%Samp5_1
n=0:0.01:2;    %頻率點
for ii=1:4     %取4種濾波器
   switch ii
   case 1,N=2;
   case 2,N=5;
   case 3,N=10;
   case 4,N=20;
   end
   [z,p,k]=buttap(N);    %設計Butterworth濾波器
   [b,a]=zp2tf(z,p,k);   %將零點極點增益形式轉換為傳遞函數形式
   [H,w]=freqs(b,a,n);   %按n指定的頻率點給出頻率響應
   magH2=(abs(H)).^2;    %給出傳遞函數幅度平方
   hold on;
   plot(w,magH2);     %繪制傳遞函數幅度平方
end
xlabel('w/wc');
ylabel('|H(jw)|^2');
title('Butterworth模擬原型濾波器');
text(1.5,0.18,'n=2')   %作必要的標記
text(1.3,0.08,'n=5')
text(1.16,0.08,'n=10')
text(0.93,0.98,'n=20')
grid on;