Analysis of an Adaptive Wideband Duplexer With Double-Loop Cancellation

An active wideband duplexer reduces the need for multiple switched duplexing filters in multiband software radio implementations. The technique is based on a two-step isolation process: 1) a low isolation device and 2) an active double-loop canceling technique that leads to two nulls, one at the desired receive frequency and the other at the transmit frequency. Two cancellation paths generate replicas of the residual transmitter interference signal and noise signal for subtraction from the output of the low isolation device. The feasibility of wideband cancellation mainly depends on the duplexing offset and the operating range of delays in the main path and the cancellation paths. The adjustment range of the coefficients in the cancellation path puts further constraints on these delays. This paper presents a theoretical analysis of double-loop cancellation using geometrical representations. The delay boundaries are derived for a given adjustment range of the vector attenuators (coefficients) used in the cancellation paths. The relationship between cancellation bandwidth and residual cancellation level is shown to be linear (6 dB/octave) and related to the loop delays and the duplexing offset.