RE: [SI-LIST] : Inner vs. Outer Layer Routing

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Tue Dec 21 1999 - 10:52:18 PST


>Chris, if you are comparing differential microstrips vs differential
>striplines it should be obvious to you that stripline, being surrounded
>by grounds would radiate less.
 
Wait a minute ... maybe it's just me, or the phase of the moon, but this
isn't obvious.

For the differential component, I agree. For the common-mode component, I'm
not so sure. In microstrip, both paths of the common-mode component are
exposed to the side of the board, both might radiate, out-of-phase, and
cancel one another in the far field to some degree.

In stripline, the common-mode component on the signal traces is shielded
from the outside world. At high frequencies, the return path takes the
inner surface of the return plane, and is shielded by the thickness of the
return plane from the outside world. But at lower frequencies, the return
path, and ONLY the return path, might be exposed to the outside world, into
which it alone can radiate. No cancellation of radiation components as you
could get in microstrip.

There may be some leaps of faith in these assumptions, but it seems that the
potential could exist for stripline to maybe cause more radiation than
microstrip. This seems flawed, but I'm not sure how.

Andy

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