Tunable filters, cancellers, and duplexers based on passive mixers

1. A tunable duplexer comprising:

one or more antennas;

an isolator electrically coupled to the one or more antennas for isolating a received signal and a transmitted signal on the one or more antennas;

a combiner comprising a first input electrically coupled to the one or more antennas; and

a feedforward path, between a coupler of the transmitted signal and a second input of the combiner, configured to cancel leakage across the isolator, wherein the feedforward path comprises a tunable delay cell comprising a plurality of passive mixers electrically coupled together for receiving an input signal and outputting a delayed signal, each passive mixer comprising a plurality of mixer switches.

2. The tunable duplexer of claim 1 , wherein the tunable delay cell comprises a control circuit for providing, to each passive mixer, a respective plurality of local oscillator (LO) signals, one to each mixer switch of each passive mixer, and wherein the control circuit is configured to vary the LO signals to cause a target frequency band of the input signal to be delayed by a target delay time in propagating through the passive mixers.

3. The tunable duplexer of claim 1 , wherein the feedforward path comprises a tunable attenuator.

4. The tunable duplexer of claim 1 , comprising a reflective-type tunable filter electrically coupled to an output of the combiner.

5. The tunable duplexer of claim 4 , comprising a second feedforward path, between the output of the combiner and one or more reflective loads of the reflective-type tunable filter, configured to cancel leakage over a directional coupler of the reflective-type tunable filter, wherein the second feedforward path is configured to cancel the leakage over the directional coupler by virtue of a down-converting mixer configured to downconvert a feedforward signal on the second feedforward path to baseband before the feedforward signal is coupled to one or more reflective loads of the reflective-type tunable filter.

6. The tunable duplexer of claim 5 , comprising a third feedforward path, between the feedforward path and the one or more reflective loads of the reflective-type tunable filter, configured to further cancel leakage over the isolator.

7. A tunable duplexer comprising:

one or more antennas;

an isolator electrically coupled to the one or more antennas for isolating a received signal and a transmitted signal on the one or more antennas;

a combiner comprising a first input electrically coupled to the one or more antennas;

a reflective-type tunable filter electrically coupled to an output of the combiner; and

a feedforward path, between the output of the combiner and one or more reflective loads of the reflective-type tunable filter, configured to cancel leakage across a directional coupler of the reflective-type tunable filter, wherein the feedforward path is configured to cancel the leakage by virtue of one or more passive mixers in the feedforward path.

8. A tunable duplexer comprising:

one or more antennas;

an isolator electrically coupled to the one or more antennas for isolating a received signal and a transmitted signal on the one or more antennas;

a reflective-type tunable filter electrically coupled to the one or more antennas; and

a feedforward path, between a coupler of the transmitted signal and one or more reflective loads of the reflective-type tunable filter, configured to cancel leakage across the isolator.