US Patent 7573839 Line driver for an adaptive hybrid circuit

A second output transmission signal (“TX2”) added to a line driver is a scaled version of the main output transmission signal (“TX1”). TX2 is scaled from TX1 by a variable scale factor K. An adaptive hybrid circuit subtracts TX1 and TX2 from a line signal carrying both a line transmission signal and a line received signal (“RX”). A programmable impedance Ztune is coupled between the TX2 output of the line driver and the RX output of the adaptive hybrid circuit. A transmission echo in the output RX signal is measured. K and Ztune are then adaptively tuned to minimize the transmission echo. The hybrid in this case becomes a 4-port network, one port specifically added to adaptively cancel the transmission echo in the RX output of the adaptive hybrid circuit. Alternatively, the hybrid may be a 3-port hybrid including variable impedances to cancel the line transmission signal.