From: Nirmal Jain (nirmalj@pacbell.net)
Date: Mon Jan 10 2000  17:33:02 PST
Long Wang wrote:
> Hello,
>
> I have a couple questions about backward crosstalk. In Ansoft 2D tool,
> the backward crosstalk coefficient can be calculated. My questions are :
> Assuming to consider 2 lines parallel each other, one line is active and
> the other is a victim.
> 1. In the result matrix of Ansoft 2D tool, the diagonal element is 0.5.
> What does that mean?
Dear Long,
In this analysis, your near end is connected to a source with a source resistance of Zo, where Zo is the characteristic impedance of the transmission
line. Due to the voltage divider action, you have 0.5*Vs across the transmission line where Vs is the source voltage.
>
> 2. The offdiagonal element, for instance, is 0.01. Does it mean 1%
> coupling from the active line? Does the backward crosstalk depend on
> risetime of a signal? If no, can you explain it?
>
Yes you are right, if the offdiagonal element is 0.01, it would mean you would have maximum 1% coupling from the active line. The picture below depicts
how backward crosstalk behaves. "a" is for a smaller rise time than "b"
V




 the crosstalk would saturate at Length = Vprop * Tr/2
 where Vprop is the propagation velocity and the Tr is the rise time

 /*/*************************** maximum near end xtalk
 a / /b
 / /
/_ /_____________________________________________
Length
Hence the slope of the line (a, b) would change if your rise time changes.
Hope it makes sense. Feel free to give me a call if you have any further questions.
Warm regards
Nirmal
>
> Thanks a lot!
>
> Long
>
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 Nirmal Jain Senior Application Engineer Signal Integrity ProductsAnsoft Corp. 2560 Oak Rd. #322 Walnut Creek, CA 94596 (925) 9386490 (Tel) (925) 2152477 (Fax)
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