主要指令：butter（产生巴特沃斯滤波器）；freqz（绘制幅频、相频曲线）；zplane（绘制零、极点图）；ss2zp（状态方程变零、极点）；zp2tf（零、极点变分式多项式）。-Main instruction: butter (produced bart wo filter)　Freqz (draw amplitude frequency, phase frequency curve)　Zplane (draw zero, poles chart)　Ss2zp (state equation is zero, poles)　Zp2tf (zero, pole change fraction polynomial).