I read in !emc-pstc that George Stults <george.stu...@watchguard.com> wrote (in <ea1b3684d553cbc96c23d33894bdfe4c3c350...@watchguard.com>) about '-2dB margin', on Thu, 3 Jan 2002: >After talking with some other folks, I've found that one can concoct a 2 dB >margin from CISPR 22 [the section I mentioned below] as follows: Using the >formula for statistically assessed compliance "Xavg + K*StdDev < Limit", >assume that the StdDev is 1.0 and that you are initially testing one device >as permitted. Then take the value of [K =2.04] for [n=3] and you have a >2.04 dB margin requirement for the initial device. The standard also >requires that if you just test one device, that you do subsequent tests from >time to time. So in effect, by going with a 2dB margin the first time, >you're betting that the statistics will still be in your favor by the time >you've tested 3 samples. Does anyone disagree that it could be done this >way?
It seems a reasonable explanation. > >An additional question would be, how is "from time to time" defined. Is it >spelled out anywhere? Possibly not, but there is a principle that you can invoke. It was developed in the context of safety testing in service, AFAIK, but it works for other things. What you do is to choose an initial re-test period based on how many units you produce per unit time and how much unit-to-unit variability you expect. If you find that re-testing at this rate throws up failures, re-test more often. If it throws up no failures and not even marginal cases, re-test less often. -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk After swimming across the Hellespont, I felt like a Hero. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
