MMIC GaAs Isolators and Quasi-Circulators With Co-Designed RF Filtering Functionality

This paper reports on the RF design and practical development of GaAs MMIC multi-functional RF isolators and quasi-circulators with embedded filtering functionality. They are based on cascaded arrays of non-reciprocal resonators (NRRs), passive resonators, and impedance inverters. A unique transistor-based architecture is used for the realization of the NRR that exhibits an one-pole type bandpass response in the forward direction and RF signal cancellation in the reversed one. By controlling the number and type of resonators within cascaded resonator arrays, different levels of gain, out-of-band selectivity and in-band isolation can be obtained. The operating principles of the multi-functional isolator/filtering and quasi-circulator/filtering concepts are presented through synthesized and linear circuit-simulated examples. Practical design aspects using a commercially available GaAs MMIC process are also discussed. For proof-of-concept demonstration purposes, four prototypes were designed and experimentally validated at X-band. They include: i) a NRR, ii) a two-resonator based frequency selective isolator (FSI), iii) a five-resonator based FSI, and iv) a two-pole/two-TZ frequency selective quasi-circulator (FSQC).