I think you had just the right amount of coffee today. Your observation is
good, if the return planes for vertical and horizontal routing are not the
same net, you essentially loose control of the return path every time you
make transition between the routing layers. Since transitions often happen
close to IC's ther is usually a lot of decoupling around, but somtimes there
are clusters of transitions away from any circuitry, and much crosstalk can
happen there.

For several years now I have used board stackups consisting of closely
spaced substacks of 3 layers with the return plane in the centre as far as
possible. The substacks are then separated with layers that are more than 2x
the distance between layer internal to the substack. This approach resembles
what you are suggesting, and it has worked very well for me in terms of
noise control and signal integrity. Applications span wireless networks, FM
radio, sensitive analog measurement circuits and GBethernet over copper.
(also switching power supplies creating large noise currents at 100 MHz
close a couple of cm away from a FM receiver with no loss of sensitivity).

I also use a single GND if at all possible, and only use GND as return. The
GND layers are stitched as tightly as space will allow, and I always use GND
stitching around the edge of the board.

Several of our customers have remarked that boards we have re-developed for
them are remarkably more quiet than earlier versions and that it has had a
real impact on perfromance. 

I know everyone cannot afford such extravagant use of copper, but I warmly
recomend the approach you suggest, and it can be realized in other ways than
the one I have suggested here.


Med vennlig hilsen/Best regards
________________________________________________
Haldor Husby, Senior Development Engineer
Data Respons Norge AS
Kongsberg Næringspark
P.O. Box 1022
N-3601 Kongsberg, Norway

Tel: +47 32 29 94 00 	Fax: +47 32 29 94 40
Dir: +47 32 29 94 18 	Mob: +47 48 04 83 68
[log in to unmask] 

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