GB665403A - Improvements relating to demodulators for frequency modulated waves - Google Patents

Abstract

665,403. Frequency modulation. SOC. ANON. DE TELECOMMUNICATIONS. July 6, 1949 [Dec. 21, 1948], No. 17903/49. Class 40 (v). A demodulating circuit for frequency-modulated waves comprises one or more transformers feeding a balanced modulator with waves whose relative phase is dependent on the instantaneous frequency deviation, the modulator output comprising the demodulated signal. The circuit operates as a limiter, and resistors are arranged for impedance-matching and ensuring balance at the transformer. If Fig. 1, the F.M. wave in the anode circuit a, b of an amplifier V feeds two transformers 2, 3 in series, of which the latter has a centre-tapped secondary coil tuned by a condenser 6 and connected to rectifiers 1, condensers 5 and load resistor R. The secondary of the transformer 2 is connected between the centre-tap of coil 3 and the junction of the condensers 5. Resistor 4 is connected between H.T. + and a tapping on the primary coil of the transformer 2, and resistor 4a across the primary of the transformer 3. In a modified circuit, Fig. 3 (not shown), the condenser 6 is omitted, the secondary 3 is connected through a phaseshift network to one diagonal of a bridge of rectifiers, the other diagonal of which is connected across the secondary 2. Output is taken between centre taps on the two secondaries. In Fig. 2, the anode circuit a, b includes the transformer 10 and the input to a phase-shift network 7. The secondary coil 10 feeds a further phase-shift network 12, the output of which is connected across one diagonal of a rectifier bridge 9, having its other diagonal connected to an output transformer 11 across which the demodulated signal is obtained. The phase-shifted output from 7 is coupled across the mid-points of the transformers 10, 11. A resistor 8 is connected from H.T.+ to the midpoint of the primary coil of the transformer 10. Specification 681,003 is referred to in explanation of the amplitude-limiting action.