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

  • From: Chris Cheng <chris.cheng@xxxxxxxxxxxx>
  • To: "'bdewitt@xxxxxxxxxxxxx'" <bdewitt@xxxxxxxxxxxxx>,leeritchey@xxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
  • Date: Mon, 20 Oct 2003 17:57:50 -0700

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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