Magnet-Less Circulator Using AlN MEMS Filters and CMOS RF Switches

A circulator with three ports allows unidirectional (or non-reciprocal) power transmission and facilitates the development of in-band full duplexers for simultaneous transmit and receive radio frequency (RF) applications. The use of ferromagnetic materials is the dominant approach for the implementation of circulators despite they are bulky and non-integrable with semiconductor electronics. Here, we present a novel magnetic-free electrical circulator using CMOS-integrable filters based on AlN MEMS resonators and CMOS RF switches. This paper describes a compact approach for a circulator that could be used in handheld devices. To break reciprocity, the AlN MEMS filter network is parametrically modulated via a switching matrix. The modulation frequency and duty cycle of the switching matrix were optimized experimentally so that the circulator can provide up to 15dB of isolation over the entire filter bandwidth. This prototype is a very compact solution for the implementation of magnetic-free circulators, which also intrinsically offer sharp pass-band filtering, thus eliminating the need for additional filters. The discussion highlights how possible paths to reduce the circulator insertion loss would make this approach more appealing for a broad range of applications.