In a private message dated 10/27/2003 2:39:27 PM Pacific Standard Time, tom@xxxxxxxxxxxxx writes: The thread started discussing how board traces could radiate causing EMC issues. The note from you that I commented about concerned full PCs and other systems that did have radiation that was cured by placing a shielded box around them. My question to you was how could you tell that the EMC from these systems was caused by the traces when there were all kinds of other emitter; cables, ICs, poorly constructed traces, power supplies, etc., that were also contributors to EMC issues. ********* Tom, since the initial questions you posed were to the open forum, I'm also sending this response to the SI List as I did not give a complete answer to your initial comments. Others probably have the same question in their mind. ********** The short answers: (a) By direct near-field measurements of the fields throughout the guts of each PC I mentioned, and (b) via empirical tests that demonstrated the microstrip emission levels. Only one of the multiple PCs I mentioned in the earlier E-mails was observed to marginally fail allowable emissions when the case was closed. All failed with the case open. The longer answer: The following will hopefully clear this question up for you (as the answer is not easily described). As you are clearly aware, open versus closed PC cases make a tremendous difference. The emissions from cables and external entities connected to a PC are primarily CM emissions. CM emissions from cables are the most common cause for EMC failures. Open (i.e., non-shielding enclosures) are a different story. The open case condition adds emissions generated by differential-mode circuits and their near-field coupling to any cables internal to the PC (or other unit) with subsequent re-radiation. Note that some of these internal cables also have CM-induced currents on them which cause radiation, but which are contained by a shielded case when closed. Therefore, quantification of the exact source(s) of the added noise for the open case condition is different for each manufacturers layout design. I have examined the designs of multiple vendors as far as trace layouts, stackups. internal cable routing, etceteras. By using near-field magnetic probes, one can successfully isolate the weaknesses and differences between different vendor designs (and generically, their design techniques). In short, there is no one answer that will fit all designs; therefore, the entire PC design and construction must be critiqued to yield an appropriate answer for that particular design. By careful modeling and analysis, I have directly correlated multiple different trace layouts and designs with the emissions they produced as a function of frequency. My take on the original discussion was based on Lee Richey's comment that (in essence) negligible (and sometimes undetectable) emissions emanated from microstrip traces. This is misleading information and has been (unfortunately) proven by many vendors who failed to pass the necessary regulations UNLESS a shielding enclosure was an integral part of the design. However, compliant, non-enclosed designs are indeed obtainable when careful design techniques are used. In my experience, a major element of ALL such designs is the incorporation of onboard shielding, guard traces, ground fills, chassis ground rings, and other techniques I have commented upon in other threads over the last few years. Not all of these techniques are mandatory for every design, of course. Good Engineering to all, Mike Michael L. Conn Owner/Principal Consultant Mikon Consulting *** Serving Your Needs with Technical Excellence *** ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu