Passing is passing. The limit is passing. Curtis-Straus will write you a report which says you pass if we observe zero dB or greater margin. Just don't expect to pass with great certainty on the next sample.
I would strongly suggest that you consider requiring a margin if the product will evolve and need to be retested or if you sell to a systems integrator who may have his/her system tested. But it is up to you. 4 dB is a nice margin. Now for the interesting comments: Consider that some are lobbying for a new limit which is lower than the old limit by the uncertainty of measurement. There has always been an uncertainty. In fact, it's been going down as the techniques of EMI measurement become better understood. And emissions problems have been fairly benign from equipment tested to the limit with the older measuring techniques (those with high uncertanty). What this discussion needs is a quantification of the old uncertainty. For argument let's say it's 7dB. If we want to take uncertainty into account in a pass/fail decision, we should compare the emission to the limit plus 7dB minus the actual uncertainty. In other words, discussions of uncertainty should be viewed as a way to raise the limit while maintaining the protection objectives of various regulations. Or, uncertainty was never considered before, so consideration of it now without a limit increase is in fact a limit decrease. Since the existing limit has been working to control interference for some time now, any decrease is likely to be a brake on economic growth (it'll cost more to comply) with little benefit to the community (no reduction in non-existant emissions problems). Jon Curtis. Chris Dupres wrote: > EMC Folk. > > I have been reading all the learned submissions of what constitutes the > acceptable emission limits for EMC purposes. Most of you are very clever, > very technical, and I'm in awe of all of you. > > But there seems to be a bit of a missed point here. EMC in Europe relates > to the EMC Directive, which was born of the SIngle Market arrangements > between Euro States, and which were born of the Treaty of Rome way back > before my kids were born. > > The ultimate purpose of EMC Standards/limits is to provide a level trading > platform for Euro countries, so that all conditions are equal in the market > place, and that no-one can steal a lead over someone else by dropping > technical standards and therefore saving costs and putting cheaper goods on > the market. It follows that perhaps we should look at these limits in the > same way that the packaging industry looks at filling cans and bottles, or > the way car drivers treat speed limits. i.e, that the EMC emission limits > are a target in absolute terms, and if you can show honest intent in > achieving them, then the legislation has achieved it's aim. > > If I carry out an honest emissions test on a piece of equipment, and the > graph is below the line by the thickness of the pen, then I believe that > the spirit of the EMC Directive has been met. If this acceptance level was > an absolute amount, such as money in banking, then I would allow a % for > measurement error, but it isn't, it's an objective. No-ones head is going > to explode if the emissions are 0.5dB over limit, and in all honesty > dropping the emissions by 0.5dB can usually be achieved by moving a cable > or snapping on a ferrite sleeve. Hardly enough to change the whole balance > of trade in Europe is it? > > So, if I carry out properly conducted tests, with the equipment working > normally, and it shows emissions right on the limit, then I think the EMC > Directive has been followed, and the equipment can be CE marked with > honesty and placed on the market. > > Just a tuppence worth (what's THAT in Euro's?) > > Chris Dupres > Surrey, UK. > > --------- > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to [email protected] > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to [email protected], > [email protected], [email protected], or > [email protected] (the list administrators). -- Jon D. Curtis, PE Curtis-Straus LLC [email protected] Laboratory for EMC, Safety, NEBS, SEMI-S2 and Telecom 527 Great Road voice (978) 486-8880 Littleton, MA 01460 fax (978) 486-8828 http://www.curtis-straus.com --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
