Abstract: A device capable of being operated under the control of a programmer for rapidly measuring the differences between the actual values of a plurality of transmitted signal frequencies and their assigned values. It comprises a constant frequency local oscillator, a modulator fed from said oscillator and from a signal the actual frequency of which is to be determined, a bandpass filter fed from the output of said modulator, another local oscillator producing a constant frequency, and means for measuring the frequency difference between said other oscillator and the output frequency of the filter. Arrangements are provided for displaying the measured frequency difference values in digital form.