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

  • From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
  • To: <MikonCons@xxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 23 Oct 2003 17:49:22 -0700

Michael

The email I responded to (see below) stated that the example complete PCs
radiated in excess of the EMC limits.  The PC has cables, power supplies and
other potential radiators besides the microstrip traces on the motherboard.

****************************************************************************
*********
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.
****************************************************************************
*********

Were the resonances predicted radiate directly or did the resonances conduct
energy to other elements which radiated the energy?  How did you  determine
the contribution from the microstrip vs. the other radiators in a complete
PC?

Tom Dagostino
Teraspeed Consulting Group LLC     Teraspeed Consulting Group LLC
2926 SE Yamhill St.                Device Modeling Division
Portland, OR 97214                 13610 SW Harness Lane
                                   Beaverton, OR 97008
http://www.teraspeed.com           503-430-1065
tom@xxxxxxxxxxxxx


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of MikonCons@xxxxxxx
Sent: Wednesday, October 22, 2003 4:10 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


In a message dated 10/22/2003 3:32:48 PM Pacific Standard Time,
tom@xxxxxxxxxxxxx writes:
In my experience, which I'm sure is not as broad as many who are
contributing to this thread, is the cables were the big problem.  Any noise
on the ground, power or signals that were connected to the cables was
radiated.  This was much higher than anything on the board.
************
The above is a true statement for many (if not most) cases as radiated
emissions increase with the square of frequency for common-mode radiation,
but not
for all EMI problems (either extenally detected or internally
experienced....with resultng unacceptable performance). Loop
circuits/antennas radiate only in
proportion to frequency and are therefore not as big a problem at higher
frequencies (where most of this discussion applies). As we all know, the
generation
of CM noise involves several other causes that have been discussed at length
on this forum.
Knowledge of ALL these phenomena (and their prevention) is what establishes
the need for SI Engineers.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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


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