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

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From: Fred Balistreri (fred@apsimtech.com)
Date: Tue Apr 25 2000 - 11:19:12 PDT


Hello Pat, you have raised some other issues. The relationship
to gnd of diff pairs should be the same. If not you won't have
a balanced differential signaling. The traces need to be symmetrical
w.r.t. gnd or the planes.

Approach 2 is theoritcally OK. The problem is in the measurement.
The impedance measured on trace 1 will depend on what trace 2
is doing. This is the fundamental problem with approach 2. One
can get many different answers with % error being a function of
coupling. Approach 1 is a more desirable way to measure since
your driving the system differentially the problem of trace 2 does
not exist.

If your needs are critical I would either have your suppliers get a
tdr system which can measure diff signals using method 1. This
means a three prong approach with the syncronized pulse launching.
OR qualify a different vendor. There are plenty of PCB vendors that
don't have this problem. If you like I could recommend some of these
to you off line.

Best Regards,

"Zabinski, Patrick J." wrote:

> 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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--
Fred Balistreri
fred@apsimtech.com

http://www.apsimtech.com

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