I have a copy of a CBEMA (Computer and Business Equipment Manufacturers' Association) report from 1977 that includes the complete development of the CE and RE limits imposed by the FCC. The limits are based purely on the effect on a radio receiver. I have myself verified that the limits are well-placed in the conducted case. I think any concern about measurement accuracy should NOT be built into the limits. The limits themselves contain a lot of slop. The limits are based on maintaining a particular signal-to-noise ratio which in turn means a particular level of signal from the broadcast station. Also, different radios have different sensitivity when receiving the same rf input, that is for the same rf input to the front end, differing amounts of rfi cause the same amount of interference. I have myself verified this across a sample of over 30 radios with large variations from sample to sample but the mean was exactly 48 dBuV, which is the CBEMA-recommended and FCC mandated CE limit. With significant variation in the response of individual protected victims, an excessive focus on minor variations in the repeatability of measurements is unproductive and misplaced emphasis. Incidentally, several years ago I determined that the response of these radios to common mode rfi was 20 dB more sensitive than to differential mode rfi, because the storage capacitor in the radio's power supply acts as a filter to dm conducted interference. I proposed changing CE limits and measurements to reflect this difference in sensitivity, but no action was taken.
---------- >From: rehel...@mmm.com >To: emc-p...@majordomo.ieee.org >Subject: RE: EMI guard bands >Date: Thu, Dec 20, 2001, 6:03 AM > > > After listening to this thread, it brings up a question that I have had > over the years regarding limit lines > and passing margins. > > Are there "EMI guard bands" already built into the limit lines? > > I find it very difficult to believe that a group of reasonable people > developing a limit line would have > determined that they had thought of every possible EMI interference issue > and would not have built a > "safety margin" into that limit. > > Does anyone have any historical development data or insights into the > creation of limits? > > Bob Heller > 3M Product Safety, 76-1-01 > St. Paul, MN 55107-1208 > Tel: 651- 778-6336 > Fax: 651-778-6252 > > > > ------------------------------------------- > 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: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: pstc_ad...@garretson.org > Dave Heald davehe...@mediaone.net > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > No longer online until our new server is brought online and the old > messages are imported into the new server. > ------------------------------------------- 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
