Date: Thu Jun 22 2000 - 08:44:15 PDT
With today's fast edge devices, unless you are adding extra shielding to
either a flex cable or a simple surface ground fill, FORGET IT.
Crosshatched ground planes were reasonable at ~50 MHz and below ONLY where
balancing of copper coverage levels were necessary to minimize board warpage.
The cross-hatching has the effect of increasing losses/damping and will
increase the generation of localized voltage drops developed in the plane;
this is a BAD thing as it excites common-mode antenna effects. Unless the
size of the crosshatch grid is minute (but then why do it?), the routing of
traces with turns or bends of any kind will abruptly (albeit on a small
scale) alter the flow of the return current (since it must follow the grid
traces) and cause more (usually unpredictable) propagation delay AND more EMI
problems through both radiation (if a surface trace) and cross-coupling
between signal traces.
YES, increased line impedance would be achieved, but the potential negatives
far outweigh the positives.
Henri Merkelo has developed and described techniques for altering/optimizing
line impedances for a full decade now. He's a brilliant guy that has reduced
theory to practice (i.e., the very essence of engineering) to achieve
superior system performance. Mark Gill's comments are right on target (i.e.,
that "methodically modifying the return plane with apertures" can tailor line
impedances); however, this systematic approach is very different than just
using a crosshatch.
Michael L. Conn
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