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Re: [Help-gnucap] Modal analysis using gnucap

From: Jasper
Subject: Re: [Help-gnucap] Modal analysis using gnucap
Date: Wed, 22 Sep 2010 08:55:06 +0200

Thanks for both of your suggestions for trying NGspice. I did but so
far it has not been a success. Under OSX the PZ analysis segfaults.
Under my Ubuntu Virtualbox it works somewhat. I can obtain the correct
pole-zero values for up to 3 RL element in series with Cs to ground in
between. For more the output values become garbage. But that problem
is probably for the NGspice mailing list.
Up until now I did the analysis in Matlab which works well for a
single line. But for more complex networks with branches /
recombinations setting up the matrices becomes very hard. In Matlab I
would build a state space system relating charge (q), current (dq) and
its derivate (ddq) which would look something like this:
[ dq] = [         0         I ][ q] + [ 0        ]
[ddq] = [ -inv(L)*C -inv(L)*R ][dq] + [ inv(L)*x ] u
With the system I can then determine the frequency response etc. With
the matrix I can then solve the eigenvalue problem for getting the
poles and eigenmodes. Are similar matrices generated in gnucap or is
the ac analysis performed differently? If you have any references on
the internal working of the circuit analysis that would be
appreciated. Best regards,


On Tue, Sep 21, 2010 at 7:50 PM, al davis <address@hidden> wrote:
> On Tuesday 21 September 2010, address@hidden wrote:
> > Very cool application.  Pole-zero analysis is not supported
> > in gnucap 0.35, but I don't know about the development
> > version of gnucap. NGSpice can do it though (.PZ command) .
> There you go recommending NGspice again ..   I must confess that
> I also say try NGspice in this case.  Gnucap doesn't (yet) have
> PZ analysis.
> It hasn't been a priority, so I have no idea when/if it will
> ever be done.  With the development version, it could be done as
> a plugin, that builds on AC like Fourier builds on transient.
> On the other hand .. things like noise ...  someone (not me) is
> working on it.  It seems to be taking longer than expected.
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