There are two factors to take into account when determining the minimum margin for compliance with an EMC emissions limit. The first is the level of uncertainty of the measurement. This number should be in your test report from your lab. If not, ask for that number. This is the amount that the particular EUT needs to be below the limit in order to guarentee that it will pass. (If the lab does not have the number, or cannot produce it, find another lab fast.)
The secend issue relates to the 80/80 rule. This can be done in two ways. First by measuring a sufficiently large sample to statistically be confident that 80 percent of the devices will pass 80 per cent of the time. However, this can become time consumming and expensive. The second method is to make a resonable assumption on how much variation you could expect from the device being evaluated. This also assumes that the test lab performed a worst case measurement. The manipulated all of the cables, etc. The Measurement Uncertainty usually has a value between 4 to 6 dB. An assumed value allowing for variation in the device can be anywhere from 3 to 6 dB. Therefore, the minimum margin would be about 7 dB and can be up to 12 dB, or more. There is no hard and fast rule as to what you should use for your margin. You will have to establish that quantity depending upon your situation. John Shinn, P.E. Manager, Lab Operations Sanmina-SCI -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of Russell Sent: Wednesday, February 27, 2002 1:41 AM To: emc-pstc Subject: EMC Emissions Safety Margins What safety margin below the statutory limits for emc emissions should one aim to achieve to ensure all product samples are likely to pass when a single, peraps two, samples only have been tested. Are there any hard and fast rules? Is being just below the limits considered good enough? Thanks in anticipation for your views and advice. Russell. ------------------------------------------- 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
