用双线性变换法设计一个Butterworth低通滤波器，要求其通带截止频率100Hz，阻带截止频率200Hz，通带衰减小于2dB，阻带衰减大于15 dB，采样频率为500 Hz。-Using bilinear transformation to design a Butterworth low-pass filter, requiring that their pass-band cutoff frequency 100Hz, stopband cutoff frequency 200Hz, pass-band attenuation is less than 2dB, stop-band attenuation greater than 15 dB, a sampling frequency of 500 Hz.