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

This might be a good place to jump in and test the group's response to a theory
I've heard circulating about this very subject.  I imagine there are desktop PC
engineers in this group -- please don't be offended by the following, but
feedback on it might be enlightening.

The typical PC has a fairly tight metal enclosure (possibly covered by an
external plastic enclosure), and is connected to its peripheral devices through
shielded wiring.  Assuming the EMI testing that's done for CISPRB and CE is done
with a base configuration (no added PCI cards, etc.), the inclosure could be
providing a great deal of the necessary shielding.  The mother board itself
would then not need to be especially clean.

What an arbitrarily-configured PC might do in terms of EMI, after third-party
PCI assemblies and wiring are added, might be quite another matter.  But that's
not what is tested to get the certification.

The reason I find this distinction important is because in some other industries
(telecom/datacom for one), it is common for a single manufacturer to supply an
enclosure as well as a number of functional modules that can be assembled in
virtually limitless combinations.  The manufacturer of these products needs to
test various combinations of modules for EMI compliance at the enclosure level.
Since these modules have different kinds of interface cables and connectors,
there are numerous ways for stray RF from one type of module to leak out of the
enclosure by coupling to unrelated cable connections.  This tends to make the
direct, unshielded radiation from individual modules in such an enclosure more
critical than it would be for a fixed configuration such as a motherboard-only
PC.

Ken Hayden



Lee Ritchey wrote:

> What was the setup for the HP experiment?  Over a continuous plane?  If so,
> I question the results.
>
> Everyone who has been party to this discussion on differential pairs used a
> work station or a PC to connect.
> If you look on the back of those products, there is sure to be a sticker
> stating that the box complies with CISPRB and has a CE mark on it.
>
> Open the box and look at the mother board.  You will find that all of them
> have bot outer layers of their mother boards covered with "high speed"
> traces.  Most of them, if not all are single ended signals with a
> combination of series terminations and parallel terminations.
>
> Hows did these boxes pass emissions tests if all of the claims being made
> about traces on outer layers being sources of EMI?
>
> Answer, traces on outer layers that are close enough to a plane to create
> 50 ohm transmission lines are not good radiators.
>
> Lee
>
> > [Original Message]
> > From: Loyer, Jeff <jeff.loyer@xxxxxxxxx>
> > To: <MikonCons@xxxxxxx>; <si-list@xxxxxxxxxxxxx>
> > Date: 10/16/2003 10:12:38 AM
> > Subject: [SI-LIST] Re: Traces don't cause EMI - really?
> >
> > 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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