Spectrum diagram [Pí n p ⅸ diagram] English: The relationship between sound frequency and energy is represented by spectrum-the graphic data of various signal frequencies are recorded and drawn on the horizontal and vertical axes in the form of ripples. Common ones are amplitude spectrogram and phase spectrogram. In mechanical fault diagnosis system, spectrogram is used to answer questions such as location, type and degree of faults. It is the main tool for analyzing vibration parameters. In practical use, there are three kinds of spectra, namely linear amplitude spectrum, logarithmic amplitude spectrum and self-power spectrum. The vertical coordinate of linear amplitude spectrum has a clear physical dimension and is the most commonly used. The amplitude of each spectral line in logarithmic amplitude spectrum is calculated by logarithm, so the unit of its ordinate is dB. The purpose of this transformation is to make those low-amplitude components higher than those high-amplitude components, so as to observe the periodic signals hidden in low-amplitude noise. Self-power spectrum first convolves the measured signal with autocorrelation, with the purpose of removing random interference noise, retaining and highlighting periodic signals, and losing phase characteristics, and then Fourier transform. The self-power spectrum diagram makes the periodic signal more prominent.