GB611564A - Improvements in or relating to radio and like receivers - Google Patents

Abstract

611,564. Frequency and phase modulation; automatic gain control. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 1, 1946, No. 13239. Convention date, May 2, 1945. [Class 40 (v)] In frequency or phase modulation reception, a stronger desired signal is received with -a minimum of interference from a weaker undesired signal on the same carrier, by applying the received carrier through selective and frequency-changing stages to the grid of a lockedin oscillator which feeds the discriminatordetector; and the strength of the carrier applied to the grid is maintained by A.V.C. methods at a constant critical value at which the inter - ference is a minimum. The incoming wave is received on the aerial 9 and passes through amplifier 5, heterodyne stage 6, 7 and I.F. amplifiers 8, 81 from which the I.F. is fed through tuned coupling 11 to the inner control grid of a heptode 40. The anode and the outer control grid of the valve 40 are backcoupled at 42 to generate oscillations at a fraction (say one-fifth) of the intermediate frequency, but locked in step therewith and following the I.F. deviations. The tuned anode circuit feeds a discriminator-detector 14 followed by audio amplifiers. The A.V.C. circuit branches between the amplifiers 8, 8<1> and feeds a diode 17 through an amplifier 41. The load resistor 19 is shunted by high resistors 53, 28 and positive delay bias is applied from a source E to the junction 54. The A.V.C. voltage at the junction is applied to the grids of the stages 5, 6, 8 and a smaller adjustable bias is applied from a slider 29 to the grid of the second I.F. stage 8<1>, the slider being adjusted until the voltage across the grid resistor 12<11> is flat. In a modified A.V.C. rectifier circuit, Fig. 2 (not shown), the delay, bias is applied to the diode cathode which is earthed through a condensershunted resistor. The cathode is also connected by a condenser to the tuned input circuit 20 which is earthed through the potentiometer 28. Full A.V.C. voltage is applied to the stages 5, 8 and adjustable partial voltage to the stages 6, 8<1>. Specification 562,993 is referred to.