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

  • From: "Grasso, Charles" <Charles.Grasso@xxxxxxxxxxxx>
  • To: "'jeff.loyer@xxxxxxxxx'" <jeff.loyer@xxxxxxxxx>,"'MikonCons@xxxxxxx'" <MikonCons@xxxxxxx>
  • Date: Thu, 16 Oct 2003 17:25:52 -0600

Hi Jeff,

It sounds like you have Dockeys paper in front of you.
If I remember rightly, the trace dimensions were scaled
to maintain a constant 50 ohm impedance.

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;  
Email Alternate: chasgrasso@xxxxxxxx
 


-----Original Message-----
From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx] 
Sent: Thursday, October 16, 2003 2:06 PM
To: MikonCons@xxxxxxx
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


Thanks for sharing!

This ("a 100-Ohm trace radiated >6 dB more than a 50-Ohm trace") seems = to
agree with Dockey's paper - the farther away you are from the = reference
plane (and, consequently, the higher the impedance), the more = EMI you
radiate.

The question I'm confronted with at this point is: do "well-behaved" =
single-ended signals generate appreciable EMI?  Dockey's paper outlined =
the effect, but with geometries (90mil wide trace over a 62mil =
dielectric) that aren't likely to be used.  He then demonstrated the =
reduction in EMI when the trace was brought closer to the reference = plane.
Given current geometries, is it likely for a bus of properly = designed
single-ended traces to cause failing EMI?  My = experience/impression is no.
=20

If that's incorrect, please let me know.

On the other hand, as I understand it, differential traces are much more =
"forgiving", EMI-wise, of non-idealities.  And these we often induce, =
knowingly or not.  So, tightly-coupled differential traces are = desireable,
EMI-wise, given our imperfect world and the issues we must = deal with.

That's not to say there aren't other reasons, besides EMI (increased = loss,
and some yet to be discovered/published), why we may NOT want to = route
differential traces closely-coupled.

Unless I'm incorrect about current single-ended traces and their = tendency
(or lack of) to generate EMI, I'd stick with my original = assertion, that
closely coupling differential signals has benefits, but = is not absolutely
necessary for their entire length (but compensation = for impedance change
due to reducing or increasing coupling might be = necessary).


Thanks for your time/energy.  Actually, I've been contracted by the =
fishees to distract you from pursuing them :-)

Jeff Loyer

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On
Behalf Of MikonCons@xxxxxxx
Sent: Thursday, October 16, 2003 11:33 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Traces don't cause EMI - really?


In a message dated 10/16/2003 10:14:54 AM Pacific Standard Time,=20
jeff.loyer@xxxxxxxxx writes: Just to be clear - are you saying that if I
connected 2 properly = designed=20 chips (driver and receiver) together with
many properly designed = single-ended=20 transmission lines, they would
likely fail FCC standards? Yes, but only with qualifications. Picture a
single-ended, 50-Ohm = microstrip,=20 not multiple closely spaced traces. If
excited with a 1 ns risetime = signal at=20 33 MHz clock frequency from a
50-Ohm generator, and terminated with a = 50-Ohm=20 chip resistor, expect
failure. For example, the EMCAD1 software = application=20 (circa 1992) is
based on radiated emissions equations and predicts ~39 = dBuV at=20 33 MHz
at 30 meters antenna distance. The first few harmonics are also=20 predicted
to be out of limits. Note that this software uses worst-case =
predictions.=20 The same software also indicates an increase in the
single-trace = emission level=20 by SQRT(N), where N =3D the number of
traces; however, the assumption is = that=20 all traces are carrying the
same excitation (which is unrealistic).=20 Now, if you place grounded planar
areas around the trace, or have a = densely=20 routed board, the additional
copper provides a substantial reduction in = the=20 measured emissions from
that single trace because many field lines that = would=20 otherwise leave
the board terminate on the added copper (even though = they may be=20
additional 50-Ohm traces). And, in a normal board design, many of the =
signals=20 on these surface traces will generate fields that cancel each
other.=20 Therefore, the bare (i.e., unshielded, and unenclosed) PCB MAY or
MAY = NOT fail an FCC=20 or CISPR radiated emissions test.=20

The IBM paper I mentioned recorded up to 20 dB variation in radiated=20
emissions from a single microstrip on a bare board as the trace was = moved
in-board=20 from the PCB edge. This measured reduction also illustrates the
= field-capturing=20 effect of surrounding copper, reference planes, or
other conductors.

The PCB I created contained significant open surface space around the =
traces=20 (including the guarded trace structures) which tended to increase
the = radiated=20 emissions relative to a real board design, but this was on
purpose to=20 illustrate the effect.

As an added note, I found that the radiated emissions from a single =
trace=20 increased more than linearly with the impedance of the line; i.e.,
a = 100-Ohm=20 trace radiated >6 dB more than a 50-Ohm trace. This result
makes sense = as a better=20 match to the 120*pi =3D 377 Ohms of free space
is achieved with the = 100-Ohm=20 line.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

*** Serving Your Needs with Technical Excellence ***


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