Abstract: An analog input signal is converted into a digital signal. The digital signal is shifted to the difference frequency region by mixing it with each of a sine wave local signal and a cosine wave local signal to obtain complex waveform data. On the other hand, a trigger signal is generated when a reference signal meets a specific requirement, and a frame composed of a fixed number of data is defined which is synchronized with the trigger signal. The complex waveform data of each frame thus defined is phase rotated so that its phase angle has a predetermined value, or the phases of the local signals are synchronized with the trigger signal so that the complex waveform data of respective frames have the same phase. The in-phase complex waveform data of plural frames thus obtained may also be averaged, if necessary.