说明：  模拟滤波器的设计包含了巴特沃斯滤波器，切比雪夫滤波器，椭圆滤波器，以及10阶贝塞尔滤波器的平方幅频率和相频响应曲线，贝塞尔滤波器的特点是：在0拼时具有最平坦的群延迟，并在整个通带群延迟几乎不变
(Analog filter design consists of Butterworth filters, Chebyshev filters, elliptic filters, as well as 10 pieces of first-order Bessel filter is the square of the frequency and phase frequency response curves, Bessel filters are characterized by: at 0, when fight with the most flat group delay, and in the entire pass-band group delay is almost constant)