Chris, This could be a problem with the automated software running the test. I have seen at least two commercial software packages that have problems stepping between frequencies.
There may be 2 or 3 (or more) dB difference in the signal generator calibration level between adjacent frequencies. (This is especially true above 150 MHz or so in the chambers I have experience with, the lower frequencies don't vary as much in level.) The software may not properly change the instrumentation between steps. For instance, the software may always change the frequency first, then the level, which is fine if the level is rising but not ok if it is dropping. So, for a fraction of a second the EUT may be overtested and trigger a false failure. The other possibility is in the instrumentation, quite often you can observe a spike in the output when the sig-gen changes frequency ranges or level changes in the stepping attenuator. If the software allows dropping the level between frequencies and then ramping up, you might want to try that in the problem areas. It could make the test run painfully slow to do it all the time, however. Bob Richards, NCT. --- "Chileshe, Chris" <[email protected]> wrote: > 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 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 > 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
