stft4

代码说明：

  对于持续时间为500s的两个LFM信号，其初始频率分别为0.1Hz、0.3Hz，中止频率分别为0.3Hz、0.1Hz，在不含噪声和含有噪声（ ）的情况下，仿真(for the duration of the two 500s LFM, the initial frequency of 0.1Hz. 0.3Hz, suspension frequencies of 0.3Hz, 0.1Hz. In noise-free and noisy () cases, the simulation)