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

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From: Brent DeWitt (bdewitt@ix.netcom.com)
Date: Tue Dec 21 1999 - 15:56:47 PST


This is interesting to me, so I'd like to ask a couple of questions to help
my understanding:

> 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.

By "lower frequencies" do you mean frequencies at which the skin depth is
significantly larger than the thickness of the return plane?

> 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.

Since I am an EMC engineer rather than a serious s.i. engineer, my first
thought is that the trace length involved would have to be rather large for
the balanced stripline return currents evident on the outside of the return
plane to be a significant radiator, although I've certainly been wrong
before!

Best regards,

Brent DeWitt
Datex-Ohmeda
Louisville, CO

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