METHOD OF AND APPARATUS FOR MEASURING THE AMPLITUDE OF A HEAVILY NOISE AFFECTED PERIODIC SIGNAL WITHOUT PHASE REFERENCE

A means is provided of measuring the amplitude of a periodic signal, whose maxima and zero crossings have a phase difference of .pi./2, which extracts the amplitude information of a desired phase in which the measurement is to be effected by digitally processing the signal itself. The technique is particularly suitable when the signal-to-noise ratio is very low and the phase reference of the periodic signal is not available, as for instance when an attenuation measurement is to be effected in the field on an optical carrier for telecommunications systems. Phase locking is achieved by taking pairs of samples in each period, the samples having a .pi./2 separation, digitally filtering one sample of each pair, separately accumulating the two samples from a number of successive pairs, comparing the signs of the accumulated samples to provide a phase correction signal, and using the accumulated filtered samples to derive the desired amplitude measurement once phase locking is achieved.