A high isolation hybrid circuit composed of a pair of transformers, a voltage control resistance (VCR) circuit and an automatic impedance control device is designed for a two-wire full duplex cable modem to adapt variable line impedance. A binary frequency shift keying (BFSK) cable modem using the new hybrid circuit with an isolation of 52 dB to 58 dB in the line impedance variation range of 400 to 950 ohm is demonstrated. The isolation of the new hybrid circuit is increased by more than 30 dB over the traditional hybrid circuit for a two-wire full duplex modem in the preset line impedance range.