Required Margin for EMI?

Jim Hulbert Mon, 26 Oct 1998 23:52:38 +0000

     This topic can open a can of worms.  (Sort of like the class B vs. 
     class A argument for Europe.)
     
     I don't believe there's a margin of compliance with the limit spelled 
     out in any regulations or standards.  The standards set a limit and 
     leave it up to the manufacturer to meet that limit.  EN55022/CISPR22 
     does refer to the statistical method of assuring with 80% confidence 
     level that 80% of the product type is below the limit, but this 
     requires testing at least 5 product samples.  I think most companies 
     prefer to test 1 sample for qualification and then from time to time 
     perform an audit test on a production sample taken at random.
     
     Given a test sample size of 1 and the measurement variables 
     (measurement uncertainty, product to product variations, test site to 
     test site variations, testing repeatability, ...to name just a 
     few...), I believe a 2 dB margin is not good enough to ensure your 
     product would pass a random audit test.  At least not for radiated 
     emissions.  (I might accept that for conducted emissions, which is 
     inherently a more stable and repeatable measurement.  However, I would 
     probably insist on a larger sample size than 1 if the margin is only 2 
     dB.)  In general, for compliance measurements, I prefer to work with a 
     5 dB or greater margin with the applicable limit.
     
     Jim Hulbert
     Senior EMC Engineer
     Pitney Bowes

______________________________ Forward Header __________________________________
Subject: Required Margin for EMI? 
Author:  <[email protected] (Scott Douglas) > at SMTPGWY
List-Post: [email protected]
Date:    10/26/98 12:50 PM

Hello all,

When doing EMI scans of ITE, I have always worked with a margin of 2 dB 
less than the actual limit. No test house I have ever been to will give a 
"meets the requirements..." without having this margin. Can anyone point 
out if this is an actual requirement built into any of the standards, i.e. 
EN 55022 or FCC Part 15, etc. and if so, where is it in writing?

I understand about measurement errors, product to product differences and 
repeatability of testing issues. Just want to know if the 2dB margin is 
actually written into law.

Thanks for your comments.
Scott
[email protected]

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Date: Mon, 26 Oct 1998 12:50:00 -0500
Subject: Required Margin for EMI?
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Required Margin for EMI?

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