Many test items fall in the category where some attached cables are long and others are short. A computer with a power cord (long) and keyboard and monitor and USB cables (short) is a perfect example.
In this case you clearly run the test on the power cord from 0.15 - 80 MHz. If the shorter cables are going to resonate at the higher frequencies then that will happen, but you don't hit the shorter cables directly with the lower frequencies. Incidentally, it should be noted that an 80 MHz wavelength is 3.75 meters long, therefore maximum pick-up occurs when a cable is 1.875 meters (one half-wavelength) long, which is longer than many keyboard, monitor and USB cables, so the potential for resonance is slight. From: Bob Richards <[email protected]> List-Post: [email protected] Date: Mon, 26 Feb 2007 10:56:12 -0800 (PST) To: ieee <[email protected]> Subject: Re: Conducted immunity lower frequency Ken, I can see the case of a battery operated device with only the one cable being connected -- if it is electrically short, it is not likely to see much current induced by a very low frequency field. However, if just the one cable is short, but there are other long cables connected, I can see where the entire system can become resonant and therefore the induced current on the short cable can be higher than it would be otherwise. Of course, I admit I am not anywhere near being an expert on antenna theory, so my thinking could very well be flawed. :-) Regardless, I believe the letter of the law should be followed, regardless of how flawed it may be. It is worded the way it is worded. May not make sense, but it is what it is. Cheers, Bob Richards, NCT. Ken Javor <[email protected]> wrote: I disagree with what I perceive is being stated here - that if any portion of a system has a connection to a long conductor, then all system-attached cables must be tested down to 150 kHz. The purpose of the Annex B wording is to allow for basic physics to be applied to the limit - short cables are inefficient pick-up at low frequencies. It is the length of the individual cable-under-test which should drive the low-frequency cut-off limit. Ken Javor - ---------------------------------------------------------------- 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/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: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ - ---------------------------------------------------------------- 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/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: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
