Re: gEDA-user: Compiling MMI (electromagnetic simulator)

[Dan McMahill <dan@xxxxxxxxxxxx>]

On Tue, Dec 14, 2004 at 01:19:08PM +0000, Daniel Nilsson wrote:
> Karel Kulhavy (clock@xxxxxxxxxxxxx) wrote*:
> >
> >On Sun, Dec 12, 2004 at 09:29:10AM -0500, Daniel Nilsson wrote:
> >
> >[...]
> >
> >> Here's a good collection of links (including fastcap):
> >>
> >> http://www.fastfieldsolvers.com/links.htm
> >>
> >> What problem are you trying to solve ?
> >
> >I am trying to determine how wide a trace must be to have impedance of
> >50, 75, 93, 100 ohm and also how much capacitance per length my ordinary traces
> >will have.


> To calculate the impedance (with reasonable accuracy for slower speed applications)
> you can just use impedance formulas such as the ones found here:
> 
> http://www.emclab.umr.edu/pcbtlc/

both the formulas in the ultracad pdf file and the ones listed there are fairly
approximate.  For an online tool, http://mcalc.sf.net is the most accurate I
know of.  Of course I'm a bit biased.  That said, I spent quite a bit of time
looking for the "best" approximations.

For mcalc, you'll get Z0 and keff.  From that you can calculate L and C.

> I get asked the question about total capacitance on a trace rather often but I have
> still not heard anyone explain to me how that number can be of any practical use. If

You may care about capacitive loading.  Also if you have say a 4 layer board
with gnd and vdd as the inner layers, you may care about parasitic coupling
from a trace to vdd.

-Dan

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