A signal detect/loss of signal (SD/LOS) circuit for a four-channel parallel TX/RX chip was designed and fabricated in a 0.35 mum SiGe HBT process. The measurement was performed in power supply range (3.0-3.6 V) and case temperatures (0-100degreesC) under the influence of cross-talk from the other RX channels and transmitter channels. The result shows a typical assert level of -20.5 dBm and a de-assert level of -21.5 dBm with a typical hysteresis of 1 dB.