When I worked for Wyse Technology we did perform audits of manufactured products. Each week we would audit one product. Statistical analysis was performed per the 80/80 rule. We only used the data points within 6 dB of the margin for the formula. Each frequency point within 6B was of the limit was treated as an entry. x-sub-n is is each entry point. This was done for conducted emissions as well as radiated emissions. (remember to apply either qp or avg as applicable, do not use peak) You need a spread sheet program or you'll be calculating for days!
Best Regards P.S., you need a minimum of three samples (the standard reccomends 5) to make it meaningfull, i.e. you need a k value for one or two samples but those rise astronomically, so three is an acceptable minimum for statistical purposes. ______________________________ Reply Separator _________________________________ Subject: Re: Measurement Uncertainty Author: Non-HP-Tony-Fredriksson ([email protected]) at hp-boise,uugw2 List-Post: [email protected] Date: 01/13/97 03:04 PM Jon et al, I helped to implement production audits per CISPR 16 @ Sun Microsystems back in 1989. When I left in 1995, they had discontinued doing the audits for ongoing production, but were still doing 5 unit audits on pilot samples. I am not sure if they do it any longer. I felt that the program improved the quality of their products and kept them out of trouble with the larger VARs retesting their systems. Regards, [email protected] ---------- From: Jon D Curtis To: HANS_MELLBERG Cc: emc-pstc; Michael_Barge Subject: Re: Measurement Uncertainty List-Post: [email protected] Date: Monday, January 13, 1997 5:09PM Dear Hans, I know of no manufacturers actually engaged in series production audits. So lets hear from them. Please respond to this forum. The companies I work with look to CISPR 22 8.2.1.1 and test one sample. Some of them are happy with 0dB margin. I advise a higher margin, but they are responsible for signing the DoC. To date it would appear to me that the 80/80 rule only has a place in making it harder to take product off the market. You can go to market with only one sample tested, but if someone wants to restrict your access they have to perform an 80/80 rule statistical test to say you fail (CISPR 22 8.2.4). As a test lab, I'd love the 80/80 rule if the market would support it (three-five times the testing, yippee!). The doctrine also seems to need a bit of clarification: Xn is refered to as the value of the individual item. Is this the value of the one point closest to the limit? Can you change the frequency? On a product do you evaluate more than one frequency? How many? - the six closest to the limit? When doing more than one test, are several 80/80 tests performed - one for radiated, one for conducted? The 80/80 test is a statistician's dream and a test engineer's nightmare. 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 [email protected] wrote: > > I take exception with the statement "passing is passing and failing is > failing". CISPR 16 and 22(section 8.2.4) (maybe others too) require that > during manufacturing sampling, the products pass the so called 80/80 rule. > A minimum sampling of 3 units is required to perform this 80/80 calculation > and products with minimal margin will discover that they fail the formula > test! Go ahead and try a sample hypothetical test! > > > ______________________________ Reply Separator _________________________________ > Subject: Re: Measurement Uncertainty > Author: Non-HP-owner-emc-pstc ([email protected]) at > HP-Boise,mimegw2 > Date: 01/13/97 05:14 AM > > > 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 > > >
