G'Day all.
I have been wondering about this (the 20H rule for planes) for some
time. I have been trying to think of practical situations (rather than
theoretical) why there may be some advantage.
First off - I agree that if both planes are equally noisy then keeping them
the same extents is likely to be the best. However this may not be the
case where both planes are not equally noisy.
Lets say that in one portion of a board I have a component with +3.3V plane
connection close to the edge of the plane but *no* nearby connection to the
GND plane (or more carefully stated - the gnd connections are further from
the edge of the plane). Allow me the further condition that maybe the main
earth return to the power supply system is in completely the opposite
direction - so the bit of circuitry we are thinking about is located in a
corner well away from any other stray main supply return currents.
So we may have a situation where the gnd plane at the very edge of the
board is not as noisy as the power plane - just in that localised area. So
in this situation there may well be a good reason to extend the gnd plane a
little beyond the noisier power plane - but only in that small region.
So, if this were the case then what we should really be doing is
considering each region on a region by region basis, attempting to
understand the paths of the induced currents on the planes and then
determining which is going to be noisier in that micro-region. Sounds
messy and in my experience more likely to make things worse than
better. Just too fiddly to get it right with sufficient reliability to
make the 0.1dB improvement in EMC worthwhile.
Me - I am keeping my planes equal until I see reliable experimental
evidence to the contrary, including full disclosure of the experimental
conditions.
Ian Wilson
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