iirfilter.m

matlab 编写的FIR iir滤波器源程序

function [b,a] = IIRfilter( ap,as,wp,ws ,fs)                     %定义函数其中返回参数b a为设计后的滤波器的分子分母的系数
T=1/fs
ws=2/T*tan(ws/2);
wp=2/T*tan(wp/2);
N=ceil(0.5*log10((10.^(as/10)-1)/(10.^(ap/10)-1))/log10(ws/wp)); %求模拟巴特沃思滤波器的阶数N
wc=ws/((10^(as/10)-1)^(1/2*N));                                %求模拟滤波器的截止频率
for k=1:N;                                                     %求模拟滤波器的极点
  sk(k)=wc*exp((0.5+(2*k-1)/(2*N))*j*pi);
end                                
aa=real(poly(sk))                                             % 求模拟滤波器的分子 分母系数                                         
bb=wc^N
W=0:5:ws;
h=freqs(bb,aa,W);
[b,a]=bilinear(bb,aa,fs);                                        %进行双线性变换 将模拟的滤波器转换成数字滤波器
w=[0:0.1:2];
H=freqz(b,a,w);
angH=angle(H);
H=abs(H);
db=20*log10(H);
subplot(2,2,1);plot(w/(2*pi)*1000,db)
axis([0,350,-25,0]);
subplot(2,2,2);plot(w/(2*pi)*1000,angH)
subplot(2,1,2);plot(w/(2*pi)*1000,H),hold on,plot(W/(2*pi),abs(h),'r')