输入信号的时域和频谱分析 输入信号为ft=[5cos(600*pi*t)+4cos(90*pi*t)+9]/18 输入信号为高频和低频信号的叠加：高频300Hz，低频45Hz- Of the input signal in time domain and spectral analysis of the input signal is ft = [5cos (600* pi* t)+4 cos (90* pi* t)+9]/18 input signal for the hi