说明：  巴特沃斯滤波器，切比雪夫滤波器，椭圆滤波器设计及比较，同样的设计参数指标，在同一张图中比较三种滤波器的幅频响应波形，并用统计图画出各滤波器所需阶数。
(Butterworth filter, Chebyshev filter, elliptic filter design, the same design parameter index, with a figure in the comparison of three filter frequency response waveform, and use statistical picture of the desired order of each filter.)