Oops, forgot to include another potential source of
variation...
I've had test labs here in the Bay Area that are incredibly
close to the ideal attenuation data, YET...
They produce variations when testing actual equipment - up
to plus minus 6 dB! The variations don't seem to track
bands, or bandwidths, but do kind of seem to track sources.
Explanation for such wild variations in testing results
seemed to relate to the "impedance" of the source and how
that impedance interacts with the site. Antennas being 50
ohm systems are well behaved, but real life sources being
some kind of obtuse values, well just don't act the same.
That "explanation" for the magic involved worked for me, so
I just kept it as a reference. However have had clients
that historically have "shopped" for beneficial sites end
up coming to me as they updated their product line and
started failing miserably. They really regretted that
original choice of shopping around.
- Robert -
On Fri, 25 Mar 2005 10:30:45 -0800
"Dan Anchondo" <[email protected]> wrote:
> US/Taiwan EMC discrepancyGeetha
> You could also have a third lab with a BSMI cert run a
> scan on all suspect
> frequencies and compare lab data. A few hundred bucks to
> the lab and a
> couple hours set up and scan at the lab. You will have a
> better picture and
> save some bucks as long as you reproduce your set ups and
> focus on the exact
> frequencies. or...
> The Taiwan lab will not change their report so I would
> just use the ferrite
> and let it go at that. IMHO.
> Regards
> Dan
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of
> [email protected]
> Sent: Thursday, March 24, 2005 2:14 PM
> To: [email protected]
> Subject: US/Taiwan EMC discrepancy
>
>
> I was thinking about the recent US/Taiwan EMC issue
> discussed here and I
> thought I would apply "Inference to the best explanation"
> to it. There was
> much conjecture about what is going on but we still have
> no more than the
> original evidence.
>
> Inference to the Best Explanation:
>
> Evidence:
>
> a.. E1: The device passed at the US lab
> b.. E2: The observed signal frequencies, at the US
> lab, correspond to
> known clock frequencies
> c.. E3: The device failed at the Taiwanese lab
> d.. E4: The observed signal frequencies, at the
> Taiwanese lab, do not
> correspond to known clock frequencies (they are
> different)
> e.. E5: The device passes at the Taiwanese lab when a
> ferrite is placed
> on the USB cable
> f.. E6: The identical device was tested at both labs
>
> Hypotheses (rank ordered by me from best to worst):
>
> a.. T0: The US lab is correct and the Taiwanese lab
> is in error
> b.. T1: The Taiwanese lab is correct and the US lab
> is in error
> c.. T2: The device has changed between the US lab and
> the Taiwanese lab
> d.. T3: Both labs are in error
>
> How one ranks the hypotheses depends on how they rate
> the evidence.
>
> Dave Cuthbert
> Micron Technology
>
>
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