Analysis and Design of Quasi-Circulating Quadrature Hybrid for Full-Duplex Wireless

This article presents a new electronic device - the four-port quasi-circulating quadrature hybrid (QCQH), which combines desirable features of electronic circulators and quadrature hybrids. The QCQH comprises three quarter-wavelength transmission lines and a 90° non-reciprocal phase shifter (NRPS) and is suitable for inband full duplex applications. We derive the four-port S-parameter matrix of the QCQH and the transfer functions under termination scenarios of interest. The resulting closed-form expressions are compared with the prior art electronic circulator. A 90° transmission line and lumped transformer alternatives are considered for designing a QCQH based on a two-port N-path circuit. TX isolation and transmission expressions are derived and verified against simulations, and a new wideband leakage cancellation approach is proposed. A TSMC 65 nm CMOS N-path chip is integrated on-board with a discrete, lumped, LCL transformer implementation. TX-to-antenna insertion loss of 1.4 dB and an RX NF of 4.7 dB at the frequency of 1 GHz were measured with a primary passive TX-RX isolation of 21 dB. Total isolation of more than 50 dB for an 80 MHz OFDM WiFi TX signal employing digitally equalized active leakage cancellation was achieved along with better than −40 dB TX EVM with SIC ON.