So far, spectral analysis method based on fourier transform is used to analyze electromagnetic emission or conductive emission noise from PCB (Printing Circuit Board) of electronic equipments. But recently time domain analysis is required in order to provide waveform information which gives clock timing and waveform transmission characteristics. One of the problems in analyzing waveform in time domain is synchronous failure because of nonstationary nature of the noise included. In this paper, the discrete wavelet transform and correlation calculations are applied, as a low pass digital filter, to extract voltage descent of power terminal of a Pentium CPU from noisy waveform. Further it became possible to make synchronous addition for reducing amplitude of random noise.