GB592556A - Improvements in or relating to radio receivers and to circuits for use therein - Google Patents

Abstract

592,556. Phase modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 18, 1945, No. 12523. Convention date, May 18, 1944. [Class 40 (v)] A demodulator for angle-modulated or amplitude-modulated waves comprises a diode rectifier, means for selecting either amplitude or angle-modulated waves, and means for applying the modulated waves to the rectifier together with a modulation-free carrier derived from the incoming wave, in such a phase relationship that the modulation signals appear in 'the output of the rectifier. Theincoming waves are reduced to a low frequency and applied to the primary of a transformer T, which is tuned to the mid-frequency of the applied waves. The pass-band of transformer T is wide enough to pass the side-bands, and the voltage across the tuned secondary is passed through a crystal filter P and neutralizing condenser Cn to a balanced discriminator 26, 27, similar to that described in Specification 580,815 to provide an automatic frequency control voltage. The filtered carrier output of the crystal P is applied to the anode of diode detector 15 through a condenser 12, which is sufficiently large to have no effect on the phase of the filtered carrier. The anode of diode 15 is also supplied with unfiltered angle-modulated waves through condenser 13 and the contact PM of switch S. Condenser 13 alters the phase of the unfiltered energy by 90 degrees and also attenuates the side-band components to give carrier-exalted reception. The audio-frequency output is taken off through filter 22, 23 and switch 801 and A.V.C. voltage is fed through filter 24, 24<1> to the prior stages. In order to demodulate amplitude-modulated waves, the unfiltered waves are fed through condenser 20, resistance 19 and the AM contact of switch S to diode 15, the combination 20, 19 being adjusted to attenuate the side-bands without affecting the phase of the waves. The carrier wave derived from crystal filter P may be amplified before being applied to diode 15. The output terminal of crystal P is connected to earth through a parallel tuned circuit 60, and to the anodes of diodes 26, 27 through compensating networks 30, 40, 41 and 50, 51, 31. The audio-frequency output of balanced discriminator 26, 27 may be taken off through switch 80<1>, when carrier exaltation is not required; and A.F.C. voltage may be derived through the filter 82, 84.