GB589968A - Gain control circuit for radio receivers - Google Patents

Abstract

589,968. Wireless receiving apparatus. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Dec. 29, 1944, No. 26119. Convention date, Dec. 30, 1943. [Class 40 (v)] In a wireless receiver, the output of the A.V.C. rectifier is connected to ground through a discharge device which is biassed to conduct only on receipt of a signal above a predetermined value. The invention is described as applied to a receiver for interrupted carrier wave telegraph signals comprising a mixer 5 fed by local oscillator 25, I.F. amplifier 8 and second detector 11. The load for the second detector comprises a resistance 17 and triode 16 in series with secondary winding 10, the output being taken from the cathode of valve 16, or from a point on resistance 17 according to the setting of switch 18. A.V.C. voltage is ta'ken from the cathode of 16 by way of short time constant circuit 23, 24 to the grids of valves 5 and 8. A triode 22 is connected between the cathode of 16 and earth, and is biassed from potentiometer 26 so as not to conduct for normal signal amplitudes. Triode 16 is less biassed, and so conducts for all but very small signals. Thus, normal signal impulses cause valve 16 to conduct and the A.V.C. line is energized to set the gain to a value which is maintained during the succeeding space period. An excessive signal such as static causes valve 22 to conduct. This not only shunts away the excessive signal thus preventing its appearance in the output, but discharges the A.V.C. circuit, thus allowing the gain to rise. Otherwise, a loud signal would set the gain to a low value which might be inadequate to reproduce a weak subsequent marking signal. The arrangement is stated to be applicable to ordinary broadcast receivers.