Radio frequency duplex filter for removing transmit signals from a receive path
View Patent ↗The present invention is an RF duplex filter that is used to remove transmit signals from the receive path of a full duplex transceiver. The RF duplex filter includes a notch filter for blocking signals at a transmit frequency and a bandpass filter for enhancing signals at a receive frequency. The notch filter is formed with series resonant elements and the bandpass filter is formed with parallel resonant elements. One embodiment of the present invention may include tunable resonant elements for tuning the notch filter to a transmit frequency, tuning the bandpass filter to a receive frequency, or both. Calibration circuitry may be included in the full duplex receiver for adjusting the tunable resonant elements. The present invention includes a method for calibrating the tunable resonant elements.
1. A radio frequency (RF) duplex filter comprising:
an integrated circuit die including a series impedance circuit and a series resonant circuit, wherein:
the series impedance circuit: includes at least one series impedance element coupled to the series resonant circuit to form at least one RF output that provides at least one RF output signal, and adapted to receive an RF input signal; and
the series resonant circuit includes
a parallel resonant circuit having a parallel resonant frequency that is approximately equal to a receive frequency, wherein the at least one series parallel resonant circuit includes a switched capacitor bank in parallel with a first parallel resonant capacitive element, a parallel resonant inductive element, and a negative transconductance circuit; and
at least one series resonant element coupled in series with the parallel resonant circuit; and
wherein the series resonant circuit has a series resonant frequency that is approximately equal to a transmit frequency, and wherein the at least one RF output signal is based on the at least one RF input signal and filtering characteristics provided by the series impedance circuit and the series resonant circuit.
2. The RF duplex filter of claim 1 wherein the at least one series resonant element comprises at least one capacitive element.
3. The RF duplex filter of claim 1 wherein the at least one series resonant element comprises at least one tunable reactive element.
4. The RF duplex filter of claim 1 wherein the at least one series impedance element comprises at least one output impedance of at least one active circuit.
5. The RF duplex filter of claim 1 wherein the parallel resonant circuit comprises a negative transconductance circuit.
6. The RF duplex filter of claim 1 wherein the series resonant circuit comprises at least one calibration switching element coupled in series with the parallel resonant circuit and the at least one series resonant element, wherein the at least one calibration switching element is configured to isolate the parallel resonant circuit from the at least one series resonant element during calibration of the parallel resonant circuit.
7. The RF duplex filter of claim 1 wherein the parallel resonant circuit further comprises a calibration oscillator circuit.
8. The RF duplex filter of claim 7 wherein:
the RF duplex filter further comprises calibration circuitry;
the parallel resonant circuit further comprises at least one tunable reactive element; and
the at least one tunable reactive element is tuned such that the parallel resonant frequency is approximately equal to the receive frequency.
9. The RF duplex filter of claim 8 wherein the calibration circuitry comprises an RF mixer that provides at least one down converted output signal, and the at least one tunable reactive element is tuned based on the down converted output signal.
10. The RF duplex filter of claim 9 wherein the RF mixer is adapted to mix at least one receiver local oscillator signal with at least one parallel resonant circuit signal to provide the at least one down converted output signal.
11. The RF duplex filter of claim 10 wherein the calibration circuitry further comprises:
an RF mixer counter adapted to receive the at least one down converted output signal and provide an RF mixer counter output signal; and
a parallel resonant circuit counter adapted to receive the at least one parallel resonant circuit signal and provide a parallel resonant circuit counter output signal, wherein the at least one tunable reactive element is tuned based on the RF mixer counter output signal and the parallel resonant circuit counter output signal.
12. The RF duplex filter of claim 1 wherein the receive frequency is approximately equal to a frequency of a receiver local oscillator signal.
13. The RF duplex filter of claim 1 wherein the RF duplex filter is used in a low noise amplifier (LNA).
14. The RF duplex filter of claim 1 wherein the RF duplex filter is used in an RF mixer.
15. The RF duplex filter of claim 1 wherein the RF duplex filter is used in a full duplex receiver comprising an RF receiver and an RF transmitter such that the RF receiver is capable of receiving RF signals and the RF transmitter is capable of transmitting RF signals simultaneously.
16. A method for tuning an RF duplex filter comprising:
providing an integrated circuit die including:
a series impedance circuit coupled to a series resonant circuit, wherein the series resonant circuit includes a parallel resonant circuit having at least one tunable element coupled in parallel with a negative transconductance circuit, wherein the parallel resonant circuit is coupled in series with at least one series resonant element; and
calibration circuitry coupled in parallel with the parallel resonant circuit, wherein the calibration circuitry includes a calibration oscillator coupled in parallel with a parallel resonant circuit, wherein the calibration circuit is adapted to measure a receive frequency and a resonant frequency of the parallel resonant circuit, and tune the at least one tunable element;
enabling the calibration oscillator and disabling the negative transconductance circuit to configure the RF duplex filter for calibration;
comparing, with the calibration circuitry, the receive frequency with the resonant frequency; and
tuning, with the calibration circuitry, the at least one tunable element such that the resonant frequency is approximately equal to the receive frequency.