[xypjack]

参考一下，网上下的
clear all;
Wp2=40;
Ws=50;
Rp2=0.5;
Rs2=40;
Fs2=200;
[N2,Wn]=buttord(Wp2/(Fs2/2),Ws/(Fs2/2),Rp2,Rs2);
[b2,a2]=butter(N2,Wn);
[H2,W2]=freqz(b2,a2);
plot(W2*Fs2/(2*pi),abs(H2));grid;
xlabel('频率(Hz)');
ylabel('幅度');