A variable-frequency main oscillator of digitally tunable high output frequency f.sub.A is controlled by the integrated output voltage of a phase discriminator receiving on the one hand a relatively low comparison frequency and on the other hand a matching feedback frequency stepped down from output frequency f.sub.A by frequency division or by heterodyning with an auxiliary frequency f.sub.H of the same order of magnitude from an ancillary oscillator, a difference frequency f.sub.D =f.sub.A -f.sub.H is fed to a frequency discriminator delivering a corrective voltage, independent of that emitted by the phase discriminator through a filter network, to the control input of the main oscillator. A pair of frequency selectors varying the feedback frequency and/or the comparison frequency enable the output frequency f.sub.A to be adjusted in coarse and fine tuning steps, the coarse adjustments being also applied to the auxiliary frequency f.sub.H to limit the excursions of the difference frequency f.sub.D. In order to minimize shifts in the corrective voltage due to changes in the setting of the fine-tuning selector, a digital command generated by that selector is converted into an analog compensating voltage subtracted from the output voltage of the frequency discriminator.