Abstract: The circuit for providing the high-frequency short signals which excite the specimen comprises a low-frequency oscillator linearly frequency-modulated during the course of time intervals of duration T.sub.o which are much shorter than their recurrence periodicity T.sub.r ; the output signal from the low frequency oscillator is frequency translated in a single side-band modulator supplied with a high-frequency wave by a high- frequency generator. An amplitude modulator makes it possible, in the course of the time intervals of duration T.sub.o, to create gaps in the output signal from the single side band modulator or, instead, to increase the amplitude of said signal during part of the frequency sweep. It furthermore suppresses the signal outside said time intervals of duration T.sub.o.