AVR-Chat

--- In AVR-Chat@yahoogroups.com, David VanHorn <dvanhorn@c...> wrote:

> [in reply to my statement] For best EMI
> performance ... you need to form a grid

> Well ... I have a lot of success with ...
> providing one and ONLY one low impedance
> path back to the source, and not allowing
> RF current to spread out and take all
> possible paths.

The problem with this is as follows. If the return path is not close 
to the signal path, you are making a loop antenna. This both radiates 
EMI and picks up EMI from other places.

The way that a ground plane works is to provide a return path for 
every signal that is very close to the signal path. The signal in the 
signal layer trace actually causes the high frequency components of 
the return current to flow right underneath the trace even though the 
ground plane covers the board.

But if you don't have a ground plane, what is best? First, make the 
antenna as small as possible. This means routing ground close to the 
signal. But, with signals running all over the place you can't do 
this unless you have ground all over too, which ends up being a grid. 
Second, split the signal to go in opposite directions around two 
adjacent antennas. Both radiation and pickup then cancel. This also 
ends up being a grid.

EMI control techniques can sometimes be counter-intuitive. I hope the 
above explanation is helpful.

Graham.