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

  • From: "Hargin, Bill" <bill_hargin@xxxxxxxxxxx>
  • To: "'leeritchey@xxxxxxxxxxxxx'" <leeritchey@xxxxxxxxxxxxx>,bdewitt@xxxxxxxxxxxxx, Charles Grasso <Charles.Grasso@xxxxxxxxxxxx>,si-list@xxxxxxxxxxxxx
  • Date: Mon, 13 Oct 2003 08:21:52 -0700

Hi Lee, et al:

Looking forward to reading your new book ... Just a couple of quick thoughts on 
this.

> 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?

I'd like to see measurement results here too, but don't think that the lack of 
a specific example would allow us to say that the problem can't exist.  In your 
measurements, were you looking at well-terminated traces?  If you were looking 
at nice, series terminated traces rather than a fast-driven 5 inch microstrip 
with no termination, or using a pull-up, I could see why radiation would be 
minimal.  I've seen a number of *simulated* results (in HyperLynx, for single 
traces) that radiated significantly, and were later correlated to lab results.  
I won't necessarily say this "proves" the point, but -- at the same time -- 
lacking a specific, smoking-gun example doesn't prove nonexistence ... just an 
inability to win in court.

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

Flux-field cancellation for sufficiently close differential signals (right-hand 
rule; a key benefit for differential signaling).  I don't have a specific lab 
example to throw into my e-mail here, but this is easy to simulate with 
HyperLynx ... Not a "proof," but a reasonable datapoint.

Bill Hargin
Mentor Graphics Corp.
Systems Design Division
High-Speed Tools
425-497-5079 - Direct
425-301-4425 - Mobile 

> -----Original Message-----
> From: Lee Ritchey [mailto:leeritchey@xxxxxxxxxxxxx]
> Sent: Monday, October 13, 2003 7:07 AM
> To: bdewitt@xxxxxxxxxxxxx; Charles Grasso; si-list@xxxxxxxxxxxxx
> Subject: [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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