Patent GB-437442-A (GB)
Title: Improvements in automatic tuning devices for radio receivers
IPC: H03J1/18
Filed: 1934-02-24
Abstract: 437,442. Wireless receiving-apparatus. JACKSON, H., 5, Maple Grove, St. Helens, Lancashire. Feb. 24, 1934, No. 6055. [Class 40 (v)] [See also Group XXXVI] In a wireless receiver wherein the tuning member is controlled indirectly, i.e. otherwise than by the agency of a direct rigid frictional or gear coupling, automatically operated fine tuning means are provided to compensate for lack of precision in the normal operation of tuning. In one arrangement, Figs. 1 and 2 (not shown), the main tuning condenser is driven by a spring driven rack, the various settings being determined by station keys which when depressed are projected into the path of an arm on the condenser shaft. The rack is retained normally at one end of its stroke by a latch, which is released when any one of the station keys is operated, to allow rotation of the condenser until the arm strikes the operated key. The rack is re-set by a re-set key prior to each tuning operation. Indicating lamps operated by the keys may be arranged above the station keys or each key may be constructed in cylindrical form and fitted with a translucent indicator which may be illuminated from behind by a lamp carried on the arm. In order that account may be taken of small changes in wave length and deformations and wear of the stop arm and keys, automatically controlled fine-tuning means are provided. The fine tuning means comprise a vernier condenser shunted across the main tuning condenser and operated by an electric motor to bring the circuits into exact tune. The motor is connected in the anode circuit of two oppositely connected valves, the grid circuits of which are controlled by tuned circuits which are tuned by the normal tuning operation to frequencies slightly above and below the correct frequency. When the set is accurately tuned, the action of the valves is matched and the motor remains inoperative but when the set is off-tune, one or other valve predominates and the motor is operated in one or other direction to adjust the fine tuning means and also re-establish the frequency differences in the tuned .circuits controlling the motor. In a modification the fine tuning means may be omitted, the motor being adapted to drive the main tuning condenser. Fig. 3 shows a modification wherein fine tuning is effected due to variation of the inherent impedance of a valve 92. The main tuning condenser 58 of the superheterodyne receiver is controlled from a remote point by a Wheatstone bridge control system comprising a remote resistance 101 which when varied disturbs the balance and effects operation of polarized relay 105 which operates the motor 108 in one or other direction to restore balance. The motor also controls the condenser 70 in the resonant circuit 64 connected in the circuit of the first control grid of the valve 59 which acts as local oscillator and mixer. The anode of the valve 59 is connected to the intermediate frequency circuit 72 and also to the grid of an intermediate frequency amplifier 75 which at the proper intermediate frequency produces in the tuned circuit 80, 81 equal voltages across the coil and condenser which supply the anodes 83, 86 of a double-diode valve 85 controlling the bias potential of a hexode 92. Normally when a station is accurately tuned, no p.d. exists between points A, A<1> but in the event of approximate tuning within the limits of the response curve of the tuned circuit 80, 81, the potential of A is varied relatively to A<1> to vary the amplification factor of the hexode 92 and consequently the " reflected " capacity in the grid-cathode circuit-which is in shunt with the tuning condenser 70 of the resonant circuit 64. The intermediate frequency is thereby increased and the effect continues until the p.d. between A,A<1> becomes zero. Under these circumstances the intermediate frequency has its correct value and the receiver may be regarded as being in tune although the signal frequency circuits remain slightly mistuned. In order that the efficiency of this arrangement may be uniform on different wave-lengths a condenser 96 is ganged to the main tuning control. Alternatively a resistance 93 may be made adjustable in accordance with the tuning control. The Provisional Specification also states that the reset and station keys may control one or other of a two-coil motor winding in order to facilitate operation of the motor in one or other direction ; furthermore direct control of the tuning condenser may be provided and arrangements are made for disconnecting the aerial during a tuning operation. Specifications 181,799, [Class 40 (v)], and 407,057 are referred to.