Re: how to find gain margin

[Doug Stewart]

Kai Shing Lee wrote:

>Does anyone know how to find gain margin in octave ?
>
>  
>

Here is one way to do it.
Doug Stewart


## Copyright (C) 2004 doug
##
## This program is free software; you can redistribute it and/or modify
## it under the terms of the GNU General Public License as published by
## the Free Software Foundation; either version 2 of the License, or
## (at your option) any later version.
##
## This program is distributed in the hope that it will be useful,
## but WITHOUT ANY WARRANTY; without even the implied warranty of
## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
## GNU General Public License for more details.
##
## You should have received a copy of the GNU General Public License
## along with this program; if not, write to the Free Software
## Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
%
%function bode1 (n,d)
%n= numerator
% d=denominator
%

## bode1w

## Author: Doug Stewart
##
## 2004-09-27 doug
## * Initial revision
## Rev #1 Nov 29 2004 Doug Stewart

function bode1w (n,d);

sys=tf2sys(n,d);
[mag,ph,w]=bode(sys);

# fix the phase wrap arownd
ll=length(ph);
[qm iqm]=max(abs(diff(ph)));
if(qm>300)
ph(iqm+1:ll)=ph(iqm+1:ll)-360;
endif


# make a 180 deg line
v=ones(ll,1)*(-180);
db=20*log10(mag);

# find the Gain Margin and its location
[x ix]=min(abs(ph+180));
if (x>1)
GM="INF"
ix=length(ph);
else
Gm=20*log10(mag(ix))
endif

# find the Phase Margin and its location
[pmx ipmx]=min(abs(mag-1));
PM=180+ph(ipmx)


#f=w/(2*pi);
semilogx(w,db,1,0,w,ph,w,v,[w(ix) w(ix)],[30 -180] ,[w(ipmx) w(ipmx)],[0
-200])
gset grid mxtics xtics ytics
replot

endfunction



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