GB551970A - Improvements in combined carrier-signal frequency detectors and control-voltage supply systems - Google Patents

Abstract

551,970. Frequency modulation; automatic volume control. HAZELTINE CORPORATION. Feb. 7, 1942, No. 1658. Convention date, Nov. 13, 1940. [Class 40 (v)] The frequency discriminator of a radio receiver is modified to provide control voltages suitable for automatic gain control, the control of a tuning indicator, or like purpose. In Fig. 1 a frequency-modulation receiver comprises a normal frequency discriminator 18 including tuned circuits 19, 20 resonant above and below the intermediate frequency, rectifiers 25, 26 and series load resistances 27, 28. The demodulated signal is taken to an audio amplifier 16 across the two resistances jointly. To provide a gain control voltage a further rectifier 30. is connected across the tuned circuit 19 with its load resistance 31 in shunt ; and the negative voltage produced jointly across resistances 28, 31 is applied through a time-constant circuit 33, 34 to the early stages in the receiver to control their gain. This voltage is substantially independent of the tuning of the receiver within the limits of the discriminator, Fig. 2 (not shown). In Fig. 3 two further diodes 35, 30 are fed from the tuned circuits 19, 20 and are provided with a common series load 38 and a common shunt load 31. The positive voltage across resistance 38 controls a tuning indicator 48, and the negative voltage across resistance 31 controls the gain. Owing to interaction between the diodes provided by the resistance 37 connecting their anodes, both the control voltages have pronounced minimum values when the set is in tune, Fig. 4 (not shown). In a modification of Fig. 3, the resistances 37 and 38 are omitted and the two diodes 35, 30 have separate shunt loads which are connected in series to provide the gain control voltage, Fig. 5 (not shown).