GB528152A - Transmission of frequency modulated valves - Google Patents

Abstract

528,152. Frequency modulation. STANDARD TELEPHONES & CABLES, Ltd. April 28, 1939, No. 12788. Convention date, May 21, 1938. [Class 40 (v)'] Frequency-modulated signals are received by combining them with locally-generated waves in phase-quadrature, and thereafter utilizing any component resulting from lack of frequency and phase-quadrature agreement to maintain the phase-quadrature relation, and at the same time to produce a demodulated current. Incoming signals are fed from a transformer 5 to balanced detectors 7, 8. The parallel input winding 9 is coupled to the output circuit of a variablefrequency oscillator 12, the back-coupled grid circuit of which includes a winding 14, with a magnetic core, and a control winding 16. The latter is in series with a coil 20 which is coupled to a network 19, tuned to the carrier-frequency and shunted across the signal input. Normally the oscillator 12 supplies a carrier-frequency oscillation to the coil 9 in phase-quadrature with the signals. This leaves the series coils 16, 20 unaffected. The arrival of a frequency-modulated signal, however, increases the spac:e-current say through detector 7, at the expense of detector 8, and so produces a control voltage across the resistances 22, 23. This alters the biasing current through the coil 16 and causes the tuning of the circuit 13 to keep in step with the changing frequency of the incoming signals, thus automatically maintaining the locally-generated oscillations in the required phase-quadrature. At the same time, voltages corresponding with the original signal appear across the series resistance 26. The band-pass action of the circuit 19 also automatically keeps in step with the shifting frequency of the incoming signals.