[SI-LIST] Re: diff pair questions

Chris,

I think you made the assumption on the drivers that
return current has to be on the planes. 
What about designing a driver to return current on
opposite trace on pcb, so we don't need to care about
reference to planes. A similar example is eithernet
Cat-5 cable, which has no ground refernce.

The reason I asked is the scenario:

diff pair with planes refernce on pcb --> coaxial 
cable --> other pcb

From pcb diff pair to cable diff pair, there seems
to have mismatch even though both have same
impendence-
one has reference to planes while the other don't.

This really puzzled me. Please advise.

Signal Hoss

--- Chris Rokusek <crokusek@xxxxxxxxxxxx> wrote:
> 
> Signal,
> 
> Try thinking about it as two single ended traces. 
> Then in terms of return
> current, a large proportion of the return current
> for each diff trace
> actually returns on the planes rather than the
> opposite trace.  So what you
> really have in terms of currents is a separate
> signal/return loop for each
> diff half.  These two loops are equal and opposite
> but spaced very near each
> other (in fact on the planes they overlap at all but
> the highest frequencies
> causing cancellation, less noise, good stuff). 
> Another way to visualize
> this is that the E/H field lines are "contained" by
> the planes.  Thus
> impedance will vary as the plane separation is
> varied.
> 
> Chris Rokusek
> Innoveda
> 


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