An Improved Adaptive Electronic Circulator for Telephone Applications

A previous attempt at realizing an adaptive circulator for telephone applications suffered from limitations due to voice switching problems, resistive part matching only, and some difficulties with adaptive operation for broadband signals. The use of pilot tones enables the circuit to be free from voice switching problems and to accomplish a close impedance match with that of a telephone line. Analysis of the circuit in an adaptive hybrid configuration produces the theoretical bounds on performance. Experimental results display the performance characteristics of the adaptive circuits and various other hybrid circuits on a selection of subscriber lines with a range of terminations.