RE: [SI-LIST] : Differential TDR "Measurements"

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From: Zabinski, Patrick J. (zabinski.patrick@mayo.edu)
Date: Tue Apr 25 2000 - 10:45:04 PDT


Fred,

To be honest, I don't know the answer to that. Approach 2
is actually implemented in commercial equipment, and I
have yet to get a complete theoretical/implementation explanation
of how it does it's 'mathematical magic'.

Under 'linear' conditions (per Dima's posting) *AND* conditions
where both traces have exactly the same relationship to ground,
I can envision where Approach 2 would work just fine. However,
we rarely work under ideal conditions.

For example, we sometimes must route differential pairs along
edges of planes, or we might try to isolate one pair from another
by routing a ground-strap between them, or we might use broad-side
coupling microstrip. These conditions (and
SEVERAL others) place a stronger relationship to ground for one
signal trace than the other, and I have yet to obtain an explanation
how the theory/equipment is able to take this into account.

If someone could clue me in, I'd appreciate it. Such an explanation
alone would solve much of my dilemma.

Pat

> Pat, in approach 2 while injecting a signal in trace 1 what is
> being done to trace 2? Is is driven with a complement, grounded,
> left open etc. To the degree that the traces are coupled method
> 2 will have some problems if the second trace is not taken care
> of.

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