Patent GB-669113-A (GB)
Title: Improvements in or relating to automatic frequency control circuit arrangements
Inventor: BARNES GEORGE WILLIAM
IPC: H03J7/16
Filed: 1949-06-22
Abstract: 669,113. Automatic frequency control systems. NATIONAL RESEARCH DEVELOPMENT CORPORATION. July 6, 1950 [June 22, 1949], No. 16583/49. Class 38 (iv). [Also in Group XL (c)] An automatic frequency control arrangement, for the local oscillator of carrier transmission systems of the kind in which the carrier wave is partially suppressed, comprises two demodulators for mixing the received waves with quadrature phases of the local oscillation, and two combiners for adding and subtracting the demodulated beat outputs after a relative phase shift, the combiner outputs being passed through filters, which pass only frequencies below the minimum essential speech frequencies, to exert a differential control effect on the frequency of the local oscillator. The arrangement is also applicable to the reception of frequency-shift carrier telegraphy. As shown, incoming waves are first reduced in frequency by a normal superheterodyne stage, and the resulting intermediate frequencies are fed in parallel to two demodulators D1, D2 which are also fed with quadrature phases derived from the local oscillator O, the frequency of which is determined by a crystal X and an associated inductance L. The beat outputs from D1, D2 are given a relative phase shift of 90 degrees at 51, 52 and are then added in one combiner, say C1, and subtracted in a second combiner C2. In this way only upper side-band signals are present in the output of one combiner, and only lower side-band signals in the other. Phones T may be connected to either or both. For A.F.C. purposes the combiner outputs are amplified and passed through filters F1, F2 which pass only frequencies below those necessary for the intelligence (say below 300 cycles per second), which frequencies result from beating of the local oscillation with the incoming carrier. According as the frequency of the source O is too high or too low, one of the combiners gives an output and the other does not. The output of the active combiner is rectified and fed through a cathode follower stage to cause the D.C. motor M to adjust the core H of the coil L and so to correct the frequency of the source O. The A.F.C. system is inoperative for frequency errors above 300 cycles per second, and for errors below 35 cycles per second the correcting voltage falls rapidly to zero owing to the effective lower limit of the phase shifters 51, 52. Relays may replace the cathode follower stages for controlling the motor M. The A.F.C. arrangement is applicable to systems in which the transmitted carrier voltage is in inverse ratio to the strength of the side-bands. The A.F.C. system may be used in receivers for frequency-shift telegraphy, which may be regarded as a suppressed carrier system with " mark " transmission of an upper side-frequency at a separation f cycles per second, and " space " transmission of a lower side-frequency at a separation f2, where f1 may be equal to f2, and f1 + f2 represents the total carrier shift. In this case it is necessary to replace the simple rectifiers R1, R2 by frequency discriminators A1, A2 centred respectively on the frequencies f<SP>1</SP> and f<SP>2</SP>. Signals are obtained at the outputs of the two combiners C1, C2, one of which gives an output for marks, and the other for spaces.