无限冲击响应滤波器 (IIR) 实现，涉及一个Butterworth低通滤波器，设计指标为：在通带内频率低于0.2π时，最大衰减小于1dB；在阻带内[0.3π, π]频率区间上，最小衰减要大于15dB -An infinite impulse response filter (IIR) implementation, involves a Butterworth low-pass filter design specifications as follows: in the passband frequencies below 0.2π, the maximum attenuation is less than 1dB　in the stopband [0.3π, π] frequency interval , the minimum attenuation greater than 15dB