A 5 GHz Frequency-Tunable SiGe Active Isolator Using a Varactor-Controlled Phase Network

This paper presents a novel integrated, frequency-tunable SiGe active isolator. Its high isolation and linearity make it well-suited for communications circuits and other systems that require frequency-agile port isolation. The topology integrates a varactor-based T-network into the feed-forward loop of a traveling-wave isolator, creating a voltage-controlled phase shifter. The fabricated circuit demonstrates a 20 dB isolation tuning range from 4.7 GHz to 5.2 GHz, with an 80 MHz instantaneous bandwidth. The isolator achieves an insertion loss of 0.9 dB, a simulated noise figure of 3.4 dB, and an input-referred P1dB (IP1dB) greater than 30 dBm.