# US 11,121,701 B2 · Tunable filter for RF circuits

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- Number: US 11,121,701 B2
- Document ID: US11121701B2
- Status: Granted
- Kind: B2
- Application number: 16/870,136
- Filed: 2020-05-08
- Published: 2020-10-08
- Granted: 2021-09-14
- Earliest claimed: 2013-12-31
- Assignees: Ethertronics, Inc.
- Applicants: Ethertronics, Inc.
- Inventors: Laurent Desclos
- Web page: https://patentgrep.com/patent/US11121701B2

## CPC

- H03H 11/1217: {using a plurality of operational amplifiers (H03H11/1204 takes precedence; parallel-T filters H03H11/1295)}
- H03F 1/56: Modifications of input or output impedances, not otherwise provided for
- H03F 3/19: with semiconductor devices only
- H03F 3/195: in integrated circuits
- H03F 3/21: with semiconductor devices only {(H03F3/245 takes precedence)}
- H03F 3/245: {with semiconductor devices only}
- H03F 3/45179: {using MOSFET transistors as the active amplifying circuit (H03F3/45278 takes precedence)}
- H03F 3/45475: {using IC blocks as the active amplifying circuit}
- H03H 11/1291: {Current or voltage controlled filters}
- H03H 19/004: {Switched capacitor networks}
- H04B 1/04: Circuits
- H04B 1/1036: {with automatic suppression of narrow band noise or interference, e.g. by using tuneable notch filters (H04B1/123 takes precedence; filter circuits H03H)}
- H04B 1/1638: {Special circuits to enhance selectivity of receivers not otherwise provided for (resonant circuits H03H)}
- H03F 2200/213: A variable capacitor being added in the input circuit, e.g. base, gate, of an amplifier stage
- H03F 2200/294: the amplifier being a low noise amplifier [LNA]
- H03F 2200/451: the amplifier being a radio frequency amplifier
- H03F 2200/546: A tunable capacitance being present in an amplifier circuit
- H03F 2203/45544: the IC comprising one or more capacitors, e.g. coupling capacitors
- H03F 2203/45594: the IC comprising one or more resistors, which are not biasing resistor
- H03H 2210/012: Centre frequency; Cut-off frequency
- H03H 2210/015: Quality factor or bandwidth
- H03H 2210/025: Capacitor
- H03H 2210/028: Resistor
- H04B 2001/1054: {by changing bandwidth}
- H03H: IMPEDANCE NETWORKS, e.g. RESONANT CIRCUITS; RESONATORS (waveguides, resonators, lines or other devices of the waveguide type H01P)
- H03F: AMPLIFIERS
- H04B: TRANSMISSION

## Abstract

A tunable filter is described where the frequency response as well as bandwidth and transmission loss characteristics can be dynamically altered, providing improved performance for transceiver front-end tuning applications. The rate of roll-off of the frequency response can be adjusted to improve performance when used in duplexer applications. The tunable filter topology is applicable for both transmit and receive circuits. A method is described where the filter characteristics are adjusted to account for and compensate for the frequency response of the antenna used in a communication system.

## Claims

### Claim 1

A tunable filter comprising:
a first operational amplifier comprising a non-inverting input and an output thereof; and
a second operational amplifier comprising a non-inverting input and an output thereof,
the output of the first operational amplifier connected to the non-inverting input of the second operational amplifier at an output of the tunable filter;
the output of the second operational amplifier connected to the non-inverting input of the first operational amplifier at an input of the tunable filter; and
a tunable capacitor coupled in between the non-inverting input of the first operational amplifier and ground,
wherein the output of the second operational amplifier is connected to a variable resistor circuit, the variable resistor circuit connected to the non-inverting input of the first operational amplifier.

### Claim 2 (depends on 1)

The tunable filter of claim 1, wherein a change in resistance of the variable resistor is operable to result in an inverse change in frequency of a filter response of the tunable filter.

### Claim 3 (depends on 1)

The tunable filter of claim 1, wherein the variable resistor circuit comprises a plurality of resistors coupled to a multi-port switch.

### Claim 4 (depends on 1)

The tunable filter of claim 1, wherein the variable resistor circuit is connected in a series configuration with the non-inverting input of the first operational amplifier prior to a junction where the tunable capacitor is connected between the non-inverting input of the first operation amplifier and ground.

### Claim 5 (depends on 1)

The tunable filter of claim 1, further comprising a second variable resistor circuit coupled to the non-inverting input of the first operational amplifier.

### Claim 6 (depends on 1)

A communication system comprising the tunable filter of claim 1.

### Claim 7 (depends on 6)

The communication system of claim 6, wherein the communication system comprises an antenna.

### Claim 8 (depends on 7)

The communication system of claim 7, wherein the antenna is an isolated magnetic dipole.

### Claim 9 (depends on 7)

The communication system of claim 7, wherein the tunable fitter is disposed in a receive chain.

### Claim 10 (depends on 7)

The communication system of claim 7, wherein the tunable filter is disposed in a transmit chain.