Patent GB-799959-A (GB)
Title: Frequency error compensation in fm systems
IPC: G08C15/02
Filed: 1956-05-07
Abstract: 799,959. Electric telemetering. BENDIX AVIATION CORPORATION. May 7, 1956 [May 12, 1955], No. 14097/56. Class 40 (1). A compensation system for reducing the effects of frequency shift errors in a frequency modulation signalling system having means for transmitting an A.C. signal wave of one base frequency which is frequency modulated with a signal and an A.C. reference wave of a second base frequency and means for receiving the signal and reference waves and separating them, is provided with means for detecting the separate reference wave and producing therefrom an output error potential proportional to departures of the received reference wave from its base frequency and signal detecting means responsive to both the separated signal waves and the error potential for producing an output signal potential proportional to departures of the received signal wave from its base frequency and inversely proportional to the error potential whereby frequency shift errors common to the signal and reference waves are reduced in the output of the signal detecting means. As shown in Figs 1 and 2, information signals from sources 10, 10n frequency modulate oscillators 6, 6n and the outputs of the oscillators and the output of an unmodulated reference oscillator 7 are combined in a unit 8 and recorded by a unit 9 on a recording medium such as a tape 9a. The recording is subsequently played back in a unit 9b and the various waves are separated by band-pass filters 11, 12, 13 and each wave after amplification is applied to a respective wave shaper 17, 18 or 19 which limits the wave and generates a trigger pulse for each cycle in the wave. The trigger pulses are applied to the detecting means, these comprise generators 20, 21 or 22 which generate pulses of constant width, the width depending upon the circuit parameters and which are separated by spaces the length of which depends upon the repetition rate of the triggering pulses. The generators are of the type having one stable and one non- stable condition such as mono-stable multivibrators or Phantastrons. The output pulses from the generators 20 to 22 are amplified and applied to limiters 26 to 28 which limit the pulse amplitudes to closely regulated values and also fix their amplitude with reference to earth potential such that their amplitudes in positive and negative directions are equal. The outputs of the limiters are fed to low-pass filters 29 to 31 where the pulses are integrated to give signals which are proportional to the input signals from the sources 10, 10n in the case of the signal waves. When no speed errors are present in the recording and play-back units 9, 9b the output of the reference frequency filter 31 will be zero but when speed errors are present, these frequency-modulate the waves and produce an error signal which after amplification is fed to the generators 20, 21 or to the limiters 26, 27 or both. When the error output is fed to the generators 20, 21 it alters the width of the pulses produced so that the signals produced after integration in the filters 29 and 30 are exactly proportional to the input signals from the sources 10 and 10n. When the error signal is applied to the limiters 26, 27 the amplitude of the pulses is altered instead of their width to achieve the same result. If necessary the error signal can be passed through delay networks 38, 39 which are adjusted to compensate for the delays in the respective signal circuits and through amplifiers 40 and 41 which enable the amplitude of the error signal to be adjusted before the signal is applied to the generators 20 or 21 or the limiters 26, 27. Any number of signal channels can be provided.