afd_butt.m

按照给定指标采用模拟低通滤波器（巴特沃思低通）去逼近理想模拟低通滤波器

%巴特沃兹模拟滤波器设计子程序
function[b,a,z,p,k]=afd_butt(Wp,Ws,Rp,As) 
if Wp<=0
    error('Passband edge must be larger than 0') %通带截止频率必须大于0
end
if Ws<=Wp
    error('Stopband edge must be larger than Passed edge') %阻带截止频率必须大于通带截止频率
end
if (Rp<=0)|(As<0)
    error('PB ripple and /or SB attenuation must be larger than 0') %阻带衰减/通带波纹必须大于0
end
N=ceil((log10((10^(Rp/10)-1)/(10^(As/10)-1)))/(2*log10(Wp/Ws)));  %求N
fprintf('\n ***Butterworth Filter Order=%2.of\n',N)
OmegaC=Ws/((10^(As/10)-1)^(1/(2*N)));      %求截止频率

for i=1:N
    p(i)=OmegaC*exp(j*pi*(1/2+(2*i-1)/(2*N))); %求极点
end
z=[];  %零点
k=OmegaC^N;  %增益
B=real(poly(z));
b0=k;
b=k*B;
a=real(poly(p));