On Tue, Dec 14, 2004 at 01:19:08PM +0000, Daniel Nilsson wrote: > Karel Kulhavy ([EMAIL PROTECTED]) 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 --
