Re: A question regarding unwrap

[Doug Stewart]

Doug Stewart wrote:

> Joe Koski wrote:
>
> > on 9/24/05 7:44 PM, Doug Stewart at address@hidden wrote:
> >
> >  
> >
> > > Ron Crummett wrote:
> > >
> > >   
> > >
> > > > Hi -
> > > >
> > > > I have a recent problem with unwrap; I tried plotting the phase of a
> > > > transfer function and it jumped halfway through the plot, from -180 to
> > > > 180.  I tried to unwrap the phase using unwrap and plotted the phase
> > > > again, but still had the same jump.  Reading the help on unwrap, it
> > > > sounds like it will only really work if the phase is positive (it adds
> > > > 2*pi).  For me, I have a negative phase, so I need to subtract 2*pi.
> > > > Any idea of what the solution could be?  I tried looking at the code
> > > > but couldn't make too much sense out of it from my few glances.
> > > >
> > > > Thanks,
> > > >
> > > > Ron Crummett
Here is my new and improved version.
(better help)

try this "filter" with bode.m and with my  bode1w.m

n=poly([ -1+100j  -1-100j   -2+150j -2-150j  -3-200j -3+200j])
d=poly([-1 -2 -3 -4 -5 -6 ]*500)
bode1w(n,d)
s=tf2sys(n,d)
%bode(s)

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     bode1w (n,d)
%n= numerator
%d=denominator
%
% This was written to help my students with Gain Margin and Phase Margin
%GM and PM lines are added to the Bode plot and
%GM and PM values are printed on the screen.


## bode1w

## Author: doug
##
## 2004-09-27 doug
## * Initial revision
## Rev #1  Nov 29 2004 Doug Stewart
## Rev #2 Sept 2005 Doug Stewart
##              - Added the If for negative phase steps.
##              - and the DO loop

function     bode1w (n,d);

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


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




# 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







-------------------------------------------------------------
Octave is freely available under the terms of the GNU GPL.

Octave's home on the web:  http://www.octave.org
How to fund new projects:  http://www.octave.org/funding.html
Subscription information:  http://www.octave.org/archive.html
-------------------------------------------------------------