Magnetless microstrip circulators based on differential spatiotemporal modulation of resonators

Abstract

This article presents a magnetless differential microstrip circulator for high frequency operation. The differential design is obtained by pairing two single-ended (SE) circulators, each based on three resonators connected in a wye topology with spatiotemporal modulation (STM). What is more, the STM biases of the two SE circulators are kept a fixed 180° phase difference. The resonance of the circuit is realized by varactor-loaded microstrip line sections rather than conventional series of inductor and varactor. The design method of the proposed microstrip circulator has been presented by analyzing its equivalent circuit, which indicates the microstrip line sections cannot be simply equivalent to inductors with the conventional method. The analysis of the time-varying circuit is verified by the simulated results of both the microstrip circulator and its equivalent circuit. A magnetless microstrip circulator was designed, fabricated and measured at center frequency of 1.99 GHz, showing maximum isolation (IX) of more than 50 dB, minimum insertion loss of 4.8 dB, and 1.4% 20 dB IX bandwidth.