Patent GB-530886-A (GB)
Title: Improvements in or relating to tuning systems for radio receivers
IPC: H03J5/14
Filed: 1939-06-23
Abstract: 530,886. Automatic frequency control. STANDARD TELEPHONES & CABLES, Ltd., BEATTY, W. A., and BRAENDLE, E. W. June 23, 1939, No. 18360. [Class 40 (v)] [Also in Group XXXVIII] In receivers provided with pre-set push button tuning corrected by an automatic frequency control circuit, in which the same motor which performs the coarse tuning is also controlled through the A.F.C. circuit to perform the final fine tuning, an induction motor having shaded windings arranged on split poles, e.g. as described in Specification 522,130, [Group XXXV], is employed. One pair of split poles may be used to control rough tuning and another pair for the fine tuning. The output of the A.F.C. error detector circuit is passed to a pair of valves connected across the shaded pole windings so that the valve impedances are varied by the rough and fine tuning circuits respectively, so as to control the rotation of the motor. The rotor shaft 42 of the ganged tuning condensers 43 of a receiver is driven through pulley wheels 41, 36 and an endless band 38 from a shaft 35 rotated through gearing 30-33 from the driving disc 29 of a motor having a split pole armature provided with shaded windings 27, 28. A selector commutator disc 45 having two insulated segments 46, 47 is mounted on the shaft 44 and provided with the usual wiping contacts 49- 51 connected respectively to one of each pair of arms of the press button-switches 54-56. The output of the error detector circuit (not shown) is connected to the opposite ends of a pair of resistors 69, 70 these ends being connected respectively to the grids of a pair of control valves 63, 65 while the common point of the resistors is joined through lead 68 and variable resistors 79, 80 to the cathodes of the valves. The error detector circuit is balanced about a point 71 connected through a relay comprising a pair of contacts 72, 73 to the resistors 79, 80. The pivoted arm of the contact 72 lies between two coils 76, 60. The cathodes of the valves 63, 65 are connected through wipers 52, 53 to the insulated segments 46, 47 while the plates of the valves are supplied with anode current from the shaded windings 27, 28. The coil 60 is connected to the remaining arms of the switch contacts of the pressbuttons 54, 56. For coarse tuning the contacts 72, 73 are open and the error detector circuit is disabled. On depression of push-button 55 a circuit is set up through the cathode end of resistor 80 via pigtail 53, segment 47, wiper 51, switch contacts 55, relay winding 60 to the other end of the resistor 80. The coil 60 is thus connected in parallel with the resistor 80 and the impedance of the valve 65 is lowered thus causing the motor to rotate until the approximate tuning position is obtained. The coil 76 is connected to the earlier valves of the set so that when the approximate tuning position is reached a current flows through the coil 76 thus causing the contacts 72, 73 to close. The error detector circuit is thus brought into operation to alter the relative impedances of the valves 63, 65 thus causing the motor to rotate in a direction which counteracts any error in the coarse tuning. Two further switches 57, 58 are provided with multiple contacts for (a) waveband switching (b) disabling the motor tuning when manual tuning is employed. A further pair of switches 93, 94 and 91, 92 which are closed on slight rotation of the manual tuning knob 37 disable the A.F.C. circuit during manual tuning, the switch opening automatically as soon as rotation of the knob ceases. The insulated gap 48 separating the selection discs 46, 47 may be made wider than usual (e.g. corresponding to a tuning error of 3 kcs. on either side of the true tuning position) and instead of the side of the gap being parallel they may be arranged to converge towards the periphery of the disc or to widen in steps towards the centre of the disc, the wipers 49-51 then being mounted at varying distances from the centre of the disc so that when they rest on the insulated insert the latter has at that point a width corresponding to the desired angular limit of rotation for the particular wavelength selected. The wiper supports or selector discs can be calibrated to indicate the correct setting relative to the wavelength concerned. The circuit prevents a stronger adjacent station from taking control of a desired weaker station. In a modification the relay winding 60 is connected in series with a metal oxide rectifier and thence to the anodes of the valves 63, 65. The motor may be used for driving a gramophone turntable. Specifications 362,001 and 511,121 also are referred to.