GB452585A - Improvements in or relating to radio receivers - Google Patents

Abstract

452,585. Valve circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Electra House, Victoria Embankment, London.-(Assignees of Beers, G. L. ; 119, Summerfield Avenue, Collingswood, New Jersey, U.S.A.) Feb. 25, 1935, No. 6010. Convention date, Feb. 23, 1934. [Class 40 (v)] In a superheterodyne receiver of the kind comprising manual tuning control and supplementary automatic tuning control for final accuracy of tuning, operation of the automatic control is prevented until the manual control is adjusted to a predetermined frequency displacement from the in-tune position. Fig. 1 shows a receiver comprising a first detector 95, oscillator 112, I.F. amplifier 97 and signal diode 99. A noise suppression circuit, comprising an amplifier 187, coupled to the I.F. stage over condenser 201, a diode 189 and suppressor valve 153 prevents automatic tuning action until the receiver is tuned to within about 1À5 k.c. of a desired frequency. The automatic tuning circuit comprises amplifier 177, diodes 163, 165 and frequency control valve 138, the diode input circuits being tuned one 3 k.c. above, the other 3 k.c. below the desired I.F., and including load resistances 173, 175 oppositely connected in the grid-cathode circuit of valve 138. The bias on the latter thus varies with the departure from exact tune, the tuning circuit of oscillator 112 being varied in compensating sense. The amplifiers 97 and 177 are biassed back to cut-off, until signals reach a predetermined amplitude, by the p.d. across resistance 151 in the anode circuit of the suppressor valve 153. As increasing signal is passed by valve 187, increasing p.d. across the load resistance 211 of diode 189 increases the negative bias on the control grid of valve 153, the resulting fall in anode current releasing valves 97 and 177. The inclusion of the signal diode resistance 137 in the suppressor valve grid circuit accelerates the release action. The automatic tuning may be effected either by varying the effect of a variable trimmer condenser 166, gang-controlled with the oscillator and main tuning condenser controls, or by varying the oscillator frequency by varying its anode potential. Switches 131, 132 effect the change from one control to the other. Fig. 2 shows an input filter suitable for use with the receiving arrangement of Fig. 1, and designed for a band width of 50-100 k.c. The filter 5 comprises variable series inductances 7, 9, 11 and shunt inductances 15, 17, all with movable powdered iron cores, unicontrolled by knob 21.