GB732798A - Improvements in or relating to telecommunication systems and apparatus - Google Patents

Abstract

732,798. Automatic frequency control systems. GENERAL ELECTRIC CO., Ltd., and ASPIN, J. J. E. Nov. 25, 1953 [Oct. 24, 1952], No. 26763/52. Class 38 (4). [Also in Group XL (c)] An oscillator, e.g. the local oscillator 3 of the receiver of a relaying station, has its frequency controlled by passing part of its output to a standard resonator 8 either whose tuning is modulated about a mean frequency equal to the frequency to be controlled, or the oscillator itself is slightly frequency modulated so that the resonator output is phase or amplitude modulated with a phase or phase sense corresponding to the sense of frequency error, which phase is then detected and applied to correct the frequency through a servo control 14. Further, the second harmonic of the modulating frequency fm present when the oscillator is at its correct frequency is detected by a filter 13 and applied to operate an alarm, indicator or control 16 when the amplitude falls below a predetermined level. Thus any failure of the local oscillator 3 or A.F.C. system causes operation of device 16. In addition a monitor 17 responsive to the D.C. output of the receiver frequency changer 2 may check the latter and the output from the local oscillator 3. The monitor 16 may operate a supervisory tonemodulated generator 18 which injects the appropriate intermediate frequency fi into the amplifier 4 if the preceding part of the relay station fails. Alternatively, as described in Specification 664,016, [Group XL (c)], the sysmay effect change-over of faulty transmitting apparatus.