A Tunable FDD Duplexer using Electrical Balance with N-Path Filters

This work presents a self-interference cancellation front-end for FDD systems. The front end utilizes an electric-balance hybrid coupler connected to N-path filters having equal and opposite reflection coefficients. This arrangement allows for wideband transmitter (Tx) to receiver (Rx) isolation with a very close offset without incurring loss present in conventional electrical balance front-ends. In addition, Tx-Rx isolation in the proposed front end is insensitive to antenna impedance. We have simulated the N-path filters using transistors from a 65 nm CMOS process. Simulation of the entire front-end shows a tunable center frequency from 800 MHz to 1.5 GHz with wideband Tx-Rx isolation exceeding >40 dB, even with a 50 MHz Tx-Rx frequency offset. The insertion loss in the Tx and Rx paths is <1.4 dB and <3 dB respectively.