Jack,

I've always gotten away with just assuming there are enough decoupling
caps throughout the board to consider power planes as identical to DC
ground.  

Your message got me thinking decoupling caps may not be the reason this
works.  In a standard six-layer "sandwiched routing layer" stackup, it's
easy for a trace on a vertical layer to switch to a horizontal layer
through a via not near a decoupling cap.  
TOP
GND
Vertical
Horizontal
PWR
BOTTOM
Perhaps it helps in this scenario that a) some of the return currents
can travel on the plane not immediately next to the trace and b) there
is some capacitive coupling between planes.  

You could solve this potential problem by putting the two planes in the
center, especially if GND and PWR are close enough (3 or 4 mils) to
provide good capacitive coupling.
TOP
Vertical
GND
PWR
Horizontal
BOTTOM.
But now there's no stripline (internal) traces AND you have to think
about Top-Vertical and Bottom-Horizontal crosstalk.

I don't know a scientific way to determine which is better.  It seems
like we always end up weighing the various tradeoffs, and the values of
the various tradeoffs are reckoned by more handwaving than science.  

Handwaving Through the Fog,

CJ
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
Olson
Sent: Thursday, January 04, 2007 9:51 AM
To: [log in to unmask]
Subject: [DC] Straddling the Return Path

I have a question about layer stackup and routing.

Most of our designs have a pair of routing layers
sandwiched between planes, one for vertical and
the other for horizontal traces.
Maybe I haven't had enough coffee yet today, but
I started wondering how the return path energy
gets from one plane to the other (unless there
happens to be a decoupling cap nearby?) If both
planes were GND there wouldn't be a problem,
since they would be stitched together in many
places, but one is GND and the other is PWR.

So I started wondering.... Wouldn't it be better to
have one routing layer ABOVE the GND plane and
one BENEATH? In this scheme all the routing would
be straddling the SAME return path and it seems
logical the it would be less noisy. There would never
be a discontiuity from the energy trying to get from
one plane to the other, right?

maybe the gains are not worth the loss in the
overall scheme, though?

onward thru the fog,
Jack

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