GB552658A - Improvements in frequency modulation receivers - Google Patents

Abstract

552,658. Frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. March 26, 1942, No. 4019. Convention date, March 26, 1941. [Class 40 (v)] Receivers for frequency-modulated waves include two heterodyne stages by which the mean incoming frequency is reduced to a frequency greater than but comparable with the frequency deviation of the incoming wave, the two stages being fed from the same local oscillator. The detector may be of the counter type. The twostage production in frequency is effected by mixer stages 5, 51 fed with oscillations from the local source 12. The input circuit 2, local oscillator circuit 13, and inter-stage circuit 21 are tuned over a given range by ganged control 22; while the final output circuit 21 is fixed tuned to a frequency say 0.2 mc. of the same order as the frequency deviation of the incoming wave, The output wave is fed through an amplifier 30 to a linuter stage 31 which provides AVC voltage for the mixer valves. The limited output is passed to a screen-grid amplifier 61 with its plate resistance 62 connected through a condenser 63 to an earthed resistance 64. A diode 65 is shunted across the resistance 64, and a second diode 66 has its anode connected through a resistance 67 to the anode of diode 65, and through a resistance 71 to the audio-frequency amplifier. With this arrangement, the average current through the resistance 64 is proportional to the input frequency; and the voltage passed to the AF amplifier varies with the frequency deviation. Interference from " image " frequencies may be prevented by interposing rejector circuits 11 between the aerial 1 and the input transformer.