Device for measuring frequencies, comprising a frequencymeter having a memory and digital outputs which programs an iterative frequency synthesizer. The latter comprises an interpolation oscillator switchable to a plurality of the decade units of the synthesizer. Means are provided for effecting a subtractive mixing of the synthesized frequency and the frequency to be measured and means are provided for controlling the frequency variation of the interpolation oscillator from the output of the mixing means. A second frequencymeter which has digital outputs measures the frequency increment of the interpolation oscillator.
