An active quasi-circulator with controllable leakage canceler and passive TX path for 77-GHz automotive FMCW radar systems in SiGe technology

A modified structure of an active quasi-circulator (QC) is proposed for receive signal (RX) and transmit signal (TX) separation in 77-GHz monostatic FMCW radars. The circuit includes an impedance tuner, operating as leakage canceler, which performs compensation for process variations, parasitics of transistors as well as antenna mismatch. Additionally, the TX path is designed to be passive, to avoid any compression in the transmit signal. The QC is fabricated in a 200-GHz fT Silicon-Germanium (SiGe) HBT technology. On-wafer S-Parameter measurements show an isolation of 43 dB at 78 GHz. The insertion loss for the TX-path is around 7dB, whereas the gain in the RX-path is around 3 dB. Due to the tuner, the center frequency of the isolation can be varied from 67 GHz to 85 GHz.