Response to Tony Fredriksson's comments: See below.
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 Mon, 13 Jan 1997, Tony Fredriksson wrote:
>
> 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?
For radiated immunity, we do a Type B uncertainty evaluation on the test.
We only
consider the factors from the test equipment as we feel our uncertainty
results from the level of the field without the EUT. The effects of the
EUT on the field will vary by EUT (size, dimensional resonances, etc.) but
between samples of individual EUTs these factors should be well behaved.
We do
ignore cable position as a factor, relying on the fact that we test 4
different sides in two polarities to give us some statistical protection
in this area.
The major contribution to uncertainty then becomes the leveling of the
field which is +-3dB. Since this is a MUST be within 0dB to 6bB, we use
Uj as 3dB and divide it by square root of three assuming a rectangular
distribution. See the NIST note.
>
> 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?
It could be a function of frequency. Certainly antenna factors and site
anomalies are generally better behaved above 200MHz. To date we use a
simple
uncertainty using estimated worst case results across the frequency
spectrum. As factors we have: antenna factor: +-1dB, NSA: +-4dB (ours is
really max 2dB so we use +-4dB, but assume a traingular distribution on
this factor), Equipment factors: SA flatness 1.5dB, cable calibration:
.3dB, test method variance (cable manipulation: +-2dB, normal
distribution). Add it up using Root-sum-of-squares=2.9dB uncertainty:
Note that there are additional factors based on the assumed distributions
of the uncertainties.
>
> 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).
Yes, cable manipulation done properly is still the over-riding factor in
repeatibility. I am able to obtain 2dB repeatibility with attention to
cable manipulation to maximize emissions at each frequency. If you don't
maximize the cables you will see variances of at least 10dB between EUT
setups that appear identicle.
>
> 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
> 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
> >
>
