Tunable Duplexer With Passive Feed-Forward Cancellation to Improve the RX-TX Isolation

The paper presents an approach that enables trading off the insertion loss versus isolation in the duplexer design. Specifically, the Insertion Loss (IL) of the duplexer filters is reduced by using low-order Bandpass Filters (BPF), while the transmitter (TX) to receiver (RX) isolation is improved using a feed-forward, passive, wideband, cancellation scheme. The proposed cancellation scheme is fully passive and hence the duplexer does not incur power consumption and noise penalties. The linearity of the proposed duplexer is very high limited only by the tunable passive components used in the design. A tunable duplexer prototype is demonstrated with TX-RX isolation better than 50 dB in both TX and RX bands (high band TX: 860-890 MHz, RX: 948-984 MHz; low band TX: 700-718 MHz, RX: 780-801 MHz). Tuning is achieved using digitally controlled switched capacitors with silicon on sapphire (SOS) switches resulting in high linearity.