JPH0766656A - Lc filter - Google Patents

Abstract

PURPOSE:To obtain a small-sized LC filter provided with a sharp attenuation characteristic without using a special element such as a transformer, etc. CONSTITUTION:A high-pass filter 1 is provided with a capacitor 2, inductors 3 and 4, and an inductor 5 for attenuation electrode generation. In the capacitor 2, one terminal part is electrically connected with an input electrode 8 and the other terminal part is electrically connected with an output electrode 9. In the inductor 3, one terminal part is electrically connected with the input electrode 8 and the other terminal part is electrically connected with a gland electrode 10 via the inductor 5 for attenuation electrode generation. In the inductor 4, one terminal part is electrically connected with the output electrode 9 and the other terminal part is electrically connected with the gland electrode 10 via the inductor 5 for attenuation electrode generation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LC filter used as a bandpass filter, a highpass filter or the like.

[0002]

2. Description of the Related Art In an LC filter, an attenuation pole may be used to obtain a steep attenuation characteristic. As an example of a circuit for generating this attenuation pole, there is known a circuit in which an LC parallel resonance circuit having a maximum impedance at a predetermined resonance frequency is connected in series between an input electrode and an output electrode. Since it is actually difficult to obtain and manufacture an inductor or a capacitor having the optimum inductance and capacitance required for this LC parallel resonant circuit, an inductor that is easy to obtain and manufacture at the expense of attenuation characteristics. And capacitors were used.

This difficulty occurs especially when a large value of inductance is required for the inductor that constitutes the LC parallel resonance circuit, and conventionally, a new transformer is added or the number of turns of the inductor conductor is reduced. It was necessary to increase the number, and the size of the LC filter tended to increase inevitably. Therefore, an object of the present invention is to provide a small LC filter having a steep attenuation characteristic without using a special element such as a transformer.

[0004]

In order to solve the above problems, the LC filter according to the present invention comprises (a) a capacitor electrically connected in series between an input electrode and an output electrode, and b) two inductors electrically connected in parallel with each other with the capacitor interposed between the input electrode and the output electrode; and (c) electrically connected in series between the two inductors and the ground electrode. The above-described inductor for generating an attenuation pole is provided.

In the above structure, for example, the LC filter of the equivalent circuit shown in FIG. 2 is formed. That is, the two inductors and one attenuation pole generating inductor forming the “star circuit” are equivalently converted to the “delta circuit”. Then, in the equivalent conversion circuit, the LC parallel resonance circuit is connected in series between the input electrode and the output electrode, and the LC parallel resonance circuit generates the attenuation pole.

[0006]

Embodiments of the LC filter according to the present invention will be described below with reference to the accompanying drawings. [First Embodiment, FIGS. 1 to 4] The first embodiment will be described by taking a high-pass filter as an example. As shown in FIG. 1, the high-pass filter 1 is a three-terminal filter including a capacitor 2, inductors 3 and 4, and an attenuation pole generating inductor 5. One end of the capacitor 2 has an input electrode 8
Is electrically connected to the output electrode 9, and the other end is electrically connected to the output electrode 9. The inductor 3 has one end electrically connected to the input electrode 8 and the other end electrically connected to the ground electrode 10 via the attenuation pole generating inductor 5. One end of the inductor 4 is electrically connected to the output electrode 9, and the other end thereof is electrically connected to the ground electrode 10 via the attenuation pole generating inductor 5. Here, the inductances of the inductors 3 and 4 and the attenuation pole generating inductor 5 are respectively L
3, L4 and L5, and the capacitance of the capacitor 2 is C1.

By the way, the filter 1 having the above structure
The inductors 3 and 4 and the inductor 5 for generating the attenuation pole are
It has a so-called “star circuit” connection relationship. Therefore, when the equivalent conversion from the well-known "star circuit" to the "delta circuit" is performed, the equivalent electric circuit of the filter 1 becomes that shown in FIG. Here, the inductances L0, L1, and L2 have values represented by the following equations (1), (2), and (3), respectively.

L0 = (L3.L4 + L4.L5 + L5.L3) / L5 ... (1) L1 = (L3.L4 + L4.L5 + L5.L3) / L4 ... (2) L2 = (L3.L4 + L4.L5 + L5.L3) / L3 (3) In the equivalent electric circuit shown in FIG. 2, the capacitance C1 and the inductance L0 form an LC parallel resonance circuit, and are connected in series between the input electrode 8 and the output electrode 9. Therefore, the high pass filter 1 is
Because of the attenuation pole generated by the parallel resonant circuit, it is possible to have a steep attenuation characteristic.

The resonance frequency f of the LC parallel resonance circuit composed of the capacitance C1 and the inductance L0.
Is expressed by the following equation (4). f = 1 / (2π√ (L0 · C1)) (4) Further, the action and effect of the high-pass filter 1 will be described using specific numerical values.

In order for the high-pass filter 1 to have a cutoff frequency at 500 MHz and an attenuation pole to be generated at 200 MHz as shown in FIG. 3, the capacitance C1 of the capacitor 2 is 3.2 pF and the inductors 3 and 4 have the same characteristics. The inductances L3 and L4 are set to 14 nH, and the inductance L5 of the attenuation pole generating inductor 5 is set to 1.1 nH.

