[SI-LIST] : Differential TDR Measurements

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From: Eric Bogatin ([email protected])
Date: Tue Apr 25 2000 - 12:38:00 PDT

Pat and others interested in differential TDR measurements:

Mike Resso, of Agilent, and I, did a paper at the last DesignCon on this
topic. We also put one together on a similar topic at the Intel developers
forum. Copies of both papers can be downloaded from my web site:

Go to the publications section and sign in. They are in the Jan and Feb
download sections. Hope you enjoy them.

The Agilent 83480 Digital Comm Mainframe with a dual TDR plug in works
really well to extract differential impedance of a pair of lines as well as
the odd and even mode impedance of each lines. With a dual channel high
bandwidth amplifier plugged into the 3 and 4 channels, you can emulate high
speed digital signals through the lines and look at the generation of common
mode signals due asymmetries. All of these issues are explained in our

I always welcome comments.


Date: Mon, 24 Apr 2000 19:45:29 -0500
From: "Zabinski, Patrick J." <[email protected]>
Subject: [SI-LIST] : Differential TDR "Measurements"

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

Eric Bogatin
Training for Signal Integrity and Interconnect Design
26235 W. 110th Terr.
Olathe, KS 66061
v: 913-393-1305
f: 913-393-1306
e: [email protected]
web: <http://www.bogatinenterprises.com/>
ftp: ftp://ftp.BogatinEnterprises.com

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