中文说明：演示采样定理，时域采样，频谱周期延拓同时演示采样频率小于2*fc时，产生的混叠效应。可作函数应用
English Description：
This paper demonstrates the sampling theorem, time domain sampling, spectrum period extension, and the aliasing effect when the sampling frequency is less than 2 * FC. It can be used as a function