中文说明：
在matlab下实现信号频谱分析：利用快速付里叶变换FFT得到下列信号的频谱，并绘制信号及频图形：1.长2s的方波信号，频率f=10Hz。
English Description：
Realize signal spectrum analysis in Matlab: use fast Fourier transform FFT to get the following signal spectrum, and draw signal and frequency graph: 1. Square wave signal with length of 2S, frequency f = 10Hz