Most of what was said below is true and I have to agree with the opinions expressed as well, but part needs serious critique. The part about exceeding RE limits and causing a problem to a non-radio receiver equipment is WRONG! No EMC engineer should make such a case!
If you scale in from 1, 3, or 10 m to a much smaller distance (inches) the scaling is highly non-linear. And I don't mean it goes as r^n, either, with n = 1, 2, or 3. As you approach the emitting source (typically a cable) the field intensity is bounded by the rf common mode potential on the cable divided by its separation from ground, or the rf potential along its length at higher frequencies. That number will always be a few millivolts per meter, max. Further, with a culprit and victim cable laying adjacent, the coupled noise potential or current will always be less (on average) than the source potential/current. Another factor people forget is huge disparities in RE/RI test setups. The RI test set up illuminates a 1.5 m square. A culprit RE emitting cable adjacent to another cable can couple the whole parallel length, but as noted above, the coupling signal is tiny. An aperture or slot radiator a few inches away only illuminates a tiny portion of the 1.5 m square and coupling is proportional to the victim physical aperture illuminated. Finally, the RI signal is 80% AM at 1 kHz, which is much more severe than the typically cw or near cw clock-based unintentional emissions covered by RE limits. Moral: Exceeding RE limits does not correlate with interference to other than antenna-connected receivers. ---------- >From: "Whitehouse, Terence (Terry)" <[email protected]> >To: "'Ken Javor'" <[email protected]>, [email protected], [email protected] >Subject: Let's not carried away (in a box) >Date: Wed, Jun 20, 2001, 7:23 PM > > Let's not carried away (in a box - as an innocent victim of a catastrophic > EMC event) is precisely Kyle's point. > > Originally, domestic and commercial EMC emissions standards were created to > minimize broadcast reception interference. More recently, a much needed > awareness on increasing the immunity of electronic systems to the massive > amounts of EMC polution that exists in our increasingly complex society has > thankfully made its way into the standards arena. Military and civil > avionics are similarly regulated. > > Notwithstanding, international aviation regulatory authorities require > management of personal electronics during "critical phases" of a commercial > aircraft's flight. (I might ask is there really a non-critical aspect of > flight when either accelerating down the runway, in flight at any height, or > decelerating to a standstill?) > > Immunity standards define electric field stengths between 1 and 10 V/m. > Clearly these levels are certainly far more severe than the fields created > by a just compliant class B product at 3 meter separation; 40 dBuV versus > 120 dBuV. Take into account a simple 10 dB distance compensation for a > reduced 1 meter separation between a laptop and wiring harness installed > under floor or overhead and add to it the "30 dB in excess of the limit" as > called out in this thread and the protection margin is significantly > reduced. > > Is this a problem? Frankly, I don't know the answer. But it is my belief > that our silence should not condone accepting a 30 dB error margin. > > A cell phone does intentionally emit at levels more closely approximating to > the immunity thresholds. A waiting room sharing a wall with a consultation > office might just provide a scenario where an instantaneous measurement > could be recorded artificially high or low with a potentially disastrous > outcome either way; preventative action not taken or mis-applied. > > I, for one, would not want to have my life put at risk by dependence on > poorly enforced requirements such as notices or requests to turn off > electronic emitters - be they intentional or unintentional. While 98% of > the population acts responsibly, it is always the 2% who cause all of the > problems with their selfish disregard for those around them. Cell phone > anecdotes are plentiful. Ever been seated in an aircraft where the person > in front of you reclines their seat immediately on take-off when the > attendants can't notice? Is that laptop actually switched off? (Ever seen > a loaded 18 wheel semi less than a car's length away at 65 mph?) Hmmm > ..... > > (BTW, I really do enjoy flying and always try to get a good window seat to > delight at the view below me. SFO to DFW routed over Southern Utah is > awesome on a clear day!) > > > Terry Whitehouse > Avaya Regulatory Milpitas > (408) 577-7714 > > > -----Original Message----- > From: Ken Javor [mailto:[email protected]] > Sent: Wednesday, June 20, 2001 8:43 AM > To: [email protected]; [email protected] > Subject: Re: FCC + FCC = FCC? - What? > > > > Let's not carried away. Those radiated emission limits protect broadcast > radio reception, period. As such, your personal electronics are turned off > when aircraft safety requires glitch-free operation of its NAV systems. > Hospitals already prohibit INTENTIONAL electromagnetic transmissions which > can affect patient safety. > > ---------- >>From: [email protected] >>To: [email protected] >>Subject: RE: FCC + FCC = FCC? - What? >>Date: Wed, Jun 20, 2001, 8:28 AM >> > >> >>>I think the answer is that the FCC allows declared >>>compliant devices to be sold in another unit >>>WITHOUT testing. This has allowed PCs on the >>>market with as much as 30dB over Class B limits. >> >> It was my understanding that testing always had to be done for the most >> common configurations of equipment as it is shipped. If this new PC card > is >> going to be shipped in more than 50% of a given line of products I believe >> that line needs to pass testing with the PC card. The intent of the >> standards is to limit testing to configurations within reason. I am >> guessing that the only reason this PC card has been tested so far is > because >> it is going to be used in a majority of at least one line of products. >> Based on this the product (a PC I take it) should pass emissions tests > with >> the PC Card, or a different PC Card should be used. >> >> I have to say that it is frightening how flippantly some of my colleagues >> accept that PCs are being released into the market up to 30 dB over the >> limit. The limits are there for a reason, and it is machines such as > these >> that can interfere with airplanes, hospital equipment and the like. >> Although it may make your job a little harder some days, I think the next >> time you are on a plane or having surgery you will be happy that a > notebook >> or a cell phone doesn't cause a failure of those critical systems. >> >> Sincerely, >> Kyle Cross >> >> ------------------------------------------- >> 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: >> Michael Garretson: [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://www.rcic.com/ click on "Virtual Conference Hall," >> > > ------------------------------------------- > 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: > Michael Garretson: [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://www.rcic.com/ click on "Virtual Conference Hall," > ------------------------------------------- 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: Michael Garretson: [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://www.rcic.com/ click on "Virtual Conference Hall,"
