The one and only reason why EMI is “ not precise”, or “uncertain” is exactly one
piece of equipment in the measurement chain…. The EUT and it’s physical set-up. *All* other aspects can very well and relatively easy be controlled under a MU system, allowing customers to compare quality and performance of different labs and and judge and compare the quality of measurement alternatives. Defining a MU figure also implies that a round robin test using a well defined EUT will get identical results everywhere +/- the margin defined by the labs uncertainty figure. It also makes it possible by accurately reproducing a real EUT setup in another lab to get comparable results. I agree that this round robin test is not the definition of daily EMI work. In real life you WILL have an undefined EUT with as many degrees of freedom of operation and setup as you can recognize, resulting in an uncertainty well beyond the MU figures this discussion is about. But that is no reason to neglect the accuracy specifications of the defined chain. There is also an additional skill required in EMI work, that is finding and characterizing cause of emissions or lack of immunity. Often a lack of EMC is caused by a loophole in the design, and a simple fix may virtually make the emissions disappear to such a degree that discussions about a few dB seem ridiculous. That is real engineering work and should not be confused with the art of measuring for which MU is meant. In Europe the EMC requirements (read limits) in the standards have been raised to an importance level (not by the European Commission but by the market stakeholders united in CENELEC/ISO/IEC) where an excess of 0.1 dB can make the difference between pass/fail and market-access or no-market-access. Hence a decision of enormous economic value. Lab shopping has been a popular method to gain access for equipment that marginally failed. An unwanted situation that disturbs a level playing field. That is why the EMC labs have been brought under the regime of ISO 17025 (also a market driven standard) and metrology entered the labs to force them to create reproducible measurements . Metrology assessors never saw an EUT and do not understand the EMI measurement chain. And if you are used to calibrate VOLTS and meters (SI !!!!), the V/m do not seem so very different. So they imposed their methods upon us. And for each of the parts they recognized, they imposed us to make a MU. How right they were and how wrong they are. But one part cannot be subject to metrology methods: the EUT. (never available when an audit is made) But that does not imply that you cannot gain in quality and performance by applying a relatively simple procedure to the inaccuracy figures your equipment providers supply you with. And it makes you think about the impact of all components in the chain that affect accuracy. But that is not the only thing to be said about EMC , as Derek emphasizes. Gert Van: [email protected] [mailto:[email protected]] Namens [email protected] Verzonden: woensdag 11 augustus 2010 18:00 Aan: [email protected]; [email protected] Onderwerp: Re: Calibration supplier for signal generator with pulse modulation Hi Deniz, this is where "real" EMI guys differ from Metrology guys. We really don't need to be piddling with little numbers.... with the exception of frequency. Most of these modulations are compromises, and measuring a compromise accurately is a silly thing to do: in reality. You may want to get on board with the proof of the pudding phrase, it's OK in EMI: otherwise we expend all sorts of effort for no VALUE in return. The compliance world is obsessed with meeting a number, when the REAL reason for doing this testing is to ensure successful operation in use. I strongly disagree with Gerts statement MU is simple, it's not. Of the 140+ labs I've visited as an assessor, only a handful have a valid effort, and less than 1/2 doz believe it was of real value. Precision and EMI do not go together. I for one like it like that. Whats really needed is a competent individual with adequate equipment. If a committee would like MU, then as test labs we should isolate the cost that adds, and identify it on peoples invoices when that test is run. It's an effective way of making the standard unpopular. The reason I'm on a soap box about this is because unless silly requiremnts like this re CHALLENGED, and not just carte-blanche accepted, they become requirements. For calibration, I believe MU is useful, but for EMI, it has no place. Sincerely, Derek. From: Deniz Demirci <[email protected]> To: Untitled <[email protected]> Sent: Wed, Aug 11, 2010 10:35 am Subject: RE: Calibration supplier for signal generator with pulse modulation Could you define how precise? What is your acceptance criteria for the modulation dept? Let's say for basic 80 % AM modulation. Is your tolerance 0.1 % or between 60 % to 90 % is good enough. There is more than a dB peak difference in the signal for 60 % AM and 80 % AM modulations. You won't realize the difference with a field probe in radiated immunity tests with modulated signal. (Simple math; 20*Log(1.6/1.8) = -1.02 dB) It seems not very insignificant to me Another case; If you are using 6 meter cable for radiated emission at 10 GHz, your cable characterization uncertainty is more than 1.5 dB even with a quite high grade cable (Experimental measurements). Check the cable vswr figures in their specs measured in ideal conditions. Even a simple cable is a significant uncertainty contributor. Your measurement antenna is not exactly 50 Ohm at 10 GHz also. Site imperfection is another story. Those figures are not accounted for in the MIL-STD-461 RE102 verification. Everything seems to be very good when you terminate the measurement cable with a signal generator (50 ohm) output. I don't agree with this <quote> an ultimate "proof-of-the-pudding," </quote> You can use all your engineering skills for the tests but you have to quantify your risk using type B measurement uncertainty analysis and do the Type A if you can afford to see where you stand. As Mr. Gremmen said " MU is not difficult" No need to be afraid, 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 From: [email protected] [mailto:[email protected] <mailto:[email protected]?> ] On Behalf Of Ken Javor Sent: Wednesday, August 11, 2010 7:55 AM To: Untitled Subject: Re: Calibration supplier for signal generator with pulse modulation Precisely. Ken Javor Phone: (256) 650-5261 > From: Cortland Richmond <[email protected]> > Reply-To: <[email protected]> > Date: Wed, 11 Aug 2010 08:27:34 -0400 > To: emc-pstc <[email protected]> > Subject: re: Calibration supplier for signal generator with pulse modulation > > Not quite what you need to know but I've used signal taps or directional > couplers with a 'scope to watch the RF waveform and set modulation depth, > and a calibrated counter, or even a receiver or analyzer will for > frequency. One always has recourse to calibrated devices to monitor > another, uncalibrated one. It's a good idea anyway! If a detector must be > used whose linearity is unknown, one can use calibrated attenuators to find > out. > > > Cortland Richmond > KA5S > > >> [Original Message] >> From: Wendy Nya <[email protected]> >> To: <[email protected]> >> Date: 8/10/2010 4:23:29 AM >> Subject: Calibration supplier for signal generator with pulse modulation >> >> Dear All, >> >> I am looking for a calibration supplier that can provide accredited > service for >> AR SG6000. It has built-in pulse modulation option (for Radiated Immunity > use). >> >> By the way - Is anyone using this model? It seems to be OEM from Agilent > (it >> came in an agilent box) but the equipment is marked AR >> and works with Agilent N5181A driver. Agilent is saying that it is not > able to >> provide accredited calibration service for this model. >> >> Thanks & Regards, >> Wendy Nya > > - > ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society emc-pstc > discussion list. To post a message to the list, send your e-mail to > <[email protected]> > > All emc-pstc postings are archived and searchable on the web at: > http://www.ieeecommunities.org/emc-pstc > Graphics (in well-used formats), large files, etc. can be posted to that URL. > > Website: http://www.ieee-pses.org/ > Instructions: http://listserv.ieee.org/request/user-guide.html > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > Scott Douglas <[email protected]> > Mike Cantwell <[email protected]> > > For policy questions, send mail to: > Jim Bacher: <[email protected]> > David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at http://www.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]>
