A Millimeter-Wave Front-End for FD/FDD Transceivers Featuring an Embedded PA and an N-Path Filter Based Circulator Receiver

This work presents an ultra-compact single-antenna FD/FDD transceivers front-end. It comprises a nonreciprocal circulator, RX, and an integrated power amplifier (PA). In the proposed circulator, we devise a ring quarter-wave transmission line topology with adjusted characteristic impedances to improve TX-to-antenna insertion loss and TX-to-RX isolation. Besides, an AND-gate switching-based N-path filter is proposed to realize the circulator's nonreciprocal gyrator while acting as a mixer-first RX. Owing to the ultra-compact N-path filter structure, the circulator occupies only 0.38mm2 core area. Over a 27.1-to-31.1GHz band, the realized front-end offers >20dB TX-to-RX isolation while its measured TX-to-antenna insertion loss is 1.7~2.2dB. The RX path tolerates the PA's blocker signal, achieving 5dBm in-band and 13dBm out-of-band B1dB.Moreover, the PA delivers 15.15dBm peak output power with 33% drain efficiency. Our front-end prototype occupies only 0.7mm2, including circulator, PA, quadrature hybrid coupler LO generators, and baseband circuits.