in the method and apparatus for the phase measurement of arbitrary signals at a measuring point , for example , of surface waves on piezoelectrical substrates , the surface waves are excited on the surface of a specimen having piezoelectrical features . the measuring point on the specimen surface is scanned by a particle beam and a secondary electrical signal at the measuring point is supplied to an evaluation circuit via a detector . a phase detector within the evaluation circuit is operated in a linear region of its output characteristic curve with the use of a feedback . this makes it possible to keep the phase at the phase detector constant . the phase of the signal to be examined , for example , of the surface wave , which is produced at a interdigital transducer is influenced due to the feedback .