说明：  理解信号与频谱之间的关系，会用Matlab绘制基本信号：方波、三角波、钟形波、单边指数衰减波、双边指数衰减波等信号图和频谱图。
(Understand the relationship between the signal and the spectrum will use Matlab to draw the basic signal: square, triangle, bell-shaped wave, exponential decay wave of unilateral, bilateral exponential decay wave signals such as maps and spectrum.)