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

  • From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
  • To: <MikonCons@xxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Thu, 16 Oct 2003 10:13:17 -0700

Hi Mike,
In your posting, you said:
"I designed a special PCB in support of a worldwide seminar tour for =
Hewlett-Packard that clearly demonstrated unacceptable (i.e., >FCC Class =
B) radiation from single, terminated, 50-Ohm traces on a 10-inch PCB."

Just to be clear - are you saying that if I connected 2 properly =
designed chips (driver and receiver) together with many properly =
designed single-ended transmission lines, they would likely fail FCC =
standards?

Sorry to force you to restate something that you were so clear about, =
but the statement is a bit surprising to me.  My impression has been =
that FCC failures were inevitably due to an error in the design - =
impedance mismatches, crossing a split plane, changing reference planes, =
etc. =20

I would expect differential routing to radiate significantly less EMI in =
the presence of these errors, but I wouldn't expect a properly designed =
board with many single-ended signals to fail EMI.

Jeff Loyer

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of MikonCons@xxxxxxx
Sent: Monday, October 13, 2003 12:20 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


In a message dated 10/13/2003 7:17:10 AM Pacific Standard Time,=20
leeritchey@xxxxxxxxxxxxx writes:
I've tried several times to measure significant emissions from a single
transmission line travelling over a plane at the distances above the =
plane
one uses to create signal paths for logic circuits, the topic of concern =
in
this thread, with no luck.
I welcome measurements from others that supports the statement that =
traces
over planes are significant sources of EMI.  So far, no one has =
presented
such evidence.  Lacking such evidence, how can anyone make such claims?

True, this thread concerns differential signals, but how are they =
different
from single ended signals?

True, Doug Brooks' paper didn't set out to measure emissions from traces =
on
outer layers, but it did state that measured emissions were 30+ db below
what a mono pole would emit.  =20

In all my measurements, the lead frames of the ICs are the big =
radiators.=20
They make nice antennas.  They stick up above the PCB and they have
significant transient currents flowing through them, especially the =
power
leads.  Check out a PLCC with a near field probe sometime.

Time for some measurements from those who want this discussion to =
seriously
come to any conclusion.  Anyone have any?

Lee
**************
WOW! I just got a chance to review the latest SI list inputs and I'm=20
astounded at such comments. Lee, the "evidence" has been around for =
decades. The IEEE=20
Proceedings published a special edition circa 1989 describing over 40=20
different surface structures on PCBs that are efficient antennas, many =
of which are=20
unintentionally formed by poor trace layouts. I designed a special PCB =
in=20
support of a worldwide seminar tour for Hewlett-Packard that clearly =
demonstrated=20
unacceptable (i.e., >FCC Class B) radiation from single, terminated, =
50-Ohm=20
traces on a 10-inch PCB. The same board was used to demonstrate both =
radiation and=20
crosstalk reductions of 6 to 14 dB by the use of guard traces between =
signal=20
traces (which you recently declared as totally unnecessary). I expanded, =

updated, and presented both analyses and confirming measurements at =
multiple client=20
companies (including IBM, Motorola, AMD, and Johns-Hopkins), and =
presented a=20
paper at SuperDesignCon 95 illustrating these same effects. IBM =
published an=20
excellent paper circa 1998 at a Southern California conference (don't =
recall=20
which) that compared the radiation from traces as a function of their =
distance=20
from the PCB edges. The bottom line is that ANY trace with current =
flowing on=20
it generates EM fields and WILL radiate at some levelof efficiency. =
That's why=20
one should always look to burying high-speed traces unless they are very =
short=20
relative to the rise time of the signals they conduct.

Surface routed differential signal traces offer opposing polarities of =
fields=20
which tend to cancel, leaving only the effect of a small loop antenna to =

radiate (but they still do radiate). That's why I also favor tightly =
coupled=20
(i.e., closely spaced) pairs for such applications (which, if I recall =
correctly,=20
you also seem to dislike). Differential pairs are definitely different =
from=20
individual traces.

I have very little time for technical activities (what with the trout =
season=20
coming to an end soon), but I could not let this thread slip by.

Respectfully to all,

Mike


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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


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