Micro-Acoustic Quad Circulator with Self-Interference Cancellation

In this paper, a micro-acoustic RF circulator with novel “quad” configuration is reported. Four micro-acoustic filters are periodically modulated by RF switches to break the reciprocity. The use of high-Q micro-acoustic filters significantly reduces the modulation frequency (14 MHz) compared to previous demonstrations based on transmission lines (e.g., 333 MHz in [1]). The low modulation frequency translates to an ultra-low power consumption (202 uW, 3600 times smaller than [1]) and one of the highest linearity ($\mathrm{P}1\mathrm{dB}=31\ \mathrm{dBm}$) reported for magnet-free circulators. Furthermore, compared to the more conventional differential configuration, this quad configuration shows advantages in terms of intermodulation products (IMPs) suppression for all the in-band IMPs, guaranteeing a pseudo-linear-time-invariant (pseudo-LTI) operation. Furthermore, the isolation of the circulator is further improved by an additional cancellation circuit, achieving more than 50 dB of isolation in a 10MHz bandwidth (BW). This high level of isolation enables the use of this magnet-free circulator in full-duplex radio, which requires large level of self-interference suppression between the transmitter and the receiver nodes.