[SI-LIST] Re: Traces don't cause EMI - really?

  • From: MikonCons@xxxxxxx
  • To: si-list@xxxxxxxxxxxxx
  • Date: Mon, 27 Oct 2003 18:59:40 EST

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 ***


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