用matlab設計的IIR濾波器源程序
（1）IIR一階低通濾波器 P576
clear;
fi=1;fs=10;Gc2=0.9;
wc=2*pi*fi/fs;
omegac=tan(wc/2);
alpha=(sqrt(Gc2)/sqrt(1-Gc2))*omegac;
a=(1-alpha)/(1+alpha);
b=(1-a)/2;
w=0:pi/300:pi;
Hw2=alpha^2./(alpha^2+(tan(w/2)).^2);
plot(w/pi,Hw2);
grid;
hold on;

（2）一階高通濾波器 P581
clear;
fi=1;fs=10;Gc2=0.5;
wc=2*pi*fi/fs;
omegac=tan(wc/2);
alpha=(sqrt(1-Gc2)/(sqrt(Gc2)))*omegac;
a=(1-alpha)/(1+alpha);
b=(1+a)/2;
w=0:pi/300:pi;
Hw2=(tan(w/2).^2)./(alpha^2+(tan(w/2)).^2);
plot(w/pi,Hw2);
grid;
hold on;

（3）Notch 嵌波濾波器
clear;
Gb2=0.5;
w0=0.35*pi;
deltaw=0.1*pi;
b=1/(1+tan(deltaw/2)*(sqrt(1-Gb2)/sqrt(Gb2)));
B=[1 -2*cos(w0) 1].*b;
A=[1 -2*b*cos(w0) (2*b-1)];
w=0:pi/500:pi;
H=freqz(B,A,w);
plot(w/pi,abs(H));
grid;

（4）Peak 濾波器
clear;
Ac=3;
Gb2=10^(-Ac/10);
w0=0.35*pi;
deltaw=0.1*pi;
b=1/(1+tan(deltaw/2)*(sqrt(Gb2)/sqrt(1-Gb2)));
B=[1 0 -1].*(1-b);
A=[1 -2*b*cos(w0) (2*b-1)];
w=0:pi/500:pi;
H=freqz(B,A,w);
plot(w/pi,abs(H));
grid;

（5）IIR低通濾波（Butterworth）
% IIR Lowpass Use Butterworth
% copyright by Etual
clear;
fs=20;fpass=4;fstop=5;
Ap=0.5;As=10;
wp=2*pi*fpass/fs;ws=2*pi*fstop/fs;
omegap=tan(wp/2);omegas=tan(ws/2);
ep=sqrt(10^(Ap/10)-1);
es=sqrt(10^(As/10)-1);
N=ceil(log(es/ep)/log(omegas/omegap));
omega0=omegap/ep^(1/N);
K=floor(N/2);
for i=1:K
theta(i)=pi*(N-1+2*i)/(2*N);
end
for i=1:K
G(i)=omega0^2/(1-2*omega0*cos(theta(i))+omega0^2);
end
for i=1:K
a1(i)=2*(omega0^2-1)/(1-2*omega0*cos(theta(i))+omega0^2);
end
for i=1:K
a2(i)=(1+2*omega0*cos(theta(i))+omega0^2)/(1-2*omega0*cos(theta(i))+omega0^2);
end
if K(N/2)
G0=omega0/(omega0+1);a0=(omega0-1)/(omega0+1);
end
w=0:pi/300:pi;
Hw2=1./(1+(tan(w/2)/omega0).^(2*N));
plot(w/pi,Hw2);
grid;