Mitch Morey wrote ...... (in part)
"Bob,

I only wished it was this easy. If it was I wouldn't have a weekly arguement
about controlled impedence.

I only put #3 above because I can't seem to get a "true" value for the dielec-
tric constant from our three main fab houses. No real problem because we allow
them to modify our trace widths and stackup to give us back our impedence
required, BUT with speeds in excess of 1.2 Ghz now, we are asking for much more
precise information. Our circuits will just not work correctly with the "fidge"
factor any longer."


Exactly ...............

Dielectric "constant" changes from board to board, from layer to layer, and
even within a layer. And, as Mitch points out, it is frequency dependent, so
it even changes with signal characteristic.

If really tight controlled impedance is necessary, we believe the only
answer is to have the fab house on board at the BEGINNING of the project
(rather than after the design has been completed.) A really good fab house
will know its processes well enough to help you define what you need to do
to hit your targets.

Another example of where lowest bid doesn't necessarily mean lowest cost!!

Doug Brooks
President
UltraCAD Design, Inc.
http://www.eskimo.com/~ultra/homepage.htm