设计一个巴特沃思低通滤波器，其技术指标为：3dB带宽为，阻带截止频率为，阻带衰减大于30dB，采样间隔T=10us。
1）用冲激响应不变法
2）用双线性变换法设计
3）用FIR的矩形窗函数设计(A Butterworth low pass filter is designed, its technical specifications are: 3dB bandwidth, stopband cut-off frequency, stopband attenuation greater than 30dB, sampling interval T=10us.
1) using impulse response invariance method
2) design by the bilinear transformation method
3) the design of the rectangle window function of FIR)