An automatic frequency control system includes a frequency measuring unit (IFM), which produces a frequency quantity representing the frequency of its input signal, and a controllable-frequency signal generator (VCO), which produces an output signal whose frequency depends on a control signal supplied to its control input. In a normal mode of operation, IFM derives its input signal from an external source connected to an input terminal (T), and the system derives from the frequency quantity produced by IFM, a control signal for VCO, using a predetermined transfer characteristic, so that the output frequency of VCO is dependent on the frequency of the input signal to IFM, being, for example, equal to the frequency of the signal at T. In order that the system may have an accurate and rapid response, said transfer characteristic is predetermined in a calibration mode of operation in which different control signals for VCO are derived from a calibration signal generator (CSG), resultant input signals for IFM are derived from the output of VCO, and different resultant frequency quantities produced by IFM are associated with respective control signals for VCO by storing digital words representing these control signals in a random-access memory (RAM) at respective addresses which are resultant frequency quantities from IFM. The system may be recalibrated at intervals to take into account drifts in the system.
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