# Re: [SI-LIST] : backward Xtalk?

From: Nirmal Jain ([email protected])
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 off-diagonal element, for instance, is 0.01. Does it mean 1%
> coupling from the active line? Does the backward crosstalk depend on
> rise-time of a signal? If no, can you explain it?
>

Yes you are right, if the off-diagonal 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 Products
Ansoft Corp.
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