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 

Ian Wilson

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