An example of the procedure for setting the inductances L3 to L5 of the inductors 3 to 5 will be described below. Consider a conventionally known π-type high-pass filter shown in FIG. This π-type high-pass filter does not generate an attenuation pole. This filter has a capacitor 12 connected in series between an input electrode 16 and an output electrode 17, and
The inductors 13 and 14 are connected in parallel between the output electrode 17 and the output electrode 17. Then, one ends of the inductors 13 and 14 are connected to the ground electrode 18, respectively. In order for this filter to have a cutoff frequency at 500 MHz, the capacitance C12 of the capacitor 12 should be 3.2.
pF, the inductance L13 of the inductors 13 and 14,
L14 becomes 16 nH, respectively. Then, the inductances L3, L4, L5 of the inductors 3, 4 and the attenuation pole generating inductor 5 are calculated by using the inductances L13 to L15 of the π-type high-pass filter shown in FIG. ) And (7).

L3 = (L15 / L13) / (L13 + L14 + L15) (5) L4 = (L14 / L15) / (L13 + L14 + L15) ... (6) L5 = (L13 / L14) / (L13 + L14 + L15) ... (7) However, L15 = 1 / (4 · π 2 · f 2 · C12) f: frequency of the attenuation pole Therefore, in (5) to (7), L13 = 16 ×
10 ^-9 , L14 = 16 × 10 ^-9 , C12 = 3.2 × 10
^The inductances L3 to L5 are obtained by substituting ⁻¹² and f = 200 × 10 ⁶ for calculation.

Now, a comparison will be made between the high-pass filter 1 thus obtained and the conventional case where an LC parallel resonance circuit is connected in series between an input electrode and an output electrode to generate a similar attenuation pole. The inductance of the inductor required to form the conventional LC parallel resonance circuit can be calculated from the equation (4). That is, the equation (4) is transformed into the following equation (8).

L0 = 1 / (4 · π ² · f ² · C1) (8) In the formula (8), f = 200 × 10 ⁶ , C1 = 3.2.
Calculating by substituting × 10 ^-12 , L0 is about 200 nH
Becomes (Note that the calculation may be performed using the equation (1).
That is, L0 = (L3 · L4 + L4 · L5 + L5 · L
3) / L5, L3 = 14 × 10 ⁻⁹ , L4 = 14
Substituting x10 ^-9 and L5 = 1.1 x 10 ^-9 , L0 is about 200 nH. Therefore, the conventional high-pass filter must incorporate an inductor having an inductance of about 200 nH.

As described above, the high-pass filter 1 can obtain a large equivalent inductance by combining inductors having a small inductance without using a special element such as a transformer. As a result, a compact high-pass filter can be obtained. Further, the high-pass filter 1 having the above configuration specifically has, for example, the structure described below.

A dielectric substrate is used, and a conductor is formed on the surface thereof. The conductor is formed of a conductive paste or the like, and functions as a capacitor or an inductor depending on its shape and arrangement. Generally, a capacitor is composed of conductors facing each other with a dielectric substrate in between. Further, the inductor is composed of a linear, zigzag, or spiral conductor. In the case of this structure, a large-capacity capacitor and a large-inductance inductor necessarily require a large area, which is an obstacle to miniaturization. Therefore, taking the circuit configuration according to the present invention is extremely effective. In this case, the attenuation pole generating inductor can be obtained by providing the conductor on the dielectric substrate or the lead terminal with the bypass portion. If the attenuation pole generating inductor is installed on the side of the circuit board to which the LC filter is attached, the attenuation pole frequency can be set by a user such as a set maker.

Further, instead of the dielectric substrate, it may be integrally formed as a surface mount component by a green sheet laminating method or a printing laminating method. In this case, the inductor for generating the attenuation pole may be formed inside the component,
It can also be achieved by providing a bypass portion on the lead-out terminal to the outside. Further, as another method, not only the integrated type as described above, but also capacitors and inductors which are separate parts may be combined and manufactured.

[Second Embodiment, FIGS. 5 to 6] A second embodiment will be described by taking a bandpass filter as an example. As shown in FIG. 5, the bandpass filter 21 includes capacitors 25, 2
8 and 29, inductors 26 and 27, and an attenuation pole generating inductor 30. The capacitor 25 is the input electrode 3
3 and the output electrode 34 are electrically connected in series. The inductors 26 and 27 and the capacitors 28 and 29 are electrically connected in parallel with the capacitor 25 sandwiched between the input electrode 33 and the output electrode 34. The attenuation pole generating inductor 30 includes inductors 26 and 27 and a capacitor 2
The electrodes 8 and 29 and the ground electrode 35 are electrically connected in series.

