I read in !emc-pstc that cherryclo...@aol.com wrote (in <132.6f59d2b.296 89...@aol.com>) about 'EMC-related safety issues', on Sat, 5 Jan 2002: > Dear Cortland > People can't simply say: "ordinary semiconductors won't demodulate RF > levels > produced by an unintentional radiator" even the smallest amount of RF > can > be demodulated there are no hysteresis or threshold effects in a PN > semiconductor junction or FET that is biased into its conduction region > (at > least not until you get below signal levels equivalent to less than a > single > electron).
The question is not whether demodulation occurs, but whether the recovered modulation is at a level to cause a problem. 1 mV of r.f. can't produce even 1 mV of recovered modulation. > > What I am sure most engineers would really mean to say is: > "ordinary semiconductors exposed to RF levels from an information > technology > product which is fully compliant with all relevant EMC emissions standards > and is at 10 metres distance will generally not demodulate a sufficient > level of interference to make an appreciable difference to most electronic > systems." I don't think most engineers would go along with a statement with such a high fog-factor. That is one of the points of contention; this subject seems to attract fog-factor like a superconducting magnet. > > Now we have a statement which has some scientific rigor and some > engineering > validity to it. Are you seriously putting that forward? It's so vague, IMO, as to be not useful; it does not help in any way to realise solutions. > (Although I do worry that in Europe our harmonised EMC standards only test > emissions up to 1GHz, so what does that say about the possible emitted > fields strengths from a PC with a 1.2GHz clock frequency?) Extension up to 3 GHz (much higher in some cases) is being studied intensively. One major problem is that repeatable measurements above 1 GHz are very difficult to achieve. > > Let's see if we can put some meat into this discussion with a real-life > example... Well, it's a very extreme case of real life! I doubt that you'll come across another one before you retire! > > I once tested a blood sample incubator for RF field immunity. When was this? Before or after 1976, when EMC of medical equipment first (AFAIK) surfaced as a matter to be studied intensively. [Big snip] > > How many people reading this would be now be quite happy to place even a > fully-compliant PC (compliant at 10 metres distance, that is) right next > to > the unmodified incubator? > > If it helps, imagine that it is your young daughter whose blood sample is > in > the incubator to discover which drugs she needs to survive. > > Shall we have a vote on how close we would be prepared to place the PC? > Might be interesting. This appeal to emotion is out of place. > > Let's not even think about the problems of proximity to cellphones and > other > intentional radiators. > > I didn't mention that the incubator was a small model used for mobile > screening, for installation in a truck adapted for medical screening > purposes which travels to various communities and parks there for a few > days > while it tests the local people for disease - hardly a very well > controlled > electromagnetic environment. > > What does the above imply for similar incubators in countries that do not > have mandatory EMC immunity standards? Or for older incubators in the EU > that have never had to meet the EMC directive? > > (Please don't reply with the old chestnut that "we haven't heard of any > problems so far, so everything must be OK" - people who should have known > better were using that phrase before September 11th. It is just not an > acceptable argument where safety issues are involved, as any expert in > safety law will tell you. Try: "I've been driving past that school at > 40mph > for ten years and haven't hit a kid yet, so it must be safe mustn't it?" > as > a test of the concept.) More emotion. This is another point of contention: as soon as any critique is offered to some pronouncement, these emotional arguments are trotted out. I had a similar experience with a militant carer of disabled people. Anything that suggested that her views were perhaps just a *little* extreme (like scrapping all London's black cabs overnight because they won't accommodate a wheelchair with the user in it) was greeted with 'Oh, so you are prejudiced against disabled people, are you?' IMO, your reasoning is utterly unreasonable. Designers are not omniscient, or had better assume they are not. So they must assume that they have not thought of every possible scenario down to that 10^-9 probability. How, then, can the designer be reasonably assured that his design is satisfactory, if he cannot rely on the absence of reports of problems? Is he to continue to refine it for years and years, before releasing it, just in case he, or someone else, may think of yet another risk scenario? > -- Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk After swimming across the Hellespont, I felt like a Hero. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. 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