GB646547A - Noise reduction circuit for radio receivers - Google Patents

Abstract

646,547. Radio receiving circuits. STANDARD TELEPHONES & CABLES, Ltd. June 4, 1948, No. 15148. Convention date, June 5, 1947. [Class 40 (v)] Muting is derived from an off-tune discriminator circuit. In a frequency-modulation receiver, Fig. 2, the discriminator 3 is set to one side of the true signal frequency so as to produce a positive D.C. potential when the receiver is in tune. This is used to unblock a lowfrequency amplifier 7, being applied to the grid in opposition to a cathode bias which otherwise biasses the valve to cut-off. The grid may be shunted by a biassed rectifier 10 which in conjunction with series resistance 11 prevents the applied positive potential becoming excessive. The anode circuit of the valve may include a relay, Fig. 3 (not shown), utilized to close a signalling circuit when the receiver is in tune. To prevent closure of the circuit by an unwanted signal close to that desired, an auxiliary valve 18, Fig. 4, may be provided, having in its anode circuit a further relay, the contacts of the two relays being in series. The discriminator 3 is modified to supply to the grid of the valve 18 a D.C. potential equal and opposite to that supplied to the valve 7, so that for signals other than the one desired the anode current of one or other of the valves is insufficient to operate the corresponding relay. In another embodiment, Fig. 5 (not shown), the valves are utilized as a push-pull L.F. amplifier ; they may alternatively be used solely for operating relays, Fig. 7 (not shown), which have additional contacts arranged in series in the anode circuit of a separate L.F. amplifier. The arrangement may also be applied to an amplitude-modulation receiver, Fig. 6 (not shown), by providing a separate discriminator circuit in parallel with the detector.