From: [email protected] (Bejan Afsharpour) Subject: Re: dBmargin 6 dB is a de-facto margin everyone references when testing only one or two samples. This does not mean that if you have less than that, you put the product on hold. It only means that if you have 6 dB , you are DONE. However as Paul Zahra mentioned, if you have less margin -again depending on how much less-you have to verify that the sample you tested represents a larger population of tet samples. And the only way to do that is to use more samples and statistically prove that you are OK. I believe CISPR 22 has the methodology for statistical analysis. If not VDE 0878 part 3 has it for sure. Theoretically you can ship the product with 0.2 dB of margin if you wanted to. Personally will not do it, but I know of some people who do. Even with small sample size. Another factor that is critical when making decisions on the margins is the system itself. For example, if you have an enclosure with only couple of cables and a power cord, and you find that your emissions characteristic is such that the cables do not contribute much into the overall picture, then I would be more apt to take margins less than 6dB. Another example would be systems I am dealings with, which can have 12-60 UTP (unshielded Twisted Pair) wires plugged into it and is very susceptible to cable movements. UTP is literally an antenna if you have any inballance in your Differential pair driver. And I have yet to see a perfectly balanced driver. So in such system that even one bad driver can make a drastic change, more units need to be tested to justify approval.
This is my opinion. However the 6 dB rule has been the same at least for the past 10 years I have been dealings with EMC. I have also found that the opinions differ slightly between the consultant groups and the manufactures groups. Naturally the manufacturers have to be more lenient because of the famous SHIP IT rule from the engineering departments. I hope this helps. Bejan At 06:00 PM 10/9/96 -0700, [email protected] wrote... > A well known EMC consultant in Silicon Valley who is now employed in a > well known company here presented (when he was still an independent > consultant) a very well justified case to show that the total > uncertainties for EMC test equipment, site attenuation, weather, > manufacturing tolerances, operator variances, test methodology, etc., > were such that a minimum 6 dB margin is justified. 10 dB would be > better. > > Lets hear it from all of you other consultants, since, I suspect, many > in this industry would not like to proclaim publicly just what the > situation is in our companies. > > Tania Grant, Octel Communications Corporation > [email protected] > > >______________________________ Reply Separator _________________________________ >Subject: dBmargin >Author: [email protected] at P_Internet_mail >Date: 10/9/96 2:59 PM > > >I don't know about all of you, but I am sick of arguing with >hardware engineering managers about what constitutes >a passing EMC test and what does not. > >I have seen companies impose a 6dB margin requirement, >some 5 dB, and some 3dB. > >I have made the recommendation that the minimum here >should be 3dB. I thought that this was fair. Now comes >the product that has 2.8dB margin. Being a stick by the >rules type of person, I listened to the engineer explain >that it was the lousy PC his card was in not his card. So >I suggested that we prove his theory, purchase a new >machine and check the old machine versus the new >one and if there is an improvement, that I would let >the 2.8dB stand. > >Of course the manager of the group tells me that as >a recommendation 3dB is good, but as a rule it >is IRRESPONSIBLE. Thus I end up with the "be >a b___ch" option of imposing the retest for the .2dB >or starting a precedence of well if .2 is okay, is .3 etc. >etc. > >What I would like to do is take a pole. (must be election >year in the USA!!!!!!!!!!!) I would like all of you to respond >as to what you feel is appropriate. Then I'll run the >stats and let you what the results are. This way I can >go back to the manager with the number of certification >experts that responded and what they thought was right. > >He or I will have a hard time arguing against stats. >(I am open to the possibility that I am wrong and >0 margin is acceptable). > >Just reply with a number and I'll let you know what happens. > >Thanks >Cynthia > >[email protected] > ======================================= Bejan Afsharpour EBU EMC Engineering Egineering Group Manager 4401 Great America Pkwy MS SC7-02 Santa Clara, CA 95052 (408)495-1537
