[SI-LIST] Re: diff pairs and spacing
- From: "Beal, Weston" <weston.beal@xxxxxxxxx>
- To: <lthemanz@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Tue, 21 Aug 2007 08:06:23 -0700
Lthemanz,
The quote from Dr. Bogatin is talking about a single-ended trace with
the second trace being a grounded conductor. Your first statement is
about differential signals. Here you must be careful in applying the
details of one analysis to the other. Dr. Bogatin's description of the
fields still applies, but the calculation of the differential impedance
is different. In the single-ended case, the signal trace doesn't
recognize much effect of having the ground trace nearby as explained in
the article. In the case of a differential pair the second trace is now
part of the signal conductor (pair) and the interaction of the EM field
between the traces has more effect on the differential impedance. Also
remember that for a symmetric differential pair the differential
impedance is 2*(Z0 - Zm) so the differential impedance (affected by Zm,
mutual impedance) decreases twice as fast as the single-ended impedance
when moving the second trace closer to the first. If you do the same
analysis as described in the article, but measuring (or calculating) the
differential impedance, you should get a very similar impedance curve,
but with a different scale.
Regards,
Weston
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of lthemanz
Sent: Monday, August 20, 2007 11:29 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] diff pairs and spacing
Hi folks,
Typically our SI / EE guys tells us, give me a diff pairs with 1x or
1.5x spacing between +/-.=20
If we apply below 'equation'=20
=20
"The proximity of the grounded second trace really does decrease the
impedance of the signal trace, and it drops it more as the ground
trace gets closer. However, the effect is very small. As long as the
adjacent trace is at least a line width away, it has an impact of less
than 0.1 on the impedance of the line. When it is within half a line
width away, the impedance drops by 1 , and then drops faster as
the ground trace gets closer."
=20
http://www.bethesignal.com/_FileLibrary/MonthlyColumn/64/BTS076_Calculat
ing_Characteristic_Impedence_0307PCDM.pdf
=20
Doesn't anything more then 1x have no proximity effect? Or am I applying
this wrongly.
=20
thanks
-lthemanz
=20
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