From help-octave-request at bevo dot che dot wisc dot edu Thu Jun 26 08:06:41 2003
Subject: Re: Bode weirdness
From: Przemek Klosowski <przemek at jazz dot ncnr dot nist dot gov>
To: help-octave at bevo dot che dot wisc dot edu, octave@wylch.fastmail.fm
Date: Thu, 26 Jun 2003 09:06:25 -0400 (EDT)


   What you see is probably the phase jumping from -180 to 180 or visa 
   versa. There is no difference between a phase angle of alpha and 
   alpha+360, so it plots everything in a comfy range on your screen. It 
   also at a glance shows you where possible stability snags might be in a 
   feedback loop.

   Not an error.

Unfortunately, in the examples brought out, the phase jumps from X to
X+180, as you can check by replotting the recalculated phase:

    [mag,phase,w]= bode(tf2sys([1],[1,1,1,1]));
    plot(phase)
    phase(phase<0)+=180
    plot(phase)

I don't know enough about the bode analysis to say whether such phase
reversal has physical consequence or not (if it does, then either
Octave or Matlab is giving a wrong result; if it doesn't then it is
just a matter of presenting the result, just like in the case of a phase
shift by 360 degrees).



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