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

  • From: MikonCons@xxxxxxx
  • To: si-list@xxxxxxxxxxxxx
  • Date: Wed, 22 Oct 2003 14:59:26 EDT

In a message dated 10/20/2003 6:02:12 PM Pacific Standard Time, 
chris.cheng@xxxxxxxxxxxx writes:
To hide behind claims that since PC's or workstations have surface trace
passing EMI and magically deduced that traces does not cause EMI problem
makes as much sense as driving in the highway seeing no cops pulling people
over for speeding and extending that to no one is speeding in the highway.
I love this analogy. Excellent comment, Chris.
Jeff Loyer stated the following.

" I don't believe a case of "well behaved" microstrip traces generating 
enough EMI to fail FCC standards has been demonstrated - all failing instances 
seem 
to be accompanied with the caveat of including a design error, or 
unreasonable geometries (.062" above the ground plane)."

Absolutely not true. Several papers over the past decade from respectable 
sources (e.g., IBM, Ott, Paul, and me, sorry--don't want to sound arrogant) 
have 
demonstrated FCC Class B failure for boards that are NOT shielded by an 
appropriately designed enclosure. Following is another personal experience that 
should put this contention to rest.

At the time of launch of the AMD K6 microprosser, I supported AMD with EMI 
evaluations that required testing of multiple vendor PCs. I performed analyses, 
near-field probe tests, and microprocessor package measurements that predicted 
not only emission levels, but CPU package resonances that would aggravate the 
expected emissions. We tested about 10 separate PCs from vendors that 
included Acer, HP, Dell, Compaq, and others. Tests were conducted at two 
different 
certified facilities (one in Santa Clara, CA and another in Dallas, TX). The 
PCI 
bus at that time was only 33 MHz, and the the microprocessor (typically 
100-300 MHZ clocks) technology used was predominately 0.25 microns or larger 
(which 
inherently produces slower edge rates than todays 0.09 to 0.18 micron 
technology). Nevertheless, ALL VENDOR PCs FAILED EMI WHEN THE CASES WERE 
OPEN---MISERABLY. The resonances I had predicted were confirmed to contain the 
worst 
offending frequencies. In the same time frame, I consulted to Motorola on their 
Power PC motherboards, with the exact same result---you had better have a good 
enclosure to pass FCC (or CISPR) Class B levels. Shipping PCs from Phoenix, AZ 
to Europe loosened marginally designed case sufficiently to cause EMI test 
failures in Europe that passed fine in the US. The cases were redesigned to 
increase seam contact ruggedness to resolve the problem.

BOTTOM LINE: It is pure folly to mislead todays junior designers with 
thoughts of successful EMI performance unless an adequate shielded enclosure is 
doing 
an excellent job of suppressing emissions generated directly from the 
motherboards. Multiple, objective tests of products from the largest PC 
suppliers 
have unquestionably confirmed this fact. And, it IS possible to analytically 
predict emission levels and their consequences (i.e., failure to pass 
regulatory 
requirements). Do your own analyses with the appropriate software, than confirm 
your results and your analytical techniques by radiated emission tests. Only 
then will you have adequate confidence in how to execute a "works the first 
time" design.

If my comments offend some, sorry; but I'm old and wise enough that I accept 
reality, whether I like it or not (at $2800/day, my clients seem to think so 
too).

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

*** Serving Your Needs with Technical Excellence ***


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