Tunable Quasi-Circulator Based on a Compact Fully-Reconfigurable 180° Hybrid for Full-Duplex Transceivers

In this paper, a tunable quasi-circulator (QC) is proposed, which consists of a compact fully-reconfigurable 180° hybrid and two amplifier stages in the transmitter (TX)- and receiver (RX)-paths, respectively. Such hybrid is implemented by six varactor-tuned phase shifters, which can not only achieve the frequency tuning operation but also suppress the signal leaking from the TX to the RX in the proposed QC. To further enhance the TX-RX isolation, a variable-impedance balance resistor based on the PIN diode is introduced and tapped on the hybrid. Meanwhile, at the TX- and RX-paths of the QC, two amplifier stages with inter-stage matching networks are connected to the hybrid, respectively, which can achieve the nonreciprocity operation of the proposed QC with high performance. To verify the mechanism of the structures mentioned above, a tunable QC operating at 2.88-3.56 GHz is implemented and fabricated. The measurement has merits of the in-band TX-RX isolation greater than 34 dB, RX gain higher than 4.3 dB, and noise figure lower than 5.5 dB. Besides, the minimum gain of the TX path is 6.8 dB, and an output 1-dB compression point of 21.7 dBm with a power efficiency of 20% at 3.2 GHz is achieved, respectively.