Tunable filter for RF circuits
View Patent ↗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.
1. A communication system, comprising:
at least one tunable filter circuit capable of dynamic adjustment of a frequency, bandwidth, or transmission loss characteristics;
a processor;
a lookup table (LUT) resident in the processor or other memory cell;
wherein the at least one tunable filter circuit comprises:
a first operational amplifier comprising a non-inverting input and an output thereof;
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 at least one tunable filter circuit;
the output of the second operational amplifier connected to the non-inverting input of the first operational amplifier at an input of the at least one tunable filter circuit;
wherein the at least one tunable filter circuit further comprises:
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 at least one tunable filter circuit is tuned by changing the tuning state of the variable capacitor.
2. The communication system of claim 1 , further comprising an antenna.
3. The communication system of claim 2 , wherein the antenna comprises an isolated magnetic dipole antenna.
4. The communication system of claim 2 , wherein the processor is configured to adjust a bandwidth of the at least one tunable filter circuit such that the at least one tunable filter circuit and the antenna provide a frequency response characteristic for the communication system.
5. The communication system of claim 1 , wherein the at least one tunable filter circuit comprises a first tunable filter circuit in a transmit path and a second tunable filter circuit in a receive path.
6. The communication system of claim 1 , wherein a first tunable filter circuit is coupled to an output of a power amplifier.
7. The communication system of claim 1 , wherein a second tunable filter circuit is coupled to an input of a low noise amplifier.
8. The communication system of claim 1 , wherein a center frequency of the at least one tunable filter circuit is adjusted by changing the tuning state of the variable capacitor.
9. The communication system of claim 1 , wherein the at least one tunable filter circuit further comprises: 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 communication of claim 9 , wherein the first variable impedance network is configured to adjust a center frequency of the at least one tunable filter circuit.