bd_asymp.m

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function [wpos,ypos]=bd_asymp(G,w)
G1=zpk(G); wpos=[];pos1=[];
if nargin==1, w=freqint2(G); end
zer=G1.z{1}; pol=G1.p{1}; gain=G1.k;
for i=1:length(zer);
    if isreal(zer(i))
        wpos=[wpos,abs(zer(i))];
        pos1=[pos1,20];
    else
        if imag(zer(i))>0
            wpos=[wpos,abs(zer(i))];
            pos1=[pos1,40];
        end, end, end
for i=1:length(pol);
    if isreal(pol(i))
        wpos=[wpos,abs(pol(i))];
        pos1=[pos1,-20];
    else
        if imag(pol(i))>0
            wpos=[wpos,abs(pol(i))];
            pos1=[pos1,-40];
        end, end, end
wpos=[wpos w(1) w(length(w))];
pos1=[pos1, 0,0];
[wpos,ii]=sort(wpos); pos1=pos1(ii);
ii=find(abs(wpos)<eps); kslp=0;
w_start=1000*eps;
if length(ii)>0,
    kslp=sum(pos1(ii));
    ii=(ii(length(ii))+1):length(wpos);
    wpos=wpos(ii); pos1=pos1(ii);
end
while  1
    [ypos1,pp]=bode(G,w_start);
    if isinf(ypos1), w_start=w_start*10;
    else, break; end
end
wpos=[w_start wpos];
ypos(1)=20*log10(ypos1);
pos1=[kslp pos1];
for i=2:length(wpos)
    kslp=sum(pos1(1:i-1));
    ypos(i)=ypos(i-1)+kslp*log10(wpos(i)/wpos(i-1));
end
ii=find(wpos>=w(1) & wpos<=w(length(w)));
wpos=wpos(ii); ypos=ypos(ii);