On the anti-sidetone telephone circuit |
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Authors: | V Belevitch |
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Affiliation: | 1. N. V. Philips and M. B. L. E. Associated S. A., Avenue Van Becelaere 2, Box 8, B-1170, Brussels, Belgium
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Abstract: | The anti-sidetone telephone circuit is a frequency-independent passive reciprocal 3-port whose scattering matrix between three prescribed resistive terminations (transmitter, receiver, line) meets a certain specification. In their 1920 paper, Campbell and Foster (CF) showed that the prescribed 3-port can be realized as a lossless 4-port consisting in two 3-winding transformers closed on one resistance (the balancing network). If one (two) relations are imposed between the terminating resistances of the 4-port, realizations involving a single 3-winding (2-winding) transformer are available, and the resulting 136 (38) distinct realizations have been listed by CF. In this paper, we reestablish the CF-results (which were stated almost without proof in their paper and have never been rediscussed in the literature) and present a genetic classification of their 136 + 38 circuits. Recently, a realization of the anti-sidetone 3-port containing more than one internal resistance (in addition to the balancing network, a light-emitting diode serves to illuminate the dial) has been described by D. R. Means; in this paper, we present a complete characterization of that wider class of circuits. |
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