按照给定指标采用模拟巴特沃斯低通滤波器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 Butterworth low-pass filter to approximate the ideal analog low-pass filter, and then through the bilinear transformation method of digitizin