GB517636A - Improvements in or relating to automatic tuning systems for radio receivers - Google Patents

Abstract

517,636. Automatic tuning correction. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. Aug. 2, 1938, No. 22872. Convention date, July 31, 1937. [Class 40 (v)] The tuning of a superheterodyne receiver is automatially corrected on the required frequency by means of a two-phase motor energized by control potentials from an error detector circuit. The latter comprises a pair of reversely connected diodes 16, 17, fed from the I.F. amplifier 5 across a series-resonant circuit C, L tuned to the I.F. and connected in series with a limiting resistance 10, L and C being so chosen that their reactances are equal at the operating frequency. The low frequency output is taken from a potentiometer 27 across the series diode load resistances 14, 15, the tune-correcting voltage across point A on 27 and ground being applied to a pair of control valves 70, 71 (Fig. 3). These valves have their output circuits in push-pull, the secondary of the output transformer 73 being connected to one phase 74 of the two-phase tuning motor 75. The grid of valve 71 is earthed and mains voltage from 72 is applied directly to the second phase 76 of motor 75 and also between the centre tap of the primary of 73 and earth. When the voltage between point A and earth is zero, valves 70, 71 are in balance and no voltage is induced in phase 74 ; but when the voltage of A becomes positive or negative with respect to earth, by reason of the frequency impressed on L, C being above or below the true value, valve 70 or 71 will preponderate and the motor will turn in the appropriate sense to restore the tuning to its correct point.