Abstract: To measure various frequency components of the jitter of the deviation of the transition times in a signal on a signal line (44) from nominal bit times, a sampler (40) samples the signal at a rate high enough to determine the transition time with the required resolution. By employing a differentiator (60), test circuitry (FIG. 3B) can detect not only zero crossings but all digital-level transitions. The timings of the maxima of the differentiator output are applied to a Fourier-transform unit (76) that computes jitter-frequency components from a resultant sequence of deviations of the maxima from nominal transition times.