Multi-path active hybrid circuit

Abstract

A hybrid circuit for a full-duplex transceiver includes a two-to-four wire ratio converter (, Z, Z) for forwarding a transmitted signal TX' on a transmission line () in response to an input signal TX. The ratio converter also produces a signal RXA in response to a received signal RX arriving on the transmission line, wherein signal RXA includes an echo of input signal TX. A passive filter (H) and a first active filter (H) each filter input signal TX to generate signals TXA and TXB summed with the RXA signal to form a signal RXB at the input of an amplifier (, Z, Z) producing an output signal RXC. A second active filter (H) filters input signal TX to generate a signal TXC summed with the RXC signal to produce the output signal RX. The impedance of the passive filter is designed so that when the transmission line has a target impedance, such as the impedance of an ideal twisted pair, the TXA output signal of the passive filter adequately offsets the TX signal echo in RXA. A control circuit monitors the residual echo in the output signal and, when the transmission line impedance varies from its target value so that the passive filter alone cannot provide adequate echo cancellation, the control circuit turns on the first and/or second active filters to provide additional echo cancellation. The frequency responses of the active filters are adjustable to accommodate a variety of transmission line impedances.