[SI-LIST] : Differential TDR "Measurements"

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From: Zabinski, Patrick J. ([email protected])
Date: Mon Apr 24 2000 - 17:45:29 PDT

We're working more and more with differential signals,
and subsequently dealing with more differential printed
circuit boards (PCBs). Over the past few years, we've
had difficulty with several PCB vendors
trying to obtain a controlled impedance 100 ohm
differential pair.

The problem generally boils down to "who's measurement
do we believe"? We measure one impedance, while the
PCB vendor measures another.

We've done some digging, and there appears to be two
approaches to measuring differential impedance, and I'd
like to hear what folks have to say about them.

Approach 1: inject two signals of opposite polarity,
one into the true and one into the complement. The
complement signal is substracted from the true, and
you read the impedance just like a single-ended

Approach 2: Inject one signal into the true trace and
record its signal. Then, inject a signal into the complement
trace and record its signal. Then, with the magic of
mathematics, compile these two different captured signals
into an effective differential measurement.

The equipment we have in-house uses Approach 1, while
nearly every board vendor we work with uses Approach 2.
Can anyone shed some light into the accuracies, sensitivities,
etc. of these two approaches? Are there cases where one
approach is better/worse than the other?


  Pat Zabinski ph: 507-284-5936
  Mayo Foundation fx: 507-284-9171
  200 First Street SW [email protected]
  Rochester, MN 55905 www.mayo.edu/sppdg/sppdg_home_page.html

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