I can only agree with Tom, limits are limits. We can assume that all factors of uncertainty have been considered when the standards were written (see the statistical method required in some and the lack of reference to measurement accuracy). So far I have not heard anything to the contrary from the competent body organizations.
matthias r. heinze TUV Rheinland ---------- From: Thomas N. Cokenias[SMTP:[email protected]] Sent: Wednesday, October 09, 1996 11:29 AM To: [email protected] Subject: Re: dBmargin Acceptable margin depends on what the measurement is for. Are you checking out a unit that is going to be sent to FCC Lab for sampling? Are you trying to qualify subassemblies or components from new vendors? Is this test another in a series of identical tests on identical equipment? Most US and EU limits are just that, limits. Stay under them and you pass. Statistical analysis might show that to maintain a certain confidence level that the nth unit will also pass, you may need to maintain a 6 or even a 10 dB margin. In a sampling situation, the regulatory agency or the party with the power to approve (maybe a customer) must account for normal site to site variations, say +/- 2 dB per site, so a measurement margin of 2 dB may be adequate, but risky. Over the years I have seen customers require margins of 10 dB, 6 dB, 4 dB, 2 dB, and 1 dB for a given test to be a "pass". The higher the margin, the better the EMC engineering, the greater the confidence level, and the better the perceived quality of the product. All this comes at a cost, and it's ultimately up to those who pay the bills to decide the level of risk they can afford. If it's one number you need, I would reluctantly put forth a 2 dB margin, subject to the caveats discussed above. I appreciate your willingness to collect this info and look forward to the results of your poll. Best regards, Tom Cokenias RFI/EMC Consultant
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