From: curtiss@tekred.tek.com (Curtis Smith )
Newsgroups: comp.dcom.lans.ethernet
Subject: Re: Timedomain reflectometer Printout
Keywords: TDR
Date: 7 Apr 92 21:34:18 GMT



In article <1992Apr1.091156.13441@crow> ccut.cc.u-tokyo.ac.jp!crow!trich writes:
.
.After much messing around I managed to borrow a timedomain reflectmeter
.which I figured out how to use from the book "Keeping the Link"
.by Martin Nemzow.   The printout shows a phenomena which isn't described
.in this book.   I see the normal negative spike for each transceiver
.every 2.5 meters, but then after a group of transcievers I see a positive
.spike exactly 2.5 meters after the last transciver in the group.
.
.In fact after the last transceiver in any group I see a positive spike
.2.5 meters later.  why ?
.
.ccut.cc.u-tokyo.ac.jp!crow!trich		home: +81 (3) 3720-4088

In Reply Alan Whinery writes

>It sure looks like a termination problem. If you're getting what looks
>like a opposite-polarity spike 11 or 12 nanoseconds after the last 
>transceiver, is there a termination or a splice or a tap or a kink out 
>there somewhere?

>TDR can be very confusing.

> Anyways, if I'm not even warm, you can try the fax method...

Second Reply from Vesa Halkka

>>There could be some clever guy drilling holes to make transceiver
>>installation easier..

>>Or you are using Tektronix TDR with some too clever compare mode :-)

Repeated, inverted events on a TDR can be echos of real events.  First a 
TDR primer:

A TDR (Time Domain Reflectometer) works by looking for reflections from
impedance changes.  Any change in cable impedance (taps, ends of cables, 
splices, etc.) will cause a reflection.  The reflection height is related 
to the amount of impedance change at the event; small changes like taps 
cause a small change, large changes such as a open cable cause large 
reflections.  

(graphics for ascii terminals)

   ______      Cable             Event        TDR              Event
   | TDR|--------------------------x           /\               /\
   ------                                    _|  |_____________|  |_ 
   Signal >------------------------>
   Echo   <------------------------<              TDR Waveform

By measureing the time between when a pulse is put into the cable until
the echo returns you can tell how far away events are.  By looking at the
echo height and shape you can tell the type and severity of cable event.

What sometimes happens is if the TDR front end is not matched to the cable
impedance is you will get a re-reflection of the returning echo off the 
front panel connection of the TDR.  Part of the echo will reflect back out 
the cable, hit the event a second time, and come back to the TDR at twice
the time as the real event.  These Re-reflected echos will appear as inverted 
events at exactly twice the distance (twice the time) as real events.

   ______      Cable             Event
   | TDR|--------------------------x
   ------                                    /\         /\
   Signal >------------------------>       _|  |_______|  |_______    _
   Echo   <------------------------<                              |  | 
 re-echo  >........................>                               \/
2nd-echo  <........................<              TDR Waveform

What you see is your first events appearing inverted at twice
the distance of the real event.  To prevent this try:

 - Match the TDR impedance to the cable impedance.  Many Tektronix TDRs
   have an impedance control to do this.

 - Terminate the other end of the cable.  This will help control double
   echos.

Curtis Smith   curtiss@tekred.cna.tek.com
               Tektronix TDR/Cable Tester technical support
               U.S./Canada (800)833-9200
               fax (503)923-4434
Newsgroups: comp.dcom.lans.ethernet