GB558260A - Automatic frequency control in electric signalling systems - Google Patents

Abstract

558,260. Automatic tuning correction; frequency modulation. MARCONI'S WIRELESS TELEGRAPH CO., Ltd. (Radio Corporation of America). June 12, 1942, No. 8079. [Class 40 (v)] In the receiver of a signalling system operating with different frequencies for marks and spaces, the frequency of a local oscillator is controlled by a D.C. voltage obtained from the two frequencies and representative in magnitude and sense of deviations from their desired values. In Fig. 1, the two received frequencies are twice reduced by local oscillators 3, 5 and are passed through a limiter to the signal detector 30. The limiter 6 also feeds a pair of filters 7, 8 which separate the mark frequency from the space frequency each being heterodyned by a local oscillator 10, 12 to give a common beat frequency. This is selected by a filter 29 and passed through a limiter 13 to a frequency discriminator, the rectified output of which regulates the frequency of the local oscillator 3. Thus deviations in the beat frequency from the normal setting of the discriminator adjust the local oscillator 3 to return the beat frequency to normal. In a modification, Fig. 3 (not shown), the separating filters 7, 8 are omitted, and the two oscillators 10, 12 are replaced by a single oscillator the frequency of which is changed by, and in the rhythm of, the detected signals; with the result that the marks and spaces yield a common beat frequency. In a further modification, Fig. 6, the incoming frequencies, after reduction by the local source 3 and limitation at 6, are passed to coupled circuits 70 tuned to the mean of the two frequencies. If the frequency of the local source 3 is correct, marks and spaces produce equal amplitude of response in the circuits 70, and a continuous current in the load resistance of the diode rectifier 70'. If there is a tuning error, pulses of current pass through the primary coils P', P<SP>2</SP> of transformers 72, 73 which are in parallel with the rectifier load resistance, the polarity of the pulses depending on the sense of the tuning error. Signals from the detector 30 are passed through other primary coils P<SP>3</SP>, P<SP>4</SP> on the transformers 72, 73, the direction of winding of one coil being reversed. Each secondary winding is centretapped and connected as a full-wave rectifier 74, 75 to a load resistance 78, 79 ; and the algebraic sum of the voltages arising across the resistances is used to operate a frequency-control device 23 associated with the local source 3, to rectify the tuning error.