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

What?!?!  You don't think I have it memorized?  Ok, you caught me...

As far as I can see, it doesn't specifically say whether the trace =
dimensions were adjusted.  In fact, it doesn't say how he brought the =
trace closer to the ground plane.  I was trying to emphasize that =
Dockey's paper concludes only that traces farther away from the =
reference plane (decreasing Mgs - mutual inductance between signal and =
ground - in his paper) create more EMI; he doesn't talk about the =
relationship between impedance and EMI (is there one?).

Jeff Loyer


-----Original Message-----
From: Grasso, Charles [mailto:Charles.Grasso@xxxxxxxxxxxx]
Sent: Thursday, October 16, 2003 4:26 PM
To: Loyer, Jeff; 'MikonCons@xxxxxxx'
Cc: 'si-list@xxxxxxxxxxxxx'
Subject: RE: [SI-LIST] Re: Traces don't cause EMI - really?


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; =20
Email Alternate: chasgrasso@xxxxxxxx
=20


-----Original Message-----
From: Loyer, Jeff [mailto:jeff.loyer@xxxxxxxxx]=20
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 =
=3D to
agree with Dockey's paper - the farther away you are from the =3D =
reference
plane (and, consequently, the higher the impedance), the more =3D EMI =
you
radiate.

The question I'm confronted with at this point is: do "well-behaved" =3D
single-ended signals generate appreciable EMI?  Dockey's paper outlined =
=3D
the effect, but with geometries (90mil wide trace over a 62mil =3D
dielectric) that aren't likely to be used.  He then demonstrated the =3D
reduction in EMI when the trace was brought closer to the reference =3D =
plane.
Given current geometries, is it likely for a bus of properly =3D =
designed
single-ended traces to cause failing EMI?  My =3D experience/impression =
is no.
=3D20

If that's incorrect, please let me know.

On the other hand, as I understand it, differential traces are much more =
=3D
"forgiving", EMI-wise, of non-idealities.  And these we often induce, =
=3D
knowingly or not.  So, tightly-coupled differential traces are =3D =
desireable,
EMI-wise, given our imperfect world and the issues we must =3D deal =
with.

That's not to say there aren't other reasons, besides EMI (increased =3D =
loss,
and some yet to be discovered/published), why we may NOT want to =3D =
route
differential traces closely-coupled.

Unless I'm incorrect about current single-ended traces and their =3D =
tendency
(or lack of) to generate EMI, I'd stick with my original =3D assertion, =
that
closely coupling differential signals has benefits, but =3D is not =
absolutely
necessary for their entire length (but compensation =3D for impedance =
change
due to reducing or increasing coupling might be =3D necessary).


Thanks for your time/energy.  Actually, I've been contracted by the =3D
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,=3D20
jeff.loyer@xxxxxxxxx writes: Just to be clear - are you saying that if I
connected 2 properly =3D designed=3D20 chips (driver and receiver) =
together with
many properly designed =3D single-ended=3D20 transmission lines, they =
would
likely fail FCC standards? Yes, but only with qualifications. Picture a
single-ended, 50-Ohm =3D microstrip,=3D20 not multiple closely spaced =
traces. If
excited with a 1 ns risetime =3D signal at=3D20 33 MHz clock frequency =
from a
50-Ohm generator, and terminated with a =3D 50-Ohm=3D20 chip resistor, =
expect
failure. For example, the EMCAD1 software =3D application=3D20 (circa =
1992) is
based on radiated emissions equations and predicts ~39 =3D dBuV at=3D20 =
33 MHz
at 30 meters antenna distance. The first few harmonics are also=3D20 =
predicted
to be out of limits. Note that this software uses worst-case =3D
predictions.=3D20 The same software also indicates an increase in the
single-trace =3D emission level=3D20 by SQRT(N), where N =3D3D the =
number of
traces; however, the assumption is =3D that=3D20 all traces are carrying =
the
same excitation (which is unrealistic).=3D20 Now, if you place grounded =
planar
areas around the trace, or have a =3D densely=3D20 routed board, the =
additional
copper provides a substantial reduction in =3D the=3D20 measured =
emissions from
that single trace because many field lines that =3D would=3D20 otherwise =
leave
the board terminate on the added copper (even though =3D they may =
be=3D20
additional 50-Ohm traces). And, in a normal board design, many of the =
=3D
signals=3D20 on these surface traces will generate fields that cancel =
each
other.=3D20 Therefore, the bare (i.e., unshielded, and unenclosed) PCB =
MAY or
MAY =3D NOT fail an FCC=3D20 or CISPR radiated emissions test.=3D20

The IBM paper I mentioned recorded up to 20 dB variation in =
radiated=3D20
emissions from a single microstrip on a bare board as the trace was =3D =
moved
in-board=3D20 from the PCB edge. This measured reduction also =
illustrates the
=3D field-capturing=3D20 effect of surrounding copper, reference planes, =
or
other conductors.

The PCB I created contained significant open surface space around the =
=3D
traces=3D20 (including the guarded trace structures) which tended to =
increase
the =3D radiated=3D20 emissions relative to a real board design, but =
this was on
purpose to=3D20 illustrate the effect.

As an added note, I found that the radiated emissions from a single =3D
trace=3D20 increased more than linearly with the impedance of the line; =
i.e.,
a =3D 100-Ohm=3D20 trace radiated >6 dB more than a 50-Ohm trace. This =
result
makes sense =3D as a better=3D20 match to the 120*pi =3D3D 377 Ohms of =
free space
is achieved with the =3D 100-Ohm=3D20 line.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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


------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:    =3D20
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages=3D20
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =3D20

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:    =20
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages=20
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
 =20
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages 
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: