Abstract: A method in signal processing of a train of pulses with a stable, high repetition frequency f.sub.r, in particular in connection with dispersion measurements of optical fibers, comprises sampling (1) of the received signal at a sampling frequency f.sub.s, wheref.sub.s =(f.sub.r .+-..DELTA.f)/n(n is an integer and .DELTA.f is several magnitudes smaller than f.sub.r). When the sampling frequency is generated by a pulse generator (3), which, through a phase-detecting circuit (4), depends on partly a reference signal (from the oscillator 6) and partly a low frequency signal derived (via 5) from the signal received and sampled at the frequency f.sub.s, a stable sampling frequency is obtained even though the signal/noise ratio on the input of the receiver is significantly smaller than was permissible in the past.

Abstract: In a demodulator for use with a multi-level phase shift keying (p.s.k.) signal having N phase levels, the p.s.k. signal which includes a carrier signal is frequency multiplied by a factor N in a multiplier filtered and frequency divided by the factor N to enable the carrier signal to be reconstituted at a terminal. To enable the demodulator to work at different data rates, and to enable filters to have sufficiently precise characteristics, the received p.s.k. signal is initially converted to base-band in mixers to which is applied a locally generated signal nominally at the carrier frequency.