Frequency Selective Ferrite Circulators with Quasi-Elliptic Transmission Response

This paper reports on a new class of miniaturized RF circulators with bandpass filtering capabilities. They are based on capacitively-loaded ferrite disk resonators that occupy a 64% smaller area than conventional RF circulator architectures. Frequency selectivity is achieved by introducing four transmission zeros (TZs) below and above the circulator passband. The spectral locations of the TZs are controlled by i) the position of the capacitive load on the ferrite-based disk and by ii) incorporating resonant-type external coupling elements to its input, output and isolated ports. In this manner, non-reciprocal quasi-elliptic type power transmission responses can be obtained, which eliminate the need for additional RF filtering elements in the RF front-end. For proof-of-concept validation, two microstrip prototypes are designed, built and measured at 2.5 GHz, achieving 2.2 and 1.4 dB minimum insertion loss, and 33.4 and 30 dB maximum isolation in the pass-band and are presented in this manuscript for the first time.