Patent GB-443423-A (GB)
Title: Improvements in and relating to the tuning of radio receivers
IPC: H03J7/14
Filed: 1934-07-25
Abstract: 443,423. Valve circuits for wireless reception. MURPHY RADIO, Ltd., BAKER, G. B., and HAWKINS, G. F., 62, Brook Street, Hanover Square, London. July 25, 1934, Nos. 21775 and 34831. [Class 40 (v)] A superheterodyne radio receiver of the kind in which the local oscillator frequency is automatically adjusted to give exact tuning following approximate manual adjustment comprises a thermionic impedance the slope of which is controlled in function of departure from exact tune and presenting a reactive or resistive impedance in its output circuit which is associated with the frequency determining circuit of the local oscillator. In Fig. 1 the tuning circuit 2, 4 of the local oscillator O, back coupled to the grid circuit at 11, 12, is shunted by a pentode TI, the control grid of which is excited from the junction of a resistance 20 and condenser 21, constituting a potentiometer shunted across the anodecathode circuit of the valve ; 22 is a blocking condenser and a further condenser 27 may be connected in parallel with 21 by a switch 28 which is uncontrolled with the wave-range switch 6 of the local oscillator. The resistance 20 should have very low self-capacity and to this end its terminals may be screened by an earthed screen 26. With this arrangement the valve TI acts as a reactive impedance across the oscillator circuit, and its reactance is varied by varying the slope through the medium of the suppressor grid bias. A pair of tuned circuits 29, 30, coupled through condensers 31 to the I.F. stage of the receiver, are tuned to frequencies respectively above and below the I.F. and supply rectifiers 32, 33 whose load resistances 34, 35 are arranged in opposing sense in the suppressor grid biassing circuit. A switch 41, operated by the tuning knob of the receiver, renders the automatic control inoperative during manual tuning. The Specification shows that the valve TI presents a reactive impedance when the voltage applied to the control grid from the potentiometer 20, 21 is in phase quadrature with the anode voltage ; the potentiometer may be constituted by any desired combination of capacity, inductance and resistance to give this result, and the coupling to the control grid may be otherwise than direct. If the arrangement is such that the control grid and anode voltagns are in phase, the valve behaves as a pure resistance, and is then connected in series with an impedance (which may be variable to give sufficiently wide range of tuning) across the tuned circuit to be controlled. The control grid of TI is biassed from 23, 24, through inductance 25 which, with condenser 21, resonates at a frequency lower than the lowest received frequency thereby keeping the range of automatic control more constant throughout the wave-range. Fig. 4 shows an alternative tune-correcting circuit in which a series resonant circuit 42, 43, tuned to the I.F., is fed from an intermediate frequency amplifier 45 of the receiver and supplies two rectifiers 48, 49, the rectified voltages across resistances 46, 47 being applied to the thermionic impedance as in Fig. 1. Alternatively, the rectifiers may be constituted by a pair of triodes with the same input system 42, 43, 44, Fig. 5, (not shown). The error detector may alternatively comprise a parallel circuit 58, 59, Fig. 6, with rectifier 61 and load 60. The circuit is tuned to a frequency differing from the intermediate frequency, and to compensate for variations in p.d. across resistance 60 due to variations in volume the system 58 .. 61 is balanced against the circuit 62, 63 tuned to the I.F. In another modification the error detector is supplied from two parallel resonant circuits coupled to separate windings in the anode circuit of the I.F. amplifier, an earthed screen being interposed between the windings, Fig. 7, (not shown). A further system, Fig. 8, employs an additional oscillator 69 which beats with the intermediate frequency stage 68. The effect of the anode choke 70 is to make the amplification of 68 vary with frequency, and its output, rectified at 72, produces across 73 a p.d. varying with the error in tuning. This may be balanced against amplitude variations by opposing thereto a system 76, 77, 78 coupled to the I.F. circuit 67 as in Fig. 6.