是利用冲激响应不变法设计一个巴特沃斯数字低通滤波器。根据滤波器的设计思想脉冲响应不变法将模拟低通滤波器变换到数字低通滤波器，利用MATLAB绘制出数字低通滤波器的系统幅频函数曲线。-Use the same impulse response is to design a digital low-pass Butterworth filter. According to the impulse response filter design thinking the same method to c