[SI-LIST] FW: Estimate min TL spacing based on NE/FE noise budget
- From: "Eric Bogatin" <eric@xxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 16 Nov 2004 06:17:57 -0600
Ken and Lei-
Coincidently, I just submitted my signal integrity column to
Printed Circuit Design and Manufacture magazine for their
Jan issue on this very topic: approximations for near end
cross talk and their accuracy compared to a verified field
solver. The bottom line is the only approximation I've seen
that is close to reasonable is the one by James Mears which
he published in the National Semiconductor application note,
AN905. Anyone can download a copy of this from their web
site.
I've compared this to the Ansoft 2D field solver, which I
have independently verified to better than 1% absolute
accuracy. The approximation for stripline is not bad-
accurate to better than 10% in predicting NEXT. For
microstrip, it is accurate to about 20%, providing the
coupling is > 5% or so, depending on the line impedance; the
higher the impedance, the more accurate the approximation.
His approximation is that the coupling decreases
exponentially with spacing. This is a pretty good fit for
stripline, but only a good fit for microstrip with tight
coupling. In my column, I have examples of both coupled
microstrip and stripline for the case of 25, 50 and 75 Ohm
lines.
I completely agree with Ken's comment that you should never
use an approximation to sign off on a design- where being
off by 10% might cost you a board spin and 4 weeks of lost
time. Always use a verified 2D field solver for stack up
design.
I found, in researching approximations in the literature
that this approximation is often cited, but unfortunately
sometimes miss quoted. For stripline, the coefficient should
be -2.9. I see it sometimes as -0.29, even in other NSM
documents. Watch out for this error.
The column will be published in the Jan issue of PCD&M. If
you do not already subscribe to this free industry trade
journal, I recommend it. You can subscribe by going to:
http://www.pcdandm.com/pcdmag/ . If you subscribe now, the
first issue you receive will have this column in it.
Hope this helps.
--eric
***************************************
Eric Bogatin
Bogatin Enterprises
OnLine Lectures on Signal Integrity
26235 w 110th terr
Olathe, KS 66061
v:913-393-1305
cell: 913-424-4333
f:913-393-0929
e:eric@xxxxxxxxxxxxxxx
www.BeTheSignal.com <http://www.BogEnt.com>
Signal Integrity- Simplified
published by Prentice Hall
*****************************************
From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Estimate min TL spacing based on
NE/FE noise budget
Date: Mon, 15 Nov 2004 08:24:23 -0700
Lei,
I have never found a closed form solution that is even
vaguely accurate when it comes to coupled TLs. They usually
fall apart on the odd mode capacitance (when compared to
field solver results). I certainly wouldn't design with
them. I use hand calculations to increase my understanding
of the fundamental concepts of coupling, but nothing more.
If anyone on the list knows of some 10% accurate,no, even
20% accurate formulas when compared to solver results, I
would appreciate links also.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of lei luo
Sent: Saturday, November 13, 2004 11:54 PM
To: si-list@xxxxxxxxxxxxx
Cc: lluo3@xxxxxxxx
Subject: [SI-LIST] Estimate min TL spacing based on NE/FE
noise budget
Hi, ALL:
I am doing hand calculation to estimate the Xtalk. I have a
cross-talk noise budget (for both backward(Near End) and
forward(Far End) noise) and want to estimate the min space
needed between two microstrip lines.
1) Based on the Xtalk noise budget, I estimated the backward
crosstalk coefficient Kb and forward crosstalk coefficent Kf
through Vb=Kb*Vm and Kf*L*Vm/Tr
2) But I don't know how to find the Min Space from the Kf
and Kb.
Can someone give me suggestions,relative papers or URL
links? Thanks in advance.
Lei Luo
Research Assistant
ECE, NC State University
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