4. 产生一个淹没在噪声中的信号，分析确定采样频率及数据分析长度，计算并绘出信号的频谱，指出50Hz和120Hz的正弦分量以及噪声分量-(4) generating a signal buried in noise, analysis to determine the sampling frequency and the length of the data analysis, calculate and plot the signal spectrum, noting that the sinusoidal component and a noise component of 50Hz and 120Hz