One sure way to REALLY irritate me is to twist my words and try to make me look stupid (I do a fine job by myself on occasion and don't appreciate any outside help). I did not say that pn junctions don't detect and rectify rf, I said that the field intensities associated with unintentional emissions from ITE are too low to cause susceptibility in circuits other than radios. Your example here is 10 V/m, and you are talking about an op-amp (gain unspecified) and that it was susceptible at that level should be no surprise to anyone.
on 1/4/02 7:34 AM, [email protected] at [email protected] wrote: Does anyone else think that ordinary semiconductors doesn't respond to RF? I have tested a product which was little more than an LM324 quad op-amp for RF immunity using IEC 61000-4-3. This op-amp has a slew rate of 1V/micro-second on a good day with the wind in its favour. It was housed in an unshielded plastic enclosure. Demodulated noise that exceeded the (not very tough) product specification were seen all the way up to 500MHz at a number of spot frequencies that appeared to be due to the natural resonances of the input and output cables. Above 500MHz this resonant behaviour vanished to be replaced by a steadily rising level of demodulated 1kHz tone as the frequency increased. I stopped testing at 1GHz, where the output error from the product was about 10% and still rising with increased frequency. OK, the field strength for the test was 10V/m (unmodulated) but the real surprise was how well this very cheap and very slow opamp demodulated the RF, and that it demodulated better at 1GHz than at 500MHz. I have done many many immunity tests using IEC 61000-4-3 on audio equipment and found much the same effects with every product I've ever tested. With most larger products there is usually a roll-off in the demodulation above 500MHz - not because the semiconductors in the ICs can't respond (they can) but apparently because larger products have higher losses above 500MHz or so between the cable ports and the semiconductors, plus a denser structure that might provide more self-screening. The transistors and diodes in all modern ICs (analog or digital) are so tiny that they make excellent detectors at UHF and beyond. As they get smaller (and they are) their frequency response increases (and their vulnerability to upset and damage decreases). Regards, Keith Armstrong In a message dated 03/01/02 23:27:19 GMT Standard Time, [email protected] writes: Subj:Re: EMC-related safety issues List-Post: [email protected] Date:03/01/02 23:27:19 GMT Standard Time From: [email protected] (Ken Javor) Sender: [email protected] Reply-to: [email protected] (Ken Javor) To: [email protected] (Robert Macy), [email protected] (Pettit, Ghery), [email protected] ('James Collum'), [email protected] Emissions from a laptop are naturally (without suppression) on the order of 10 uV/m to 100s of uV/m. 1000 uV/m would represent at least a 20 dB outage at frequencies that could possibly interfere with sensor electronics. The coupling is lossy: 1 mV/m will generate far less than 1 mV signal in the electronics, and this at rf. Does anyone really see this as a remotely possible mechanism? I don't. ---------- >From: "Robert Macy" <[email protected]> >To: "Pettit, Ghery" <[email protected]>, "'James Collum'" <[email protected]>, <[email protected]> >Subject: Re: EMC-related safety issues >Date: Thu, Jan 3, 2002, 3:25 PM > > > Perhaps, it merely interfered with the "sensor" electronics, not the true > magnetic field that was being sensed. > > - Robert - > > Robert A. Macy, PE [email protected] > 408 286 3985 fx 408 297 9121 > AJM International Electronics Consultants > 619 North First St, San Jose, CA 95112 > > -----Original Message----- > From: Pettit, Ghery <[email protected]> > To: 'James Collum' <[email protected]>; > [email protected] <[email protected]> > Date: Thursday, January 03, 2002 11:46 AM > Subject: RE: EMC-related safety issues > > > I still have a hard time believing it was a compass that was affected by > a laptop computer. ADF indication, could be. VOR, maybe. Magnetic > compass? I wouldn't want a magnetic source that strong in my lap! My belt > buckle would be stuck to it. There is quite a distance between a magnetic > compass in the cockpit of an airliner and anything a passenger is carrying. > Not so in a Cessna 172, but in a DC-10? > > Ghery Pettit > > -----Original Message----- > From: James Collum [mailto:[email protected]] > Sent: Thursday, January 03, 2002 10:47 AM > To: [email protected] > Subject: Re: EMC-related safety issues > > > > > ********* > A routine flight over Dallas-Fort Worth was disrupted when one of > the compasses suddenly shifted 10 degrees to the right. The pilot asked if > any passenger was operating an electronic device, and finding that a laptop > computer had just been turned on requested that it be turned off, whereupon > the compass returned to normal. Following RTCA guidelines the pilot > requested that the laptop be turned on again 10 minutes later, when the > compass error returned. > Ref: Compliance Engineering (European edition) Nov/Dec 1996 p12 > ********* > > I am fascinated by this amazing story (which must surely be an urban > myth) and went in search of more info on the internet. > I had never heard of the RTCA ( a private corporation) before, but > noticed via their web site that you have to be a member company (i.e. pay) > to receive the wisdom that it contains. Aviation is merely a hobby of mine > but I'm interested in reading a copy of the RTCA's DO-233/214 and 196 > documents without shelling out hundreds for the privilege, can anyone > advise? Also does anyone know what recommendations have they made to > modifying FAR 91.21 (as per their web site). > In reading this again, I'm curious as to how the pilot would have known > about a private companies convoluted guideline for fault finding on errant > radio direction equipment involving locating industrious passengers and > commandeering their computers at 10 minute intervals. > Surely he would have done what any professional engineer would do, beat > or kick the 10 degree error out of the RDF equipment? > Or maybe just wonder to him/herself about how strange things happen in > the Dallas Fort Worth area? > > Tounge in cheek, my comments and not those of my employer etc. > > Jim
