GB473618A - Improved automatic tuning control means for modulated carrier wave receivers - Google Patents

Abstract

473,618. Wireless receiving circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. April 16, 1936, No. 10991. Convention date, April 16, 1935. [Class 40 (v)] In a radio-receiver having automatic fine tuning control, the said control is rendered inoperative by means responsive to received signal strength, when the latter falls below a predetermined value, the receiver tuning then remaining as it was immediately before the fall in signal strength. The Figure shows a superheterodyne receiver in which the output from the I.F. amplifier 3 is applied to a filter stage 4, whereby the intermediate frequency is stripped of its side-bands and is applied in phase opposition to a pair of differential rectifiers 7, 8. The output of the I.F. amplifier 3 is applied to the rectifiers 7, 8 in phase agreement through transformer T2 and also to an A.V.C. rectifier 19 from the cathode resistance 21 of which A.V.C. potentials are fed to the radio frequency amplifier 1. The output circuit of the rectifiers 7, 8, comprises two resistances 17, 18 in the grid-biassing circuit of a frequency control valve 6, variation in whose anode impedance varies the effect of capacity 15, on the tuning circuit of the local oscillator 5. Departure from exact tune produces a resultant voltage across the resistances 17, 18 thereby varying the grid bias of 6 in compensating sense. If the desired signal fades, the corresponding variation in p.d. across resistance 21 energizes valve 9 thereby opening contacts 11, 12 by relay coil 10 and disconnecting the grid of valve 6. The receiver thus remains at the same effective tuning position till the signal reappears, when the springs S re-close contacts 11, 12. The demodulator and final signal stages are connected to terminals 14. In modifications, the frequency control from valve 6 may be effected by inductive coupling with the oscillator 5 ; or the capacity 15 may be replaced by an inductance, Figs. 1 and 4 (not shown). The relay coil 10 may be connected directly in the cathode return circuit of the A.V.C. valve 19, Fig. 3 (not shown).