From: Chileshe, Chris [mailto:[email protected]] Sent: Thursday, August 18, 2005 9:52 AM To: [email protected] 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 [email protected] 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 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 [email protected] 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 [email protected] Mike Cantwell [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.ieeecommunities.org/emc-pstc
