fp=0.2KHz,Rp=1dB, fs=0.3KHz,As=25dB,T=1ms；用脉冲响应不变法设计一Butterworth数字低通滤波器，观察所设计数字滤波器的幅频特性曲线，记录带宽和衰减量，检查是否满足要求。-fp=0.2KHz,Rp=1dB, fs=0.3KHz,As=25dB,T=1ms；The same method to design a digital low-pass Butterworth filter, observe the design of digital filter frequency characteristic curve, recording the amount of bandwidth and attenuation, check with the impulse response meets the requirements.