Racal Instruments, Inc.

Michael D Gregory

Derek R Gittins

A circuit responsive to an input signal having an unspecified frequency for converting it to a predetermined and constant means frequency, and particularly suitable for incorporation in a modulation measuring instrument, comprises a sampling mixer controlled by a variable frequency oscillator for sampling the input signal to produce an intermediate signal. The mean frequency of the intermediate signal is then compared with a reference representing a predetermined frequency value whereby to produce an error signal, and means responsive to the error signal adjusts the frequency of the oscillator in such a direction as to tend to hold the mean value of the frequency of the intermediate signal at the predetermined frequency value while preserving therein variations in amplitude and/or frequency due to amplitude and/or frequency modulation.

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Windsor, United Kingdom

Michael D Gregory