用三阶低通IIR的传递函数来绘制频率响应图：
H ( z ) = 0.05634 ( 1 + z − 1 ) ( 1 − 1.0166 z − 1 + z − 2 ) ( 1 − 0.683 z − 1 ) ( 1 − 1.4461 z − 1 + 0.7957 z − 2 ) . H(z)=\frac{0.05634(1+z^{-1})(1-1.0166z^{-1}+z^{-2})} {(1-0.683z^{-1})(1-1.4461z^{-1}+0.7957z^{-2})}.H(z)=(1−0.683z−1)(1−1.4461z−1+0.7957z−2)0.05634(1+z−1)(1−1.0166z−1+z−2).
以多项式卷积的形式表示分子和分母。
b0 = 0.05634;
b1 = [1 1];
b2 = [1 -1.0166 1];
a1 = [1 -0.683];
a2 = [1 -1.4461 0.7957];
b = b0*conv(b1,b2);
a = conv(a1,a2);
[h,w] = freqz(b,a,'whole',2001);
plot(w/pi,20*log10(abs(h)))
ax = gca;
ax.YLim = [-100 20];
ax.XTick = 0:.5:2;
xlabel('归一化频率 (\times\pi rad/sample)')
ylabel('幅度(dB)')
