Tunable filter for RF circuits

• US 10,069,479 B1
• Filed: 12/31/2014
• Issued: 09/04/2018
• Est. Priority Date: 12/31/2013
• Status: Active Grant

• Current Assignee: Kyocera AVX Components Corporation (Kyocera Corporation)
• Original Assignee: Ethertronics, Inc. (Kyocera Corporation)
• Inventors: Desclos, Laurent
• Primary Examiner(s): Nguyen, Hai L
• Application Number:
  US14/588,311
• Time in Patent Office: 1,343 Days
• CPC Class Codes: H03F 1/56 Modifications of input or output impedances, not otherwise provided for
• CPC Class Codes: H03F 2200/213 A variable capacitor being added in the input circuit, e.g. base, gate, of an amplifier stage
• CPC Class Codes: H03F 2200/294 the amplifier being a low noise amplifier [LNA]
• CPC Class Codes: H03F 2200/451 the amplifier being a radio frequency amplifier
• CPC Class Codes: H03F 2200/546 A tunable capacitance being present in an amplifier circuit
• CPC Class Codes: H03F 2203/45544 the IC comprising one or more capacitors, e.g. coupling capacitors
• CPC Class Codes: H03F 2203/45594 the IC comprising one or more resistors, which are not biasing resistor
• CPC Class Codes: H03F 3/19 with semiconductor devices only
• CPC Class Codes: H03F 3/195 in integrated circuits
• CPC Class Codes: H03F 3/21 with semiconductor devices only H03F3/245 takes precedence
• CPC Class Codes: H03F 3/245 with semiconductor devices only
• CPC Class Codes: H03F 3/45179 using MOSFET transistors as the active amplifying circuit H03F3/45278 takes precedence
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit
• CPC Class Codes: H03H 11/1217 using a plurality of operational amplifiers H03H11/1204 takes precedence; parallel-T filters H03H11/1295
• CPC Class Codes: H03H 11/1291 Current or voltage controlled filters
• CPC Class Codes: H03H 19/004 Switched capacitor networks
• CPC Class Codes: H03H 2210/012 Centre frequency; Cut-off frequency
• CPC Class Codes: H03H 2210/015 Quality factor or bandwidth
• CPC Class Codes: H03H 2210/025 Capacitor
• CPC Class Codes: H03H 2210/028 Resistor
• CPC Class Codes: H04B 1/04 : Circuits
• CPC Class Codes: 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
• CPC Class Codes: H04B 1/1638 : Special circuits to enhance selectivity of receivers not otherwise provided for resonant circuits H03H
• CPC Class Codes: H04B 2001/1054 : by changing bandwidth

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

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;

characterized by the tunable filter further comprising;

a variable capacitor connected in shunt between the non-inverting input of the first operational amplifier and ground;

wherein the variable capacitor is configured to receive control signals for changing a tuning state of the variable capacitor; and

wherein the tunable filter is tuned by changing the tuning state of the tunable capacitor.

2. The tunable filter of claim 1, further comprising:

a first resistor (R1) connected between the input of the tunable filter and the variable capacitor.

3. The tunable filter of claim 1, further comprising:

a second resistor (R2) connected between the output of the second operational amplifier and the variable capacitor.

4. The tunable filter of claim 1, further comprising:

a third resistor (R3) connected between the output of the first operational amplifier and an inverting input of the first operational amplifier.

5. The tunable filter of claim 1, further comprising:

a fourth resistor (R4) connected between the output of the tunable filter and the non-inverting input of the second operational amplifier.

6. The tunable filter of claim 5, further comprising a fifth resistor (R5) having a first end thereof connected between the non-inverting input of the second operational amplifier and the fourth resistor, and a second end thereof connected to ground.

7. The tunable filter of claim 1, wherein a center frequency of the tunable filter is adjusted by changing a tuning state of the variable capacitor.

8. The tunable filter of claim 1, wherein the variable capacitor is selected form the group consisting of:

a MEMS device, a switched capacitor assembly fabricated in complementary metal-oxide-semiconductor (CMOS), a switched capacitor assembly fabricated in silicon on insulator (SOI), a switched capacitor assembly fabricated in silicon on sapphire (SOS), a switched capacitor assembly fabricated in gallium arsenide (GaAs), a barium strontium titanate (BST) capacitor, and a varactor diode.

9. The tunable filter of claim 1, further comprising:

a first variable impedance network connected between the output of the second operational amplifier and the non-inverting input of the first operational amplifier, the first variable impedance network comprising a switch coupled to a plurality of resistors, wherein each of the plurality of resistors comprises a distinct impedance relative to another.

10. The tunable filter of claim 9, wherein the first variable impedance network is configured to adjust a center frequency of the tunable filter.

11. The tunable filter of claim 9, wherein the first variable impedance network and the variable capacitor are each tuned in conjunction to adjust the frequency response of the tunable filter.

12. The tunable filter of claim 9, further comprising:

a second variable impedance network connected between the input of the tunable filter and the variable capacitor, the second variable impedance network comprising a switch coupled to a plurality of resistors, wherein each of the resistors comprises a distinct impedance relative to another.

13. The tunable filter of claim 12, wherein the second variable impedance network is configured to adjust a bandwidth of the tunable filter.

14. The tunable filter of claim 12, wherein each of the variable capacitor, the first variable impedance network and the second variable impedance network is independently controlled for varying a tuning state thereof.

15. The tunable filter of claim 14, wherein the first and second variable impedance networks and the variable capacitor are each tuned in conjunction to adjust the bandwidth and frequency response of the tunable filter.