[SI-LIST] Re: general belief about Xtalk

Mick,

As David pointed out, the near-end crosstalk coefficient is proportional 
to the sum of capacitive and inductive couplings
(normalized to self capacitance and self inductance), whereas far-end 
crosstalk coefficient is proportional to the
difference of these same factors.  One more key element to keep in mind: 
the actual crosstalk signal will also depend
on the coupled length (assume homogeneous coupled lines).  Both can be 
non-saturated, where the crosstalk magnitude
is proportional to the coupled length, or saturated, where increasing 
the coupled length further does not increase the
resulting crosstalk signal any further.  The interesting twist is that 
the far-end crosstalk saturates at  higher levels
than near end crosstalk.  So for typical PCB stripline traces, where the 
far-end crosstalk coefficient is very low, we
can still get very high far-end crosstalk signal if the coupled length 
is significant.  Eventually losses will reduce the
crosstalk, but for instance in surface microstrips you can easily create 
a scenario where far-end crosstalk exceeds
near end crosstalk. 

Regards,

Istvan Novak
SUN Microsystems


Mick zhou wrote:
> 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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