说明：  典型连续信号和离散信号的时频分析，包括：单位冲激信号、单位阶跃信号、矩形脉冲信号、正弦信号、指数序列等等
(Typical continuous signal and discrete signal frequency analysis, including: unit impulse signal, the unit step signal, rectangular pulse signal, sinusoidal signal, the index sequence, etc.)