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

Brent,

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.   

In all my measurements, the lead frames of the ICs are the big radiators. 
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


> [Original Message]
> From: Brent DeWitt <bdewitt@xxxxxxxxxxxxx>
> To: <leeritchey@xxxxxxxxxxxxx>; Grasso, Charles
<Charles.Grasso@xxxxxxxxxxxx>; <si-list@xxxxxxxxxxxxx>
> Date: 10/12/2003 6:44:22 PM
> Subject: RE: [SI-LIST] Re: Traces don't cause EMI - really?
>
> Hello Lee,
>
> I suspect much of the difference of opinion lies (as often is the case) in
> subtleties of the English language.  If, in your statement, the words
> "matched transmission lines" had been exchanged for "traces", I doubt
anyone
> would disagree.  In the extreme devil's advocate case, the trace could be
a
> resonant patch antenna over a ground plane.  Clearly not what you
intended,
> but a pretty good radiator.  As an intermediate case, I expect you would
not
> disagree with the concept that mis-matched transmission lines, even over a
> continuous ground plane, may radiate significantly.
>
> I have read and agree with Doug Brooks' well developed paper, but I'm not
> sure it fully covers what Charles is concerned with.
>
> With respect,
>
> Brent G DeWitt
> GE Medical Systems
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Lee Ritchey
> Sent: Sunday, October 12, 2003 7:00 PM
> To: Grasso, Charles; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Traces don't cause EMI - really?
>
>
> Charles,
>
> One of the papers I refer to is on my web site as well as That of Doug
> Brooks.  It is titled "90 Degree Corners, The Final Turn".
>
> The measurements were made in the lab at UMR.
>
> If there is data out there that contradicts this, it would be good to
share
> it.
>
> It's not hard to take a near field probe and run it along traces that are
> on outer layers of a PCB with solid planes and get similar answers.
>
> Lee
>
>
> > [Original Message]
> > From: Grasso, Charles <Charles.Grasso@xxxxxxxxxxxx>
> > To: Lee Ritchey <leeritchey@xxxxxxxxxxxxx>; si-list@xxxxxxxxxxxxx
> <si-list@xxxxxxxxxxxxx>
> > Date: 10/10/2003 9:20:28 AM
> > Subject: [SI-LIST] Traces don't cause EMI - really?
> >
> > Lee,
> >
> > I was thumbing through some of the diff pair thread and ran across
> > this comment that you had made:
> >
> > "Second, traces that run over continuous planes don't cause EMI,
> > differenrtial or otherwise. Again, there are many papers that
demonstrate
> > this as well."
> > I am sure you didn't mean this. There a many papers, notably by Bob
> Dockey,
> > that clearly
> > show the diople-like profile of a trace over a continuous ground plane.
> The
> > paper
> > also showed that, depending on the size of the board, the emissions can
> > exceed the
> > FCC Class B limits - thats one trace over a continuous ground.
> >
> > Can you please expand on your comment ?
> >
> >
> > Best Regards
> > Charles Grasso
> > Senior Compliance Engineer
> > Echostar Communications Corp.
> > Tel:  303-706-5467
> > Fax: 303-799-6222
> > Cell: 303-204-2974
> > Email: charles.grasso@xxxxxxxxxxxx;
<mailto:charles.grasso@xxxxxxxxxxxx; >
> >
> > Email Alternate: chasgrasso@xxxxxxxx <mailto:chasgrasso@xxxxxxxx>
> >
> >
> >
> >
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