说明：  Sigma_delta调制器的matlab模型和完整代码，可通过仿真观察输入输出信号波形以及输出的频谱，最后还说明了如何计算信噪比
(Sigma_delta modulator of matlab models and complete code, can observe the simulation input/output signal waveform and the output spectrum, finally also illustrates how to calculate the SNR)