iir

代码说明：

  按照给定指标采用模拟低通滤波器（巴特沃思低通）去逼近理想模拟低通滤波器 ，然后通过双线性变换法对模拟滤波器进行数字化，得到数字滤波器 。(Indicators in accordance with a given analog low-pass filter (Butterworth low-pass) to approximate the ideal analog low-pass filter, and then through the bilinear transformation method of digitizing analog filters, the digital filter.)