构造级联型Butterworth滤波器，可以是任意阶数，并计算出频率幅度响应，非常实用，封装成为一个类，以便大家使用。开发环境为VC 6.0-tectonic Cascade Butterworth filter, can be arbitrary order. to work out the frequency response range, very practical, Packaging, as a category, so t