of the divided trap forming sections, in which the inductance element is a lumped constant element, substantially determines the cutoff frequency of the low-pass filter or the high-pass filter. , It will function as a bandpass filter having steep filter characteristics, The other of the plurality of divided trap forming portions is formed by a capacitance element that is a lumped constant element and an inductance element that is a distributed constant element, so that the The adjustment work for each is reduced, and the cost is reduced.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram showing an example of a filter circuit according to any one of the first to third aspects of the present invention.

FIG. 2 is a characteristic diagram showing a low-pass filter characteristic of the example shown in FIG.

FIG. 3 is a circuit configuration diagram showing another example of the filter circuit according to any one of claims 1 to 3 in the claims of the present application.

FIG. 4 is a characteristic diagram showing high-pass filter characteristics of the example shown in FIG. 3;

FIG. 5 is a circuit configuration diagram showing an example of a filter circuit according to any one of claims 1 to 6 in the claims of the present application.

FIG. 6 is a characteristic diagram showing a low-pass filter characteristic and a high-pass filter characteristic of the example shown in FIG. 5;

[Explanation of symbols]

11, 31 ... signal input terminal, 12, 32 ... signal output terminal, 13, 14, 15, 16, 16, 22, 22,
24, 40, 42, 44 ... inductance element,
17, 18, 19, 37, 38, 39 ... trap forming part, 21, 23 25, 33, 34, 35, 36,
41, 43, 45 ... capacitance element, 50
Common output terminal

Claims (6)

[Claims] 1. A device comprising a plurality of trap forming portions having different resonance frequencies from each other and functioning as a bandpass filter, wherein a capacitance element forming each of the plurality of trap forming portions is a lumped element. An inductance element forming at least one of the plurality of trap forming portions is a lumped constant element, and an inductance element forming other than the at least one of the plurality of trap forming portions is a distributed constant element. A filter circuit characterized by the following. 2. The filter according to claim 1, wherein at least one of the plurality of trap forming sections, wherein the inductance element is a lumped element, includes a trap forming section that substantially determines a cutoff frequency. circuit. 3. A lumped constant element serving as an inductance element forming at least one of the plurality of trap forming portions is formed by an air-core coil, and a lumped constant element other than the at least one of the plurality of trap forming portions is formed. 3. The filter circuit according to claim 1, wherein the distributed constant element serving as an inductance element forming the filter circuit is formed by a microstrip line disposed on a substrate. 4. A plurality of trap forming units are divided into a first group forming a low-pass filter and a second group forming a high-pass filter, wherein said first group and said second group are formed. In each of the groups, a capacitance element forming each of the plurality of divided trap forming portions is a lumped constant element, and an inductance element forming at least one of the divided plurality of trap forming portions is a lumped constant element. 2. The filter circuit according to claim 1, wherein an inductance element that forms an element and forms an element other than at least one of the divided trap forming portions is a distributed constant element. 5. A trap forming unit wherein at least one of the divided trap forming units, wherein the inductance element is a lumped element, substantially determines a cutoff frequency of a low-pass filter or a high-pass filter. The filter circuit according to claim 1, further comprising: 6. A lumped constant element, which is an inductance element forming at least one of the plurality of divided trap forming portions, is formed by an air-core coil. The filter circuit according to claim 3, wherein the distributed constant element serving as an inductance element forming one other than the at least one is formed by a microstrip line disposed on a substrate.