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Re: gEDA-user: problem

Stuart Brorson wrote: 
Is there a free software tool to perform a layout extraction to get the
parasitics associated with the layout?


For IC design/layout, I believe that the open-source tool Magic will
extract the parasitics to a SPICE netlist. 

For PCB layout, there is no tool, at least as far as I know.  The
reason is that the PCB fab process is less well characterized & more
variable than the IC process.  Also, real models for the parts used
don't exist.  Most op-amps, etc., provide only macromodels, which
don't capture this kind of info.

If in fact magic does (which I'd be suprised if it did or if it does I'd be suprised if it is anything beyond some very basic stuff), then it should be pretty easy to adapt what they do to board layout. Anyone around with inside knowledge of magic who can comment on this?

Be careful of what you mean by "the parasitics". In the IC parasitic extraction tool I use a lot, you can do C only, R only, R+C, RLC, and RLC with mutual inductances. For C extraction you can lump all capacitances to "ground" (substrate) or do coupled capacitances (trace to trace). There are all sorts of knobs to turn to control how lines are broken up for resistance, how to deal with vias, etc.

In terms of C extraction, which is often times (but certainly not always) the most important I _think_ the way this particular commercial tool works is to run a field solver (like fastcap) on a whole bunch of structures and then use the results as part of a lookup table when you run on your actual design. That way the actual extraction is not running any field solvers but rather interpolating between field solutions. The up front run of the field solver can take several days on a fast machine while the actual run on a moderately sized analog layout is just a handful of minutes.

I actually think such a tool would be most useful even though I generally don't bother much with board level simulation. I do wonder how it should all fit together. Anyone here have experience with any of the commercial board level parasitic extraction tools who could comment on how the extraction tool interfaces in with everything else?

-Dan