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

  • From: "jon powell" <jonpowell@xxxxxxxxxxxx>
  • To: <leeritchey@xxxxxxxxxxxxx>, "Jeff Loyer" <jeff.loyer@xxxxxxxxx>,<MikonCons@xxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 17 Oct 2003 11:01:38 -0700

What I find when I open many PC boxes is a lot of EMI prevention hardware
added by either the EMI guys or the mechanical engineers presumably to make
the thing pass. I see expensive metal gaskets and elaborate PCI board
connection schemes. I see filters on the wires leaving the system. I also
look at the boards and note that have been built with lots of places to
place filters and such on outgoing signals if such filters are needed.
So EMI is coming from someplace and people are spending money on the
enclosure to contain it. Money that they tell me they would rather not
spend.

Doesn't mean EMI is coming from surface traces but it also doesn't mean that
a simple enclosure is making the thing pass standard radiation tests.


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey
Sent: Friday, October 17, 2003 9:33 AM
To: Jeff Loyer; MikonCons@xxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


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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