[SI-LIST] general belief about Xtalk
- From: "eric bogatin" <eric@xxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>, <mick.zhou@xxxxxxxxx>
- Date: Tue, 24 Feb 2009 09:18:24 -0600
Mick-
Thanks for sharing your myths about cross talk. I also have heard the same
misconceptions you mentioned that near end is greater than far end cross
talk for single ended and diff signals. I'll throw in another one that near
end differential cross talk between differential channels is dramatically
reduced when you increase the coupling between the lines that make up the
differential pair.
I have found that just about every "myth" has some basis in reality. In the
case of cross talk, as has been pointed out, its about the distributed
capacitive and inductive coupling, the propagation direction of the active
signal and the fact that the induced noise in the quiet line is dynamic and
also propagates. These features give rise to the different signatures of
noise at the near and far end of the victim line.
While the most common answer to all signal integrity questions is, "it
depends", it is possible to make a fair generalization that in stripline,
where the dielectric is relatively uniformly distributed around the signal
lines, near end noise is almost always greater than far end noise- for
single ended as well as differential signals.
Likewise, in microstrip traces, where there is air above, dielectric below,
generally, far end noise can be much larger than near end noise. Of course
far end noise also scales with coupling length and inversely with rise time.
This means that near end noise could be larger than far end, for short
coupling and longer rise time. The only way to know is to "put in the
numbers". I have some examples of how to estimate the magnitude of near and
far end cross talk in my book.
If you want to read about the myth of differential noise and coupling, I
wrote a piece for PCD&F magazine on this a few months back. You can download
it for free from:
http://www.bethesignal.net/bogatin/bts110-cross-talk-differential-pairs-p-28
3.html?cPath=23.
I wrote a longer piece about cross talk in single ended and differential
pairs in EDA Tech Forum, which you can download from:
http://www.bethesignal.net/bogatin/bts108-four-tips-reduce-cross-talk-p-269.
html?cPath=23
As a final teaser, if you really want to understand the origin of far end
cross talk and why there is such a big difference between microstrip and
stripline, you have to understand the difference in speed between a common
signal and a differential signal in microstrip. If you understand this
subtle feature, the origin of why there is far end noise in microstrip and
no far end noise in stripline will be obvious. (hint, it is in my book)
Enjoy
--eric
*******************************************************
Dr. Eric Bogatin, Signal Integrity Evangelist
Bogatin Enterprises
Setting the Standard for Signal Integrity Training
"No Myths Allowed" webinar series
"Practical Differential Pair Design Feb 25, 1pm EST
26235 W 110th Terr
Olathe, KS 66061
e: <mailto:eric@xxxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx
v: 913-393-1305 cell: 913-424-4333 f: 913-393-0929
<http://www.bethesignal.com/> www.BeTheSignal.com
Blog: <http://www.bethesignal.com/blog> www.beTheSignal.com/blog
***********************************************
Msg: #6 in digest
Date: Mon, 23 Feb 2009 16:58:47 -0700
Subject: [SI-LIST] general belief about Xtalk
From: Mick zhou <mick.zhou@xxxxxxxxx>
Hi,
Recently, I run into some basic beliefs about Xtalk from my colleagues. They
seem match intuiations but may not be right.
Statement 1: Single-ended near-end Xtalk is always greater than far-end
Xtalk, both in f and t domains.
Statement 2: Differential/Common near-end Xtalk is always greater than the
far-end, both in f and t domains.
I do not remember any general proofs of the above statements. They may be
true for many practical cases but may not be generally true. One example in
S. H. Hall's book, p.50. disproves the statement 1 in t-domain.
Any more helps?
Mick
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