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

From: Doug Smith ([email protected])
Date: Thu Jan 06 2000 - 10:27:43 PST

Hi Jon,

I would tend to build up a test board to try something like this. I
learned a long time ago not to trust simulators unless I could build (or
calculate by hand if possible) a similar but simpler test case and get
the simulator to agree with calculations or measurement. I build a lot
of test circuits and do measurements on them. I call that "putting the
problem on the analog computer." Many of the test boards or other
apparatus I build eventually end up in published papers or my courses as
demos.

I have seen cases where simulation output was believable, but not
accurate because of some problem, either an input problem (garbage in ->
garbage out) or a problem within the simulator itself.

Given the above, does anyone know of any "hard" data to support the
simulation?

DougS (as opposed to DougM who posted the original 20H item)

Jon Powell wrote:
>
> 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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