Elevator car modem with transhybrid rejection

摘要

A telephone transmit circuit is connected through a resistor to a junction at which a telephone line interface circuit is connected, a transhybrid cancellation circuit is connected to the transmit circuit and includes a high frequency shunt matching network having capacitive and inductive components and a low frequency matching circuit which comprises a lead lag network having capacitive components, the output of the transhybrid circuit being connected to a summing junction at the input of a telephone receiver. The other input of the summing network is connected between the resistor and the interface circuit, whereby a portion of the transmit signal is fed back out of phase to the receive amplifier so as to cancel out undesirable side tones, across a predetermined frequency range.