Full duplex transhybrid circuit for voice communications through a single crosspoint

With the deregulation of the telephone industry, it has become essential for system designers to develop competitive solutions with better performance. The circuit described here offers a better solution for analog private branch exchanges (PBX) than the typical discrete transformer solution. The Full Duplex Transhybrid (FDT) circuit combines a Subscriber Line Interface Circuit (SLIC) with two operational amplifiers. This combination results in superior circuit performance with full duplex transhybrid voice communication through a single crosspoint. The two operational amplifiers convert the two port network of the SLIC (transmit output TX and receive input RX) into a one port network for connection through a single crosspoint. They also serve to cancel channel to channel and inter-channel echo and are the main focus of this paper. A brief comparison between the discrete transformer solution and the FDT circuit is presented along with the design equations for calculating the proper feedback