用射频频谱分析仪观测一个已调波的频谱成分，记录上边带或下边带中各条谱线的幅度值，从而可以根据有关算式计算出调制信号（音频信号）的失真度。
Spectrum of the amplitude-modulated wave is observed by a radio frequency spectrum analyzer and amplitude values of every spectrum are recorded at the upper sideband or lower sideband.
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