Re: [Help-gnucap] can gnucap output impedance as a function?

[Dan McMahill]

al davis wrote:
> On Monday 02 April 2007 17:32, Wen wrote:
>
> > Is there a a possibility to make gnucap output the impedance
> > of a part of an electric circuit as a function of the
> > frequency?
> > it would output something like
> > z(freq)=
> > R1/(1+(R1*2*pi*freq*C1)^2)+R2/(1+(R2*2*pi*freq*C2)^2) + j*
> > (-(R1)^2*2*pi*freq*C1/(1+(R1*2*pi*freq*C1)^2)-(R2)^2*2*pi*fre
> > q*C2/(1+(R2*2*pi*freq*C2)^2))
>
>
> > Is gnucap able to give me such output? If not, can you
> > recomend another free program thats able to do that?
>
>
> No.

ages ago there was a program called "xfunc" which would do this.  I've 
tried to track down the author to see about bringing xfunc back to life 
but without any luck.

> I have thought about a way to do it, based on something like the 
> AC analysis with a "polynomial" type instead of "complex".   I 
> think it is fairly simple in concept except that the complexity 
> of the result builds up fast.  When you look at it you see 
> common expressions that can be eliminated.  That's hard to do.  
> Also, when you do it manually you will drop insignificant 
> terms.  That also is difficult to do.

this is correct.  xfunc is only good for smaller networks.

Unless you have a good starting point, this optimization can be rather 
ill conditioned.  If you have an ok starting point a circuit optimizer 
may be the easiest.  Not as satisfying as really trying to apply some 
filter theory, but it can get the job done.  You can probably code up 
something fairly quickly  in your favorite scripting language and use 
gnucap for the circuit calculation.

-Dan