Re: gEDA-user: Trace impedance calculations in PCB

[Gabriel Paubert <paubert@xxxxxxx>]

On Thu, Feb 12, 2009 at 01:27:37PM -0500, Stuart Brorson wrote:
> > Are there any plans to add trace impedance calculations to PCB?  If
> > not, is there an argument against it or just that no one has time and
> > interest?  What alternatives do people use?
> 
> As Joe pointed out too, PCB doesn't not know about the metal layer
> separation distance, nor does it know the dielectric constant of your
> insulating material.  Therefore, it cannot do impedance calcs.

It's even insufficient, in my last microwave board the distance to 
the cover slightly affects the impedance (but also the coupling between
tracks). Actually the cover is very low  above the circuit, 1mm
except in the area of the largest capacitors and above the microwave
signal tracks where it is machined. 

It is really 3D and depends on things that PCB does not even
want to know...

Typically I also work the otehr way around. I compute all the physical
parameters for the impedances I need and then I set the line width
and clearance accordingly.

Now I would like very much to have a way to tell PCB that two nets
form a diffential pair and to be able to draw them at once.

	Regards,
	Gabriel


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