An Improved Circuit for the Telephone Set

Abstract

A telephone set known as the 500 type has been in production for Bell System use for some time1. The successful outcome of an intensive study has made it possible to simplify the circuit and some of the components of this set, and thereby to increase dependability and life and significantly to reduce the manufacturing cost. This change now has been completed and telephone sets embodying the necessary modifications are in production. This paper discusses some of the problems involved in this work and outlines the improvements which have been effected. Presented also is information concerning the superior performance of the 500 set over the preceding telephone set when used in noisy locations.

New Data Bus Interface Technique

“…These windings are typically one-turn windings because higher turns will result in excessive insertion loss. By choosing proper transformer turn ratios and isolation resistors, a directional coupling characteristic can be obtained similar to what is available from a hybrid transformer in a telephone set [3,4]. When the transformer is used as a transmitter interface, the RU's output signal is sent toward the MU and no signal is sent toward the far end of the cable termination, making termination irrelevant.…”

“…If R1 is too small, it will have an (3) increased insertion loss and mismatching. The input impedance is given by (4) .in = Z2(R1 + R2) (RI + Z1)2+ R1R2Z12 (RI + Z2) (RI + Z1)2 + R12R2 (12) where A = det(A).…”

Pushbutton Calling with a Two-Group Voice-Frequency Code

“…PUSHBUTTON CALLING WITH VOICE-FREQUENCY CODE 249 appears across the [8][9] winding, one end of which is grounded through RVl • Neglecting the small ac drop from base to emitter, more than e. appears at the emitter, the assumed starting point. Thus, the loop gain exceeds unity and the amplitude of oscillation must grow until the loop gain stabilizes as a result of some nonlinearity.…”