GB515940A - Frequency modulation system - Google Patents

Abstract

515,940. Frequency modulation. STANDARD TELEPHONES & CABLES. Ltd. June 10, 1938, No. 17323. Convention date, July 9, 1937. [Class 40 (v)] [Also in Groups XXXV and XXXVI] Relates to frequency-modulating systems of the kind in which the frequency of an oscillator is controlled by a thermionic reactance, and comprises means for increasing the degree of modulation, and its linearity, and for preventing undesired amplitude-variations. As shown the oscillator 1 has a parallel frequency-determining circuit comprising capacity 5 and inductances 2<1>, 31 joined by a resistor 17 arranged in the anode branch. The grids of the " reactance " valves 26, 36 are connected through condensers 25, 35 respectively to the resistance 17, and also in push-pull to the modulation transformer secondaries 46, 47, while their anodes are connected to the outer ends of inductances 2<1>, 31. Owing to the parallel connection across the resistance 17, the reactance of the valve 26 appears positive to the main oscillator circuit, whilst that of the valve 36 appears negative, so that frequency-variations cancel out, except those impressed by the modulating coils 46, 47, which are additive. Capacities 22, 32 are included in the cathode circuits of valves 26, 36 to reduce amplitude modulation, and are shunted by chokes to provide a D.C. connection. The mean frequency of the oscillator 1 is stabilized by error-detector circuits 44, 45 tuned above and below the desired mean frequency, and feeding detectors 42, 43, the D.C. voltage output of which is applied to the grids of the valves 26, 36 respectively, for frequency correction.