This excerpt from Mr. Demirci‘s message below is the kernel of the issue:
“I believe a proper measurement uncertainty assessment will decrease the different measurement / test results between accredited laboratories for the same product. Then, manufacturer / designer will not shop for a laboratory which he / she will get an easy pass test report.” In the days of OATS and ANSI C63,4, an NSA correlation of +/- 4 dB sufficed. In the days of SACS and FACS, some sort of similar measurement is clearly required. But this facility certification in no way extends to mundane measurements that are made with an ordinary instrument such as an o’scope, or handheld meter, or for that matter, an EMI receiver. Those measurements are controlled by using a calibrated device, and even better, much better: the measurement system integrity check in MIL-STD-461 in which a calibrated input signal corresponding to a level 6 dB below the limit is applied at the transducer end of the measurement system, and the EMI receiver must measure that signal accurately within +/- 3 dB. That is the ultimate “proof-of-the-pudding,” with the exception that it doesn’t account for chamber effects for radiated emissions. There are a couple reasons that military and automotive and commercial aerospace EMI testing do not require measurement uncertainty. One is the ancient dictum, “If you can’t stand the answer, don’t ask the question.” That is, we know a priori, before performing the exercise, that one meter separation radiated measurements from an extended test set-up in an imperfectly anechoic chamber are not going to result in pretty uncertainty numbers. The reason that this is ultimately acceptable goes back to the above excerpt >from the message below from Mr. Demirci. In the commercial world, you must EMI qualify before going to market. In that case, you need all EMI test facilities to be equal. In the world of military, automotive and aerospace, the EMI qualification is usually done as a collaborative effort between vendor and customer, and is performed after a decision has been made that the customer will do business with the vendor. That is all background. The unalterable fact, regardless of how MU has been misapplied and perverted, is that it applies to the measurement facility itself; something that inherently will have an uncertainty on the order of several dB. It does not apply to things that can be measured to tenths of a dB, or are not even spec’d in dB, such as the modulation waveform of a signal generator, or its frequency accuracy. Ken Javor Phone: (256) 650-5261 ________________________________ From: Deniz Demirci <[email protected]> List-Post: [email protected] List-Post: [email protected] List-Post: [email protected] Date: Tue, 10 Aug 2010 10:39:37 -0700 To: <[email protected]> Conversation: Calibration supplier for signal generator with pulse modulation Subject: RE: Calibration supplier for signal generator with pulse modulation I guess the equipments in Apollo project hasn’t only pass the tests with no margin :-) They may have required quite a big margin so MU is not an issue at all… Commercial world can be quite a bit different in that matter. They can’t afford 10 dB margin. Has anyone seen a consumer electronic product will pass 10 dB Class B emission limits ? I believe a proper measurement uncertainty assessment will decrease the different measurement / test results between accredited laboratories for the same product. Then, manufacturer / designer will not shop for a laboratory which he / she will get an easy pass test report. OOO (Own opinions only) Best regards, Deniz Demirci National Technical Systems (NTS Canada) Phone: 403-568-6605 ext 244 fax: 403-568-6970 email:[email protected] web: http://www.ntscorp.com/about/locations <http://www.ntscorp.com/about/locations> <http://www.ntscorp.com/about/locations> From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Tuesday, August 10, 2010 11:07 AM To: [email protected]; [email protected] Subject: Re: Calibration supplier for signal generator with pulse modulation HI John, just stressing Ken's use of the word unwarranted. I'm personally vehemently opposed to MU in EMC measurements: the reason why is that end to end even 10 dB is a trivial error... Just because the Stds committee says so does not make it warranted. All MU contributes is an academic exercise and lines pockets of individuals/organizations pushing it. We as an EMC community need to push back and say enough already. Our efforts are needed to address more important problems. Just for the record, Automotive EMC does not require MU, US Military to not require MU. If MU wasn't needed to put a man on the moon it sure as heck isnt needed in washing machines, computers etc. I'm guessing this is more than 10 cents worth! Derek Walton L F Research. From: John Woodgate <[email protected]> To: [email protected] Sent: Tue, Aug 10, 2010 11:46 am Subject: Re: Calibration supplier for signal generator with pulse modulation In message <c886e31f.841a0%[email protected]>, dated Tue, 10 Aug 2010, Ken Javor <[email protected]> writes: >Uncertainty for modulation parameters of depth or pulse on-off ratio >and time duration? Uncertainty applies to things like field intensity, >where the construction of the room or OATS and near field effects >combine to provide significant uncertainty. > >But something easily measured with an o'scope of modest (audio) >performance? That is a totally unwarranted extension of the concept. > Unfortunately, it isn't. The metrologists have inflicted formal uncertainty assessment on us and it's like death and taxes - always with us. Everything has to have its uncertainty assessed. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk <http://www.jmwa.demon.co.uk> <http://www.jmwa.demon.co.uk> and www.isce.org.uk <http://www.isce.org.uk> <http://www.isce.org.uk> John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK If at first you don't succeed, delegate. But I support unbloated email http://www.asciiribbon.org/ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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