GB503497A - Improvements in or relating to electrical signalling systems - Google Patents

Abstract

503,497. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd. Oct. 5, 1937, No. 26971. Convention date, Nov. 14, 1936. [Class 40 (iv)] [Also in Group XL] A receiving circuit for supervisory &c. signals in the form of 60-cycle modulated 1,000-cycle currents and selecting impulses in the form of unmodulated 1000-cycle currents comprises a circuit 1222, 1223 tuned to 1000 cycles and a circuit 1218 .. 1221, comprising series and shunt resonant circuits tuned to 1100 cycles, which peaks at 900 and 1100 cycles. The voltage developed across the coil 1223 is rectified by the right hand one of a pair of diodes 1212 and applied across a resistance 1229 in the grid circuit of an amplifier 1213 against the normal bias derived from the series heater circuit. Voltages developed across the coil 1221, when they are of sufficient amplitude to overcome a normal negative voltage on the anode, are rectified by the left-hand diode and applied to resistance 1230 in the grid circuit of 1213 in the same direction as the normal bias. By this means the response of the signal relays to wrong frequencies is prevented and the input to 1213 is subjected to a limiting action since very strong signals of 1000 cycles produce a big enough drop across 1221 to produce a biasincreasing voltage at 1230. The circuit forms part of a tone unit identical with that described in Specification 500,737.