# US 2017/0288632 A1 · Variable filter circuit, rf front end circuit and communication device

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- Number: US 2017/0288632 A1
- Document ID: US20170288632A1
- Status: Published application
- Kind: A1
- Application number: 15/492,190
- Filed: 2017-04-20
- Published: 2017-10-05
- Earliest claimed: 2014-11-11
- Assignees: Murata Manufacturing Co., Ltd.
- Applicants: Murata Manufacturing Co., Ltd.
- Inventors: Hirotsugu MORI
- Web page: https://patentgrep.com/patent/US20170288632A1

## CPC

- H03H 7/12: Bandpass or bandstop filters with adjustable bandwidth and fixed centre frequency (H03H7/09 takes precedence)
- H03F 1/565: {using inductive elements}
- H03F 3/189: High-frequency amplifiers, e.g. radio frequency amplifiers
- H03F 3/24: of transmitter output stages
- H03H 7/38: Impedance-matching networks
- H03H 9/542: {including passive elements (H03H9/545 takes precedence)}
- H03H 9/605: {consisting of a ladder configuration}
- H03H 9/6483: {Ladder SAW filters}
- H03F 3/195: in integrated circuits
- H03F 3/245: {with semiconductor devices only}
- H03F 2200/294: the amplifier being a low noise amplifier [LNA]
- H03F 2200/451: the amplifier being a radio frequency amplifier
- H03H 2210/012: Centre frequency; Cut-off frequency
- H03H 2210/015: Quality factor or bandwidth
- H03H 2210/025: Capacitor
- H03H: IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS (waveguides, resonators, lines or other devices of the waveguide type H01P)
- H03F: AMPLIFIERS

## Abstract

Provided is a variable filter circuit that can control the bandwidth and center frequency of a pass band, can realize steep attenuation characteristics in bands close to the pass band, and enables the total number of variable reactance units to be reduced. A variable filter circuit includes an inductor (Ls1) and a capacitor (Cs1), which are connected in series between a first input/output terminal (P1) and a second input/output terminal (P2), and resonators (Re_p1, Re_p2, Re_p3, Re_p4) and variable capacitors (Cc1, Cc2, Cc3, Cc4), which are connected in series between two ends of the inductor (Ls1) and the capacitor (Cs1) and ground connection terminals.

## Claims

### Claim 1

A variable filter circuit comprising:
a plurality of reactance elements that are connected in series between a first terminal and a second terminal; and
a plurality of parallel arms, wherein each one of the plurality of parallel arms is connected between an end of at least one of the plurality of reactance elements and a ground terminal and comprises:
a resonance circuit; and
a variable reactance connected in series with the resonance circuit,
wherein the plurality of reactance elements includes a capacitor having a capacitive reactance in a first frequency band and an inductor having an inductive reactance in a second frequency band.

### Claim 2 (depends on 1)

The variable filter circuit according to claim 1, wherein each of the parallel arms further comprises:
an inductor connected in series with resonance circuit.

### Claim 3 (depends on 1)

The variable filter circuit according to claim 1, wherein each resonance circuit includes a plurality of resonators and a switch that selects and connects one of the plurality of resonators.

### Claim 4 (depends on 1)

The variable filter circuit according to claim 1, further comprising:
a matching network that is connected between the inductor and the capacitor of the plurality of reactance elements.

### Claim 5 (depends on 1)

The variable filter circuit according to claim 1, wherein each of the parallel arms further comprises:
a reactance circuit connected in parallel with the resonance circuit.

### Claim 6 (depends on 2)

The variable filter circuit according to claim 2, wherein each of the parallel arms further comprises:
a reactance circuit having a reactance element connected in parallel with the resonance circuit.

### Claim 7 (depends on 6)

The variable filter circuit according to claim 6, wherein the reactance circuit connected in parallel with the resonance circuit is connected at a first end between the resonance circuit and the inductor connected in series with the resonance circuit, and at a second end to the ground terminal.

### Claim 8 (depends on 6)

The variable filter circuit according to claim 6, wherein the reactance circuit further comprises:
a switch that selectively connects the reactance element to be connected in parallel with the resonance circuit.

### Claim 9 (depends on 1)

The variable filter circuit according to claim 1, comprising first, second, and third parallel arms,
wherein the first parallel arm is connected at a first end between the first terminal and the inductor of the plurality of reactance elements, and at a second end to the ground terminal,
wherein the second parallel arm is connected at a first end between the inductor of the plurality of reactance elements and the capacitor of the plurality of reactance elements, and at a second end to the ground terminal,
wherein the first parallel arm is connected at a first end between the second terminal and the capacitor of the plurality of reactance elements, and at a second end to the ground terminal.

### Claim 10 (depends on 1)

An RF front end circuit comprising:
the variable filter circuit according to claim 1;
an antenna connection terminal that is provided on a first end of the variable filter circuit; and
a signal processing circuit connection terminal that is provided on a second end of the variable filter circuit.

### Claim 11 (depends on 10)

A communication device comprising:
the RF front end circuit according to claim 10;
an antenna that is connected to the antenna connection terminal; and
a signal processing circuit that is connected to the signal processing circuit connection terminal.