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

  • From: "Nagel, Michael" <Michael.Nagel@xxxxxxx>
  • To: si-list@xxxxxxxxxxxxx
  • Date: Tue, 21 Oct 2003 13:15:19 +0200

Hello All!

Reading the different contributions to this thread I hat to think back
to my studies. Our Professor lecturing High Frequency Design talked of
Microstrip Antennas.

After a short search on google I found the below article (sorry for clipping

the URL):
http://www.navicpmart.com/advice/advicetmp.cfm/s/C7A13B2C9F173CF2E034080020B
4BBE117272197/v/001~~%2E%2E%2Fnews%2Fmw%5F200103%5F1%2Ecfm~~~~~~~~~~~~.html

The article describes well the principle of radiation of this type of 
antenna.

I think that the frequency range is an important.

This could help to sort out at which frequency which part of a PWB will 
possibly act as source.

Best regards,
Michael


> -----Original Message-----
> From: Chris Cheng [mailto:chris.cheng@xxxxxxxxxxxx]
> Sent: Dienstag, 21. Oktober 2003 02:58
> To: 'bdewitt@xxxxxxxxxxxxx'; leeritchey@xxxxxxxxxxxxx;
> si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Traces don't cause EMI - really?
> 
> 
> Brent & Lee, 
> 
> Nice spin on the issue but unfortunately you both tried to avoid the
> original problem that Lee stated :
> 
> >As far as EMI is concerned, it has been demonstrated many 
> times, once in
> >the paper done by Doug Brooks with the staff at UMR, that 
> traces traveling
> >over planes are not a detectable source of EMI.  Therefore, 
> constraining
> >the routing of differential pairs to prevent them from 
> creating EMI is not
> >appropriate or necessary.
> 
> I have seen and explained how surface trace can both fail EMI 
> and signal
> integrity even if it is referencing to a solid plane and perfectly
> terminated. If the edges are fast enough and the reference plane is
> unrelated to the I/O power, the image current will be denied 
> a low impedance
> return path and will exhibit strong EMI and power/ground 
> bounce. This is the
> exactly reason why tight coupling differential traces can 
> help. This is also
> the original claimed by proponents of coupled differential 
> signals and Lee
> has chosen not to response to me. Whether the image current is flowing
> between the differential traces or on the reference plane, 
> they cancel each
> other out at both the driving and receiving end and thus 
> minimize both EMI
> and power/ground bounce. Whether the reference plane is 
> related to the I/O
> power or not does not affect the outcome in differential case 
> but make a big
> difference in the single end signal case. 
> 
> To hide behind claims that since PC's or workstations have 
> surface trace
> passing EMI and magically deduced that traces does not cause 
> EMI problem
> makes as much sense as driving in the highway seeing no cops 
> pulling people
> over for speeding and extending that to no one is speeding in 
> the highway.
> Let me ask you this way, have you seen a highspeed system 
> that has surface
> trace referencing to the wrong voltage plane pass EMI with an 
> open enclosure
> without those crazy highspeed decoupling caps or thin core 
> planes to return
> the image current ? I have seen plenty of them fail that way. 
> And yet I have
> seen plenty of coupled differential signals route that way 
> and pass EMI or
> signal quality. This is the fundamental advantage of coupled 
> differential
> signals that you are so ignorant about.
> 
> Lee,
> 
> You seem to like to make claims that I can easily counter but 
> when face with
> the rebuff you like to ignore it and continue to repeat your 
> flawed claim as
> if no one can give you a counter example. 
> 
> In this forum, you have been asked many times the following question :
> 
> "If tomorrow you are going into a client's office to consult 
> on designing a
> 2.4GB/s differential signal system. Will you recommend them to "routed
> thousands of differential signal where each member of the pair is on a
> different layer". Do you think that is good engineering 
> practice ? Do you
> think you can still keep your job as a consultant after making that
> statement ?"
> 
> And you seem to ignore the fact that this is how all these 
> differential
> traces vs. single un-coupled lines discussion starts.
> 
> This is a simple yes or no question. I don't even want to throw in any
> technical point or science into it. You either do it or 
> don't. If you do, I
> would propose you put that in your lecture notes and whatever 
> book you are
> writing and call it "Lee Ritchey technique" or may be even 
> patent it as I
> sure haven't seem anyone designing >GHz signals doing that. 
> As an inventor
> of that technique, you deserve to publize it and make sure 
> people follow it.
> 
> 
> On the other hand, you silence suggests you may have 
> something to hide in
> you claim. It is plain wrong and even you yourself won't dare 
> to do that.
> 
> Which way is it ?
> 
> -----Original Message-----
> From: Brent DeWitt [mailto:bdewitt@xxxxxxxxxxxxx]
> Sent: Sunday, October 19, 2003 8:28 PM
> To: leeritchey@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Traces don't cause EMI - really?
> 
> 
> It seems this has become something of a tempest in a teacup.  
> As I mentioned
> in an earlier post, I believe much of the energy of the dispute has
> developed over differences in terminology.
> 
> First, in the vast majority of cases, I agree with Mr Ritchey.  In the
> twenty five years or so I've been involved in EMC, I've never seen the
> radiation from  surface trace fail a product.  That said, 
> I've only worked
> on old/slow boards with fundamental clocks less than 1.5 GHz, 
> although I've
> worked with 900 MHz intentional radiators to their 10th 
> harmonic.  Rules of
> thumb only work until the thumb is too fat to see the problem 
> underneath it.
> As I mentioned in an earlier post, resonant patch antennas and other
> intentional radiating pcb structures are nothing more than fat, well
> designed traces.  Somewhere between them and our ideal EMC 
> designs lies
> practicality.  As frequency increases, I find myself needing 
> increasingly
> closer inspection of detail.
> 
> In somewhat oblique support of Mr. Ritchey, most failing EMC 
> issues I've
> observed have been associated with L di/dt induced voltages 
> on the reference
> plane caused by surface traces.  I/O cables, using said 
> poorly controlled
> reference planes out to the world, are often a major 
> emissions issue, but
> that is entirely another subject for discussion.
> 
> Finally,  I believe Mr. Ritchey is correct, but using the following
> assumptions:
> 
> - The trace structure and geometry does not approach a 
> resonant structure at
> the fundamental or appreciable harmonics of the operating frequency.
> - The reference plane structure supporting the return currents of the
> surface trace does not significantly contribute to reference 
> plane resonance
> and induced voltage on attached cables.
> 
> Respectfully,
> 
> Brent DeWitt
> 
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