Re: gEDA-user: basic anti-EMI design q

[sdb@xxxxxxxxxx (Stuart Brorson)]

DJ --

[ ... snip! ... ]

> So, the big question is - ground/power planes.  I could probably
> squeeze the design onto two layers, but power and ground traces would
> be going all over the place.  Going to four layers gives me power and
> ground planes, with easier signal routing.  

I usually do 4 layers.  You're going to send this board out to a fab
house, right?  They can do four layers, and the cost delta is at most
two digits.  So why waste time and increase risk to save two digits of
cost? 

If you're still etching your own boards at home, then two layers is
a constraint.  But if you're sending stuff out to a fab house, I
say you should just go to four laters.  Why be a hero to save $50?
Nobody cares that you've stuffed everything into two layers except
some hard-core pcb fanatics.  And if you screw up your two layer
layout, then you're out the $X00 you spent on the boards. 

> An auxiliary question is,
> should I split the planes?  I'm thinking, isolating the power/gnd
> going to the I/O drive FETS and thermo power (i.e. the stuff going
> around the house) back to a common point near the power supply, and
> maybe splitting the 10baseT analog power similarly.

The latest thinking on this is: Use one plane.  Keep analog and
digital components physically separated, but don't split the plane.

If you split the plane, you run the risk of running tracks over slots
and other GND structures in your board which can radiate/receive & can
contribute to SI problems.   Yes, with extremely careful planning you
can eliminate these problems, but how careful do you want to be?  Or
how careful can you possibly be?  If you have digital and analog
signals on the same connector, or on the same part anywhere, then you
end up gerrymandering the GND planes to accomodate the two signal
types, and you have the risk of getting into SI trouble.  Better to
have the one ground plane, and not worry about it. 

This was the wisdom imparted at the last National Semiconductor design
conference I attended, and it makes sense to me.  (I also got to see
Bob Pease there, so it was a good conference.)  

Here are some references about this issue:

http://www.hottconsultants.com/techtips/split-gnd-plane.html
www.national.com/appinfo/ adc/files/questweb_dec_2001.pdf

Stuart

p.s. I will try your latest and greatest PCB version with the
autotools fix tomorrow.  Thanks for working on this!