Physical questions

Jack Wathey wathey at salk.edu
Tue Jan 21 19:38:05 EST 2003



On Tue, 14 Jan 2003, Robert G. Brown wrote:

> On Tue, 14 Jan 2003, Ken Chase wrote:
>
> > Without metal you get way too much RF. We found that stacking nodes
> > really close together vertically with no shielding caused many to not
> > boot properly (seemingly the most sensitive time) and many to crash
> > randomly (perhaps 10% of nodes every 2 weeks, mysterious crash).
> >
> > Moving them to twice the distance between, my guess, reduced the crash
> > rate by a factor of 8ish (cube of distance? :)
>
> Hmmm, sounds like some unsurprising mutual inductance in the GHz range,
> doesn't it?  Reminds me of the very old days and my first IBM PC.  I
> used to work listening to the FM radio in these pre-ogg days when I
> didn't have my entire collection of music on my hard disk.  If I was
> tuned to anywhere in the vicinity of 100 MHz while the PC was running,
> the radio buzzed like crazy.  Don't ask me why -- it had a 4.77 MHz
> clock, IIRC, so it must have been harmonics of some sort.  I could
> clearly hear the computer "work" -- the buzz was nicely modulated when I
> ran a small floating point program, for example.  Made me truly believe
> that someone could read your keystrokes off your power supply line or
> local system EMF with sufficiently delicate equipment a la
> Cryptonomicon...
>

Aside from the risk of fire from a non-OTS beowulf enclosure, are there
any risks to life and property if the thing leaks RF as discussed above?
Can a shelf of radiating motherboards slowly cook (like a microwave oven)
the poor nerd standing in front of it?  Can it interfere with
communication/navigation equipment in low-flying aircraft?  My hope and
expectation is that the power of such RF signals is MUCH too low to do
such damage, but it would be comforting to hear an authoritative answer
from someone who knows more physics than I.

Along those same lines... Is there a handy-dandy (and not too expensive)
gadget for measuring RF emissions from an electonics enclosure in a
standard, quantitative way?  I'm hoping for something that can tell me
whether or not a prototype or DIY enclosure is sufficiently RF-tight that
it would pass the official certification tests.  Are those tests simple,
or way to complex and expensive for an amateur to duplicate?

Best wishes,
Jack


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