John, Emissions may be harmonically related to clocks, they may be intermodulation products or they may be non-harmonically related to any of the above. For example, ringing on an improperly terminated transmission line will be related to the length of the line and the propagation velocity of the line and may or may not have anything to do with the fundamental frequency of the signal sent down the line that starting the ringing in the first place (other than the rate at which energy is pumped into the ringing signal). I've seen this ringing problem crop up when a board was relayed out from a hand layout (nice curved corners, etc.) to a machine layout with no component changes (type, value, location). No way to quickly predict the presence of the emission until it showed up in the lab one day. Previously compliant 9 track tape drive suddenly went out of compliance with a new "identical" unit. Took some real digging to find the problem and we had an interesting discussion with ! the drive vendor. That was about 20 years ago, but it really sticks in your mind.
Ghery From: John Woodgate [mailto:[email protected]] Sent: Tuesday, March 22, 2005 9:09 AM To: Pettit, Ghery Cc: [email protected] Subject: Re: Radiated Emission Testing "Pettit, Ghery" <[email protected]> wrote (in <468F3FDA28AA87429AD807992E22D07E04A3E1C5@orsmsx408>) about 'Radiated Emission Testing', on Tue, 22 Mar 2005: > You question the frequencies from the lab in Taiwan, but not the >frequencies from the lab in the US. How would 43 MHz and 52.38 MHz >come from 4 MHz and 12 MHz clocks if harmonics are the only way to >generate emissions (which, by the way, they are not) I guessed that 43 was a typo for 48, and 52 is a sum frequency 48 + 4. Is your reference to 'they are not' about intermodulation of harmonics or does it include anything else? Multiple addressing removed. -- Regards, John Woodgate, OOO - Own Opinions Only. There are two sides to every question, except 'What is a Moebius strip?' http://www.jmwa.demon.co.uk Also see http://www.isce.org.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
