Re: [SI-LIST] : What's your favorite Screwy SI Concept?

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From: Jon Powell (jonp@pacbell.net)
Date: Wed Jan 05 2000 - 16:12:49 PST


Doug Wrote:
Screwy SI Concept #2: The 20H Rule

Why It's Screwy: Highly debatable concept and it's
                      not really an SI concept at all.

Why It's Hung Around: Easy to remember

Jon Writes:
About 6 months ago I did a test to see what was going on with this "20H"
rule. I simulated some simple ground planes with simple sources using a
3D simulator. (I am not making any claims, just telling what I saw), the
results: the 20H boards quickly put all of their energy into the
enclosure. And the regular one retained the signal (a lot longer) Now,
perhaps this doesn't sound like I needed a simulation to know this but
this is my guess:

1) If you have a good enclosure, (or don't care about EMI off the board)
th 20H rule can reduce noise on the board. (And perhaps EMI off the
connected cables?)
2) If you don't have an enclosure, and you care about EMI off the board,
the 20H rule may not be a good thing.

--
Jon Powell
Director of HSSD Consulting Services
Viewlogic Systems, INC.
805 988 8250

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