Re: [SI-LIST] : Lab procedures for TDR

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From: Bob Lewandowski (Bob.Lewandowski@Vixel.com)
Date: Fri Mar 24 2000 - 09:32:34 PST


Agilent has a fairly snazzy CD-ROM with videos, called "Time Domain
Reflectometry with the HP 54750A Digitizing Oscilloscope" (P/N
5968-2517E) that covers the basics. The video presentation is excellent
for visualizing what is going on. Their instruction manual for the
54754A Oscilloscope plug in has a theory of operation section that
covers a lot of practical details. Better than Tek's SD-24 manual.

When looking at a TDR result, one must be very careful about the
character of reflections downstream of a major discontinuity, or change
in impedance. A downstream reflection will be "attenuated" by the
reflections caused by the upstream discontinuities (multiple reflection
interactions). This can be best observed by one of the "demo" pc boards
that come with TDRs (Beatty standards) that have an SMA
launch to a short section of 50 ohm micro strip line, a long section of
25 ohm micro strip line, and another short section of 50 ohm line to
another SMA. They are usually symmetrical lengthwise about
the center. Put one of these on a TDR and look at the far transition
from the low impedance line back to the higher impedance line. It looks
different than the transition from higher Z to lower Z at
the input side. Such that one might think they are actually different
electrically. Then swap ends on the board, and the picture will stay
the same.

It is usually a good practice to provide a matched termination at the
far end of the DUT to reduce issues with multiple reflection
interactions (triple travel). Open lines have ~100% reflection and
things can get cluttered pretty easily. Also matched terminations
provide a reference impedance that allows one to better understand the
effects of distributed series resistance (skin effect), as
distributed R puts an up-slope on the TDR trace, making impedance
markers seem inaccurate.

Differential TDR also adds another significant layer of complexity.

---Bob Lewandowski
    Vixel Corp.

Kevin Dale Kirmse wrote:

> I am looking for references on procedures for using TDR equipement.
> I am taking measurements using a Tek CSA803 + SD-24 and don't trust
> some of the results I am getting. It is quite possible I am simply
> not interpreting what I am seeing correctly. The 75 ohm single /
> 150 differential impedance of fibre channel makes life "interesting".
>
> ---------------------------------------------------------------------
> | Dr. Kevin Dale Kirmse, PhD EE
> | Portable System Design, High Speed Serial Links
> | FPGA Design, Video Hardware, Graphics Hardware
> |
> | King of Prussia, PA 19406
> | kirmse@netaxs.com
> ---------------------------------------------------------------------
>

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