Adaptive hybrid circuit

An adaptive hybrid circuit for interfacing a four-wire communication channel with a two-wire communication channel and comprising:

• a first electronic amplifier having a first input connected through a first resistor to a receive port of a four-wire communication channel, the receive port being defined by a first pair of terminals, said first resistor having a resistance such that the four-wire receive channel is terminated in its characteristic impedance;
• fixed impedance means connected between an output of said first amplifier and a port of a two-wire channel for termination thereof in its nominal characteristic impedance, the two-wire channel port being defined by a second pair of terminals;
• a second electronic amplifier having a first polarity input terminal to which is applied a first signal which represents a sample of the signal received from the four-wire receive channel, said second amplifier having a second polarity input terminal to which is applied a fixed reference potential, a selected one of the first and second polarity input terminals connected to said fixed impedance means for applying to the selected polarity input terminal a second signal sensed at said fixed impedance means when said four-wire receive channel is activated and for applying only to the selected polarity input terminal a signal supplied to the two-wire channel port from the two-wire channel when said two-wire channel is activated, the selection of the polarity input terminal of said second amplifier being determined by the phase relationships of the first and second signals at inputs to the amplifier terminals in order to obtain cancellation therebetween at said second amplifier, said second amplifier having an output connected through a second resistor to a transmit port of the four-wire channel, the transmit port being defined by a third pair of terminals, said second resistor having a resistance such that the four-wire transmit channel is terminated in its characteristic impedance, said second amplifier providing a nulling of the first and second signals applied to the input terminals thereof when said four-wire receive channel is activated so as to isolate the four-wire transmit port from the four-wire receive port independent of variation in the impedance characteristics of the two-wire channel due to signal loss in said fixed impedance means swamping out such impedance variations;
• impedance synthesizer network means having a first input connected to the output of said first amplifier and a first output connected to a selected one of said first and second polarity input terminals of said second amplifier for providing a high degree of precision of the nulling at the output of said second amplifier so as to obtain improved isolation between the four-wire receive and transmit ports in the order of 12 dB or more over that obtained with the conventional transformer hybrid circuit and without the need for an externally adjustable compromise termination network, the first output of said impedance synthesizer network means producing the first signal applied to said second amplifier;
• and
• correlation control means having an input signal first input connected to a second output of said impedance synthesizer network means, an error signal second input connected to the output of said second amplifier, a third input connected to a third output of said impedance synthesizer network means, and an output connected to a second input of said impedance synthesizer network means for adaptively controlling the operation thereof so that the high degree of precision of the null at the output of said second amplifier is maintained throughout the operation of the adaptive hybrid circuit.