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

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From: Phares, Charles C ([email protected])
Date: Thu Apr 27 2000 - 13:15:58 PDT

with respect to using coaxial cable when doing a DIFFERENTIAL tdr
measurement... ideally you use need to use a DIFFERENTIAL sampling head on
your tdr scope, which will have two coax connections. the two coax shields
become common with the ground plane of the DUT and the two center conductors
attach to the differential pair. as with our tektronix tdr scope, there is
a sampling head function menu which allows you to set the polarity of the
differential heads channels.
The only problems that I know of come into play when either your interface
to the differential pair is messy, or when the differential pair is routed
in a stripline mode with one of the reference planes being a power plane as
opposed to using two ground planes. on a de-populated board, this isn't a
problem, you just have to remember to tie the power plane and ground plane
I have made many TDR measurements, both s.e. and differential. I always use
coax cable as an interface from the scope to the DUT, and I have yet to be
disappointed. As far as the origianl topic about "Method 1" vs. "Method
2"... the questions seems irrelevant, since you always want to make your
measurent as close to how the device was meant to function while still
maintaining a controlled environment... only method 1 does that. If your
PCB vendor is using method 2, it is only for cost reasons, not accuracy.

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