A 1.2–1.25-GHz Multifunctional Filtering Circulator With Flexible Frequency, Phase, and SIC Control and Its Application in Full-Duplex Beamforming

This article presents a reconfigurable multifunctional filtering circulator based on spatiotemporal modulation (STM), enabling tunable center frequency, insertion phase, and self-interference cancellation (SIC) isolation. The device integrates filtering, circulation, phase control, and enhanced transmitter (Tx)–receiver (Rx) SIC in a single structure. A 170° phase tuning range is achieved via phase control networks (PCNs) at the Tx and Rx ports, maintaining constant amplitude for Tx–antenna (Ant) and Rx–Ant channels. Secondary Tx–Ant–Rx leakage is exploited to cancel direct Tx–Rx leakage using an adjustable reflection coefficient network at the Ant port, achieving up to 60-dB isolation. With the proposed structure, phase tuning and SIC can operate simultaneously. A comprehensive theoretical analysis of the proposed PCN and SIC network is provided. For proof of concept, a $1\times 4$ filtering full-duplex beamforming network using the proposed multifunctional filtering circulator is demonstrated, capable of independent Tx and Rx beam scanning with improved Tx–Rx isolation. Prototypes of both the reconfigurable multifunctional filtering circulator and the $1\times 4$ full-duplex beamforming network were fabricated and tested, validating the proposed concepts and demonstrating the expected performance.