GB559780A - Improvements in or relating to signal-receiving systems - Google Patents

Abstract

559,780. Wireless receiving circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. June 26, 1942, No. 8769. Convention date, June 26, 1941. [Class 40 (v)] The output stage of a receiver is arranged to be muted in the absence of a received carrier of adequate strength by means of rectified noise obtained from the output of the signal detector. On the reception of an adequate signal, the gain of the early stages is reduced so that insufficient noise is transmitted to effect muting of the output. The Figure shows the later stages of a superheterodyne receiver including a frequency modulation channel 5 and an amplitude modulation channel 6. The frequency modulation channel comprises an I.F. amplifier 7, limiter 9 and discriminator circuit including rectifiers 10 and output resistances 11, 12. The amplitude modulation channel comprises an I.F. amplifier 8, A.V.C. circuit 20 and signal detector 21 with output resistance 22. Signals are selected from one circuit or the other by a switch 25 and fed through volume control potentiometer 30 to the first audio frequency amplifier 26 A second switch 43 coupled to switch 25 allows the detected output from the two channels to be selectively applied to a control rectifier 40 through a filter 42, 41, 34 which is responsive to noise frequencies above the useful range of the receiver. The negative output voltage from resistor 33 is applied through a smoothing circuit 32, 31 as bias to amplifier 26 and is arranged to bias this valve to cut-off in the absence of a desired carrier signal thus reducing the transmission of noise. On the occurrence of a desired carrier signal, the sensitivity of channel 5 or 6 is reduced by the respective limiter 9 or A.V.C. circuit 20. This reduces the noise voltage available at 33 and so restores amplifier 26 to a condition for transmitting a signal.