The subject of non-repeatability in radiated emissions (and its lesser brother, non-repeatability in radiated immunity) provides perennial entertainment. IMHO none of the posts to date has put a finger on the number 1 source of variability: cable termination.
A 2m long cable is a quarter wavelength at 75MHz. If it is terminated to the ground plane with a low impedance then at this frequency it'll be a high impedance at the EUT; vice versa if it has a high impedance to the ground plane it'll be a low impedance at the EUT. Hence the radiated emissions due to a cable, typically below 200MHz as Geetha has described, will be directly related to the source impedance of the EUT, the length of the cable in wavelengths, and its far end termination impedance. S B Worm did a very clear paper a few years ago describing this experimentally and theoretically, if anyone's interested I'll dig out the reference. The work that has been ongoing on the FAR method for radiated emissions, and also the GTEM, has barfed on precisely this problem. None of the standards makes any attempt to control the termination impedance. For as long as this situation continues, variations between test houses will be a fact of life. In the test training courses that I run, it is incredibly easy to demonstrate over 20dB difference just through this one parameter. The ferrite clamps amendment (A1 to CISPR22:1998) did try to address the problem and was roundly condemned by just about all parties, each of which had their own set of interests to defend. That being the case, there is unlikely to be appreciable improvement in the near future. There are other factors of course: the OATS is conceivably the worst possible test method (in terms of repeatability) that could have been devised and there are now better methods, at least for small EUTs, easily available. Unfortunately they all suffer from one overwhelming disadvantage, which is that politically they are required to show correlation with measurements on an OATS, whose correlation with itself is the subject of such contention. This applies particularly for the FAR, the GTEM and the reverb chamber. I understand that some CISPR comittees are beginning to come around to the view that correlation between the results of different methods might actually be unachievable and so insisting on it would be a wasted effort. The light at the end of the tunnel? Tim Williams Elmac Services, Chichester, UK http://www.elmac.co.uk 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
