A Frequency Tunable Circulator With 70-dB Isolation Using Reflection Coefficient Controller for Arbitrary Load Impedance

This article presents a method to increase the isolation and control the operating frequency of a circulator. The internal signal flow of a circulator is analyzed, and the through-port reflection to cancel out the leakage signal is investigated. A reflection coefficient controller (RCC) is designed to ensure that such reflection occurs at the operating frequency. In addition, the operating frequency can be tuned by using two varactor diodes of the RCC for an arbitrary value of the antenna impedance. The experimental results from the proposed method show isolation of 70 dB and an insertion loss of −1.4 dB at the operating frequency. Moreover, the operating frequency can be changed from 2.2 to 2.8 GHz with high isolation for an arbitrary antenna impedance.