An Active Quasi-Circulator for 77 GHz Automotive FMCW Radar Systems in SiGe Technology

One of the key functions of transmit and receive modules with a single antenna is the separation of the weak receive signal (RX) from the strong transmit signal (TX). In state-of-the-art FMCW radar MMICs, this function is usually implemented using passive devices, which have relatively large physical dimensions compared to the chip area and exhibit insertion losses. In this letter the design of an active quasi-circulator (QC) is presented with a special emphasis on TX/RX isolation. An analysis is presented to aid in the circuit design, so that the desired isolation requirements can be achieved. The implemented active QC is fabricated in a 200 GHz fT Silicon-Germanium (SiGe) HBT technology. On-wafer measurements show a TX/RX isolation of 16 dB over a bandwidth of 8 GHz. The measured insertion loss for the TX and the RX signal is less than 1 and 1.5 dB, respectively.