GB474682A - Improvements in or relating to wireless and like receivers - Google Patents

Abstract

474,682. Wireless receiving circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. May 4, 1936, No. 12623. Convention date, May 3, 1935. [Class 40 (v)] A superheterodyne receiver in which automatic fine-tuning is provided following manual adjustment comprises a frequency control valve, the grid bias of which is varied in dependence on departure from exact tune by an error detector, and a local oscillator, the circuit of which includes an impedance, the voltage across which is in phase quadrature with the anode current of the control valve, which voltage is impressed on the input circuit of the control valve. The output from the I.F. stage 5, Fig. 1, is applied to an amplifier 21, the output of which is applied over condenser 27 to an A.V.C. diode 25 and also over a pair of transformers S1P1, S2P2 to an error detector comprising circuits 35, 36 tuned 2 k.c. above and below the I.F. and feeding a pair of diodes 32, 33 whose load resistances 34, 34<1> are connected in opposing sense as regards voltage drop in the grid biassing circuit of a frequency-control valve 38. The anode of 38 is connected to the tank circuit 13, 14, R of the local oscillator 12, the junction of 14 and R, which is in phase quadrature with the anode current of valve 38, being connected over condenser 43 to the control grid of valve 38. The local oscillations are applied from the tank circuit 13, 14 to the inner grid of the frequency changer 2. With the arrangement shown, the control valve 38 will present an inductive reactance across the inductance 14, and this reactance is variable by varying the mutual conductance of 38 by varying its control grid bias. The arrangement is such that variations in grid bias, derived from the error detector, act to vary the oscillator frequency so as to restore exact tuning. The automatic frequency control may be rendered inoperative by switch 42. The resistor R may be in series with the condenser 13 instead of inductance 14, the common point of 13 and R being taken to the grid of 38 as before, Fig. 2 (not shown) ; the valve 38 will then present a capacitative reactance to the tank circuit. Alternatively, the phasequadrature voltage may be obtained by connecting the lower side of reaction coil 14<1> through a condenser to earth, a choke being provided in the H.T. + lead ; the junction of coil 14<1> and condenser is then connected through a further condenser to the grid of 38, Fig. 3 (not shown). Or the anode side of coil 14<1> may be connected through a condenser to the grid of valve 38, Fig. 4 (not shown). With the two latter arrangements the valve 38 acts respectively as a negative inductance and negative capacity ; the connection A.F.C. must thus be made to the lower terminal of resistance 341 for the modifications of Figs. 2 and 3. To prevent development of an inphase voltage component across coil 141, when used to produce the phase-quadrature voltage, the condenser coupling to the grid of 38 referred to with reference to Fig. 4 may be replaced by a transformer, Figs. 5 and 6 (not shown). Fig. 9 shows an arrangement in which approximate tuning is effected by a tuning motor operated by control switches, finetuning being automatically effected by any of the arrangements previously described. The automatic frequency control is rendered inoperative while tuning is taking place by switch 42 carried by an armature 90, the coil 92 of which is connected in the motor armature circuit and co-operates with coil 93 in the motor field circuit. When the motor is energized, the switch 42 is closed. Specifications 443,423 and 471,738 are referred to. Reference has been directed by the Comptroller to Specification 443,423.