Tony,
Radiated immunity tests set up a situation where only the unloaded
field is known. You're correct we cannot know for sure what is
actually getting into the EUT. If we had to know what current was
actually entering the ports we would need to monitor it real-time.
For radiated emissions, we maximize emissions -- within the allowable
configuration -- by movement of cables and peripherals. This would
(if we did it properly) eliminate cable placement as an issue. We
would always end up with the maximum emission possible for a
configuration. However, that's' not so easy to forecast and results
depend on the person doing the test. It does NOT eliminate
site-to-site or lab-to-lab differences. Using a different kind of
headphone cable may shift results 10 dB. Can we call this
"measurement uncertainly"? It's possible to keep this error down on a
given site by use of a uniform set of peripherals, but that does not
help those of us who need to compare our results against those
gathered elsewhere.
Regards,
Cortland
The above does not reflect opinions or policies of my employer
______________________________ Reply Separator _________________________________
Subject: Re: Measurement Uncertainty
Author: Tony Fredriksson <[email protected]> at internet
List-Post: [email protected]
Date: 1/13/97 10:00
Jon,
Thanks for the info on the NIST Technical Note. Looks like I need
to get a copy and undertand it.
It is good to know that Curtis-Straus estimates uncertainty.
I am particularly interested in the EMC side of the discussion
and have a couple of questions:
1. In the case of Immunity, I suppose the lab would only be able
to estimate uncertainty of the disturbance from the test generator.
Is that correct? Wouldn't the loading of the signal source from
EUT have a dramatic affect on the test result uncertainty? If this
is the case, how is it factored in such that uncertainty of the end
result is quantified to any practical degree?
2. In the case of EMI, what is the range of uncertainty that one of
your tests can provide? I would think it is a function of frequency.
Does it attempt to take into account the uncertainty due to a change
in cable or preripheral placement from one setup to the next of the
exact same EUT? If so, how was that uncertainty derived?
The reason that I am curious about this is that I have seen some cables
so hot (headphone on a CD ROM port for example) that moving them an
inch or two in either direction can vary emissions by 10dB or more. That
would seem to be quite unpredicatable by statistical methods and would
seem to dwarf any uncertainties from other sources. This even considers
test setup methods that have been designed to minimize test variation
(such as ANSI C63.4).
I can see that using the lab's stated uncertainty in combination with
a CISPR 16 style sampling test would be a significant improvement over
other procedures.
Thanks,
[email protected]
----------
From: Jon D Curtis
To: Barge, Michael
Cc: '[email protected]'
Subject: Re: Measurement Uncertainty
List-Post: [email protected]
Date: Monday, January 13, 1997 9:14AM
In the USA, NIST has published Technical Note 1297 1994 ed. "Guidelines
for Evaluating and Expressing the Uncertainty of NIST measurement
results."
Our NVLAP accreditation requires us to estimate uncertainties in our test
reports. Every Curtis-Straus EMC or Telco test report contains
uncertainty
estimates. As to the passing margin, passing is passing and failing is
failing. Before you take measurement uncertainty into the limit, first
consider that technique has improved (and therefore unceratinty is lower)
than it was when the limits were formulated. Second consider that the
regulators which accepted the limit were well aware that uncertainty
exists and in all likelyhood accounted for it in their choice of the
limit.
That said, I advise all clients who are within our uncertainty of the
limit (but passing), that they should be aware that they may fail next
time.
If they are at the prototype stage, or building a product which will
become the platform for future development, it is advisable to seek a
larger margin.
Jon D. Curtis, PE
Curtis-Straus LLC [email protected]
One-Stop Laboratory for EMC, Product Safety and Telecom
527 Great Road voice (508) 486-8880
Littleton, MA 01460 fax (508) 486-8828
http://world.std.com/~csweb
On Fri, 10 Jan 1997, Barge, Michael wrote:
>
>
> FROM: [email protected]
>
> Item Subject: Measurement Uncertainty
>
> Greeting Tregers;
>
> There seems to be a requirement that, when giving a measured value, there
> must be an uncertainty associated with that value describing the
confidence
> of that value.
>
> (1) Do most labs report an uncertainty measurement in the test report,
on
> the data sheet, on a certificate of compliance?
> (2) How did you generate the measurement of uncertainty for emission
> tests? For immunity tests?
>
> AND MOST IMPORTANTLY
>
> (3) What do you tell the customer when he is below the limit by less
than
> the measurement uncertainty? When he is above by less?
>
> J Michael Barge
> Alliant Techsystems
> Annapolis, MD
>
