Morning Geetha,
Although it certainly makes sense to measure the uncertainty of measurements (?!?) it remains the question what measurements are exactly evaluated. The CISPR 16-4 evaluation is made using a standard EUT ( antenna used during site attenuation measurements) , standard cable and standard equipment, which on their turn are regularly calibrated and of which uncertainty is known. A simple addition (??) is required to calculate the total uncertainty as you will find a very detailed example in CISPR 16-4. You will also note that the Site attenuation (+/- 4dB) is the dominant factor in the calculation and if your antenna is 2 dB off or your measurement receiver 1dB out of CAL, will NOT have a major impact on the result ( unless you have a VERY smooth site) So now you know that your lab can measure ONE type of EUT (send antenna) with the claimed uncertainty. This says not much in how accurate YOUR EUT will be measured. YOUR EUTand it’s cabling is part of the test chain, and unlike a simple voltmeter to a calibrator, the interface >from EUT to the emission test SET UP is unknown, and at its best accurately documented. What you do know howver, is that any measurement error (can any emission measurement error actually be called an error ??) is not caused by the equipment and room that the measurement was made in. I will not disagree with any of you that recording measurement accuracy is a relevant parameter, but one needs to know it’s meaning and even more, the limitations of such a figure. I definitely disagree with you that measurement uncertainty is a key element in emission measurements. The arguments as delivered in this topic earlier are of much more relevance to the final result as uncertainty. Gert Gremmen _____ From: [email protected] mailto:[email protected]] On Behalf Of Taylor, Michael Sent: woensdag 23 maart 2005 16:30 To: Geetha Balasubramanian; Grasso, Charles; Bob Richards ; [email protected] ; [email protected] Subject: RE: Radiated Emission Testing uncertainty I have watched this subject unfold with interest. However it seems the one key element at the heart of this interchange has been left out of the discussion. That being Uncertainty of measurement. We are assuming that the lab in question did perform signal maximization of the cables and did set up the EUT correctly. The purpose of CISPR 16-4 (2002) " Uncertainty in EMC Measurements", was to help bring the issue of measurement deviations between different measurement sites into some kind of relational understanding. In Paragraph 4.1 it states: "...... the expanded measurement uncertainty for the test lab shall be stated in the test report" This is the only way a customer has to evaluate whatever margin to limit exists with that particular product. To the best of my understanding most labs do not state their uncertainty of measurement for reasons best known to them. When I send out a product that I can't do in my lab I insist on adherence to CISPR 16-4. If the uncertainty of measurement is not included in the report its of little value to me. If more customers insisted on this information (backed up by real numbers in the calculations) these sort of problems would resolve themselves in short order. Just one guys opinion. Michael Taylor - NCE Loveland, CO _____ From: [email protected] mailto:[email protected]] On Behalf Of Geetha Balasubramanian Sent: Tuesday, March 22, 2005 8:52 AM To: Grasso, Charles; 'Bob Richards '; '[email protected] '; '[email protected] ' Subject: RE: Radiated Emission Testing Hi Gurus, As per the US lab report, the frequency at which the device had minimum pass margin is 68 MHz. At 68 MHz in vertical polarisation the margin is 4.7 dB & in horizontal polarisation the margin is 1.6 dB. Measurable emissions were found only in 40 , 43, 52.38, 64 & 68 MHz. But as per the Taiwan lab report no measurable emission is found >from the device at the above frequencies. The device failed in 122.61 MHz at horizontal polarisation. There are 2 clock sources in the device, one runs at 12 MHz and the other runs at 4 MHz. Frequencies showing up in the US lab report are atleast the harmonics of these 2 frequencies. But frequencies mentioned in the Taiwan lab report are looking funny (111.27, 135.3, 285.69 MHz. etc.) US lab tested in OATS but the TAIWAN lab tested in a anechoic chamber. I heard that chambers are not the correct way to measure ( always the results will be wrong) that is why FCC never accepts chamber measurement. Is it so? Thanking you all for the time & efforts spent on this. Thanking you all in anticipation Geetha "Grasso, Charles" <[email protected]> wrote: Hi Bob, Don't you ALWAYS maximize cables. I found that with cable maximization my test repeatabilty improved dramatically. HAs cable maximization fallen out of favour? From: [email protected] To: [email protected] Sent: 3/22/2005 6:58 AM Subject: Re: Radiated Emission Testing It is not an unusual occurance. This is one good reason why you want a good margin under the limit. Most people like to see a margin of at least 6dB. One variable is the site itself. A good OATS is the best. Sites can have up to 4dB variation from theoretical, and one chamber I've been involved with had a problem in the 50-60MHz range, where it was 3.9dB out -- barely acceptable. You might want to ask to see the site attenuation data from the labs involved. There can ! also be a big difference depending on the measuring distance. Some standards allow extrapolation to be done when measuring at some distance other than the distance spelled out in the standard, but looking at the difference in theoretical site attenuation between a 3m and a 10m site will show that linear extrapolation is not accurate through much of the frequency range. It is not unusual to get different results at the SAME lab, depending on the person conducting the test. Like it or not, some technicians are better or more diligent when it comes to maximizing the emission by playing with cable placement. When I worked at a compliance test lab, I ticked off a few clients when their product was passing by 2dB until I moved the cables and rescanned only to fail them by 1dB. Bob Richards Square D. --- Geetha Balasubramanian wrote: > Dear Experts, > > One of our products had b! een tested by an > accredited lab at US on May 2004 and found to be > compliant with FCC, CE & VCCI. We started mass > manufacturing the product and it is being sold in > the US, Europe & Japan markets for the past 6 > months. Then came a requirement for that product in > Taiwan market. Since the May 2004 testing at the US > lab was not run in a BSMI approved system it became > necessary for us to retest the same product. This > time the product has been tested in an accredited > lab in Taiwan. The Taiwan lab tests are showing that > the product is not compliant to the CISPR22 and need > a ferrite core in the USB cable to make it compliant > to the standards. Product tested by the US lab & > tested by he Taiwan lab are absolutely identical. > As an engineer responsibled for compliancy I > am a bit confused now. How to resolve this issue? I > request you experts to g! uide me. > > Sincerely > > Geetha Balasubramanian > This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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