GB613026A - Improvements in or relating to automatic gain control for radio, television or like receivers - Google Patents

Abstract

613,026. Automatic gain control. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 29, 1945, No. 22274. Convention date, Aug. 29, 1944. [Class 40 (v)] In an automatic gain control system, the control voltage is derived from a point of positive potential via means for subtracting a constant voltage and a limiter is used to reduce the effect of noise on the gain control. As applied to the reception of television signals in which the maximum carrier amplitude occurs during the synchronizing period, I.F. signals from the stage 14 are rectified in the full-wave circuit 17 which includes the limiting diode 33. The limiter 33 is connected between the signal rectifiers 18, 19 and the load impedance 34, 38, the limiting level being determined by the negative bias applied to the system through the resistance 23. In order to render the diodes 18, 19, 33 conductive in the absence of signals, a further negative bias is applied to the cathodes 35, 36, over the resistance 46. Series and shunt inductances 39, 38, 41 may be employed to correct frequency-distortion of the video signal, which is applied to the amplifying stage 44 and over the lead 54 to the A.G.C. system. The anode of the valve 51, which is biassed to cut-off in the absence of signals, is connected to a source of positive potential over the resistance 61. A potentiometer comprising the gas-discharge tube 66 and the resistance 69 is connected between a source of negative potential and the point 76, the control potential being taken off through the filter 71, 72, from the point 74. The potential of the line 68 in absence of signals is adjusted by means of the potentiometer 62. In the presence of signals, conduction in the valve 51 during peak synchronizing periods causes the potential of the point 76 and thus of the line 68 to fall.