Re: gEDA-user: 3 questions on a 4 layer board

[Rick Collins <gnuarm.2007@xxxxxxxxx>]

At 11:57 AM 4/29/2008, you wrote:

> > Turns out that for the purpose of impedance a power plane is exactly
> > the same as a ground plane.
>
>That's true unless you have a split power plane.

Only if the split power planes are nowhere near the ground plane.  If 
there is significant overlap and the ground plane is not split at the 
same location as the power planes, then the power planes AC couple to 
the ground plane and continue to provide impedance control to the 
signals.  It turns out that in most designs it is hard to create a 
power plane that *is not* a good AC ground.

BTW, it is also very important to put the power and ground planes as 
close together as possible.  This increases the capacitance which is 
useful at very high freqs.  No coupling capacitor is very useful 
above 200 MHz or so.  Beyond that you need to have tightly coupled 
power and ground planes to provide a low impedance to power spikes.


>The whole field of high speed PCB layout is much more complex than is
>warranted on this list, which is why I made a sweeping generalization.
>I think for people who don't know better (and don't need to know
>better), keeping ground near most of the signals is a simple rule that
>will cover most of the problems.

It *can* be complex.  But most of it is actually very simple.  What 
seems to make it complex is the large number of (sometimes 
conflicting) "guidelines" that are not based in fact.  That is why 
the Lee Ritchey course was so valuable.  Everything he discussed in 
the class, he had analyzed in theory, modeled in simulation and 
tested in practice with measurements.  The only thing he said in 
three days that I didn't find 100% correct was which restaurant was 
best in the area.

I find that when I stick to the old rules that I had learned before 
this illuminating course, I only make my life more difficult, 
*especially* when doing layout.

Rick




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