编写一个函数，设计给定３ＤＢ点的ＩＩＲ滤波器，这样的方法简单方便-The preparation of a function, the design of a given 3DB point IIR filters, this method is simple and convenient

进行数字滤波器的设计，滤波器的性能指标可以根据实际情况作调整, 要求用窗函数法和双线性变换法设计以下三种数字滤波器： -Digital filter design, the performance of the filter can be adjusted according to the actual situation, the three digital filters using the window function method and the bilinear transf