Abstract: The apparatus for detecting and discriminating phase jumps in a periodic signal comprises, arranged in cascade between an input terminal (1) and an output terminal (2), a frequency converter (3) transforming the frequency spectrum of the input signal e(t) containing a line at the frequency f.sub.e in a continuous band spectrum whose frequency is centered around f.sub.e, an interrupter (4), a resonator (5) having a resonant frequency f.sub.0 near f.sub.e and a phasemeter (6) having detection and discrimination means. The ensemble of these elements (3-6) is controlled by a sequencer (7) which, at each period .alpha. such that .alpha.=t.sub.1 +t.sub.2, allows the phase detection and the discrimination of each phase jump of a predetermined value .DELTA..phi..sub.e which may be contained in the input signal at the frequency f.sub.e.