The inductors 26 and 27 and the attenuation pole generating inductor 30 of the filter 21 having the above structure are
It has a wiring relationship of "star circuit". Therefore,
When the equivalent conversion from the "star circuit" to the "delta circuit" is performed, the equivalent electric circuit of the filter 21 becomes that shown in FIG. In the figure, C2, C3 and C4 represent the capacitances of the capacitors 25, 28 and 29, respectively, and L6, L
Reference numerals 7 and L8 represent equivalent conversion inductances of the inductors 26, 27 and 30, respectively. In the equivalent electric circuit shown in FIG. 6, the capacitance C2 and the inductance L6 form an LC parallel resonance circuit, and the attenuation pole generated by this LC parallel resonance circuit can have a sharp attenuation characteristic.

In FIG. 7, the capacitances of the capacitors 25, 28 and 29 are set to 5.5 pF, 42 pF and 42 p, respectively.
6 is a graph showing the insertion loss characteristic measurement results when F is F, the inductances of the inductors 26 and 27 are 2.2 nH and 2.2 nH, and the attenuation pole generating inductor 30 is 0.045 nH. An attenuation pole due to the LC parallel resonance circuit occurs at 200 MHz.

By the way, from the graph, the frequency is 1.4
Another attenuation pole is generated at the position of GHz. This includes the inductor 30 for generating the attenuation pole, the inductor 26,
27 and the capacitors 28 and 29 are attenuation poles generated by the series resonance circuit when the relative circuit conversion is performed. As a result, each of the low frequency side and the high frequency side has an attenuation pole, and a bandpass filter having steep attenuation characteristics can be obtained.

[Other Embodiments] The LC filter according to the present invention is not limited to the above embodiments, but can be variously modified within the scope of the gist thereof. The LC filter is not limited to the basic circuit of the high-pass filter or the band-pass filter, but includes the high-pass filter shown in FIGS. 8, 9 and 10 and the band-pass filter shown in FIGS. 11 and 12, that is, a complicated circuit. It may be a filter.

Further, the LC filter is not limited to the lumped constant circuit filter as in each of the above embodiments, but a high pass filter having the distributed constant circuit shown in FIG. 13 and a band having the distributed constant circuit shown in FIG. It may be a pass filter. Further, in the LC filter, its constituent elements may be replaced with another element equivalent to a capacitor or an inductor.

[0024]

As is apparent from the above description, according to the present invention, an inductor for generating an attenuation pole is provided between two inductors electrically connected in parallel between an input electrode and an output electrode and a ground electrode. Since it is electrically connected in series to, a small LC filter having a steep attenuation characteristic can be obtained without using a special element such as a transformer.

In particular, in the case of a bandpass filter, by providing one attenuation pole generating inductor, attenuation poles are generated on both the low frequency side and the high frequency side, and an excellent attenuation characteristic can be obtained.

[Brief description of drawings]

FIG. 1 is an electric circuit diagram showing a first embodiment of an LC filter according to the present invention.

2 is an equivalent electric circuit diagram of the LC filter shown in FIG.

FIG. 3 is a graph showing the relationship between frequency and insertion loss of the LC filter shown in FIG.

4 is an electric circuit diagram for explaining a procedure for setting the inductance of the inductor that constitutes the LC filter shown in FIG.

FIG. 5 is an electric circuit diagram showing a second embodiment of the LC filter according to the present invention.

6 is an equivalent electric circuit diagram of the LC filter shown in FIG.

7 is a graph showing the relationship between frequency and insertion loss of the LC filter shown in FIG.

FIG. 8 is an electric circuit diagram showing another embodiment of the LC filter according to the present invention.

FIG. 9 is an electric circuit diagram showing another embodiment of the LC filter according to the present invention.

FIG. 10 is an electric circuit diagram showing another embodiment of the LC filter according to the present invention.

FIG. 11 is an electric circuit diagram showing another embodiment of the LC filter according to the present invention.

FIG. 12 is an electric circuit diagram showing another embodiment of the LC filter according to the present invention.

FIG. 13 is an electric circuit diagram showing another embodiment of the LC filter according to the present invention.

FIG. 14 is an electric circuit diagram showing another embodiment of the LC filter according to the present invention.

[Explanation of symbols]

 1 ... High-pass filter 2 ... Capacitor 3, 4 ... Inductor 5 ... Attenuation pole generating inductor 8 ... Input electrode 9 ... Output electrode 10 ... Ground electrode C1 ... Capacitor L0, L1, L2 ... Inductance 21 ... Bandpass filter 25 ... Capacitor 26 , 27 ... Inductor 30 ... Attenuation pole generating inductor 33 ... Input electrode 34 ... Output electrode 35 ... Ground electrode C2 ... Capacitor L6, L7, L8 ... Inductance

Claims (1)

[Claims] 1. In an equivalent circuit, an LC parallel resonance circuit is connected in series between an input electrode and an output electrode, and an inductance is connected in series between the input electrode and the ground electrode and between the output electrode and the ground electrode. In the LC filter that realizes the circuit connected to, a capacitor electrically connected in series between the input electrode and the output electrode, and sandwiching the capacitor between the input electrode and the output electrode. An LC filter comprising: two inductors that are electrically connected in parallel; and an attenuation pole generation inductor that is electrically connected in series between the two inductors and a ground electrode.