Adding to Ed's comments: I test AC power protection relays. Whenever I suspect susceptibility, I switch the AM modulation to either 59 Hz from the sampling rate, or to 59 Hz. If there is a problem, this causes a 1 Hz beat frequency between the applied power frequency signals and the modulation frequency. Helps me find problems much more quickly.
Don Borowski Schweitzer Engineering Labs Pullman, WA USA "Price, Ed" <ed.pr...@cubic.com> wrote on 08/18/2005 10:49:14 AM: >> From: Chileshe, Chris [mailto:chrischile...@eaton.com] >> Sent: Thursday, August 18, 2005 9:52 AM >> To: emc-p...@ieee.org >> Subject: RF Susceptibility: Sweep v/s spot check >> Group, >> >> On several occasions during radiated immunity EMC testing, I have >> observed products deviate >> or even fail and subsequent manual spot checks at the frequencies of >> interest have revealed no >> anomalous behaviour. >> >> I saw such behaviour on a product I was testing recently, and spent >> a long time conducting spot >> checks at the frequency and the immediate neighbourhood without much >> success replicating >> the failure. I tried turning the modulation on and off and even >> applying the modulated field instantly >> on and off without much success. I did a sweep again and the >> deviation was back like clockwork! >> >> What is the explanation for this and there are techniques for >> getting round this 'problem'? >> >> Rgds >> >> - Chris > > This sounds like the product might be responding to the rate of > sweep, that is, to the apparent frequency modulation caused by the > sweeping test signal. You might check to see how the device responds > if you sit on one of those spot frequencies, and then turn on FM > with a moderately slow deviation rate. > > It may also be that the device is taking a long time to respond, and > the frequency your test sweep happens to be at (when the device goes > nuts) is actually way beyond the frequency that actually caused the > malfunction. Try sweeping the problem range at a much slower rate, > or sweep in the opposite direction. > > You may have to depend on building a "body of knowledge" about how > some products respond. I once tested a device, that used digital > signal processing, and it would pass radiated immunity just fine > using a standard 1 kHz 50% pulse modulation. But if I moved the > modulation frequency down to about 965 Hz, the device would fail at > very low field strength. The key was that a harmonic of 965 Hz (and > there were plenty with that square-wave modulation) dropped right > onto one of the digital sampling rates in the device. When that > happened, the device software couldn't reject the interfering signal > because it was "there" at every sample interval (as explained to be > by the software code gurus). The lesson is that sometimes, you find > things by accident. > > Regards, > > Ed Price > ed.pr...@cubic.com WB6WSN > NARTE Certified EMC Engineer & Technician > Electromagnetic Compatibility Lab > Cubic Defense Applications > San Diego, CA USA > 858-505-2780 (Voice) > 858-505-1583 (Fax) > Military & Avionics EMC Is Our Specialty This e-mail may contain SEL confidential or legally privileged information. The opinions expressed are not necessarily those of SEL. Any unauthorized disclosure, distribution or other use is prohibited. If you received this e-mail in error, please notify the sender, permanently delete it, and destroy any printed copies. This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to emc-p...@ieee.org Instructions: http://listserv.ieee.org/listserv/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org 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: http://www.ieeecommunities.org/emc-pstc
