GB420387A - Improvements in alternating current signal receivers - Google Patents

Abstract

420,387. Automatic exchange systems. RYALL, L. E., 52, Crescent Gardens, Eastcote, Ruislip, Middlesex. July 5, 1933, No. 19105. [Class 40 (iv)] [See also Group XL] An A.C. signal receiver, e.g. for responding to numerical impulses in a telephone system, comprises a selective network over which signals of a particular frequency are passed to a rectifier which is biassed by rectified signals derived from the input side of the network through an untuned circuit and is thereby rendered ineffective when signals of other frequencies are received. The rectifier may be of the dry plate type or a thermionic diode or triode. In the latter case the bias derived from the unwanted signal may be made to make the grid more negative so as to suppress the anode current or more positive so as to make the valve act as an amplifier instead of a rectifier. The circuit shown is for dealing with 500 cycle signals modulated or interrupted at 20 cycles. In the absence of other frequencies a certain proportion of the signal is rectified at 10 and produces a biassing voltage across the condenser 11 which is not sufficient to prevent the actuation of relay 9 by signals passing through the selective network 4 to the rectifier 7. When however other frequencies are received, the biassing voltage is larger that that of the fraction of the A.C. passing the selective network and the relay 9 is unaffected. The invention is applicable in systems in which the wanted signal is not interrupted or modulated but in this case the bias voltage on 11 must not change the anode current in the same direction as a normal signal.