I completely agree with Ken Javor. Solid theory and solid conclusions. Dave Cuthbert
From: Ken Javor [mailto:ken.ja...@emccompliance.com] Sent: Thursday, February 20, 2003 4:53 PM To: pwell...@csw.l-3com.com; 72146....@compuserve.com; emc-p...@ieee.org Subject: Re: RE02 cabling problem There is another implied concept of questionable validity in this latest posting. The way I read it, Mr. Wellington is talking about filtering signals emanating from the support equipment as it passes through a bulkhead between control and test chambers. Such signals usually require no filtering whatsoever, because if they have any bandwidth at all, they are run with dedicated returns (such as a twisted pair or a twisted shielded pair) and have no radiation efficiency to speak of. What requires filtering, and what is ameliorated by proper PCB layout, as he alluded to in an earlier post, is common mode emissions. These can be filtered to a very high degree with no impact on intentional signals. Common fixes are snap on ferrite beads and line-to-ground caps. Clearly the line-to-ground caps should not attenuate the desired signal, but in most if not all cases the undesired cm current is orders of magnitude higher in frequency than the intentional signal. In those cases where this is not the case, the military would run intentionally high frequency signals within shielded cables (think MIL-STD-1553 and fibre channel) which, if properly terminated, will provide all the protection necessary without resort to either filtering or over-braids. Specifically, if the support equipment or its environment resulted in high frequency cm currents conducted on the outside of a shielded cable, that cable should be terminated peripherally to a connector at the bulkhead as it passes into the test chamber. If the cable is not shielded and the source is the ambient, then shielding of the cable external to the test chamber is both proper and necessary, and has no effect on the validity of the test set-up within the chamber. Further, it requires no input from the customer, because it does not affect the delivered product configuration. If the test support equipment itself is the cm source, then any cm filtering necessary to attenuate those emissions before they enter the test chamber is again proper and necessary, external to the test chamber. It might be said that such filtering could reduce cm currents emanating from the test sample, but this is not a big problem for a couple of reasons. First it is easy to determine whether it is support equipment or the test sample which is driving the currents, by sequentially de-energizing suspected sources and noting the effect on the emissions. Secondly, the standard effectively requires at least 3 meters of cabling between test sample and bulkhead. Above 10 MHz the cable is electrically long and the effect of a filter at the bulkhead does not directly impact the level of cm current on the cable, but only indirectly as its impedance is transformed by the electrical length and distributed characteristic impedance of the cable in question. If the mil-std EMI test set-up were so well-controlled that every test chamber and every test bench were of precisely equal size and configuration and no matter where the test was conducted the entire test set-up including cable layout were identical within inches, then it might be productive to worry about changing a common mode impedance at the end of a three meter cable. In my experience, such is hardly the case. This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
