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

I think there are many of us who lived through these early PC days who can
substantiate Mike's (and many others) assertions that a good enclosure is
required to contain emissions from microstrips. My experiences (at another
company) was with motherboards, video cards and monitors (even back when VGA
Video cards had 6 oscillators on them), SCSI, and ethernet. We ran into the
same problem with vendor provided product and tried to find compliant
"hanger queen" devices to perform our FCC Class-B tests with. Enclosures
were always an EMC reliability problem.

Surface microstrips do radiate and their emissions need to be contained
either by planes or an enclosure. The emissions from microstrip traces fill
the enclosure looking for a return path. The image plane can't contain all
the EM field lines. The emissions couple to internal cabling, ungrounded
heatsinks, sub-assemblies, etc, and cause self compatibility and/or external
compatibility problems. This can also happen with internal cabling connected
to motherboards that do not have local decoupling to keep those emissions
off of the cables. They are essentially microstrips without a reference
plane nearby.

A low impedance return path must be provided for a PREFERRED return path for
those emissions inside the cabinet. If you don't provide it, currents split,
and they will surely find the return path that you least desire (Reset
circuits, Analog/RF circuitry, Power traces, etc). It may not be a failure
at the time of test, but degraded noise margins may lead to "spooky"
problems in adverse environments (Electro-magnetic, thermal, voltage margin
sensitivities, etc). Good Stripline (symmetrical and asymmetrical) design
solves many EMC problems. As with all designs, there are trade-offs based on
multiple pain thresholds.

Often, surface microstrips are a susceptibility path for ESD failures as
well.

This has been a good thread with a lot of good input from seasoned, very
talented people. Congratulations.

Ross Wellington
L-3 Communications CSW


-----Original Message-----
From: MikonCons@xxxxxxx [mailto:MikonCons@xxxxxxx]
Sent: Wednesday, October 22, 2003 12:59 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


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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