On Wed, 21 Sep 2005 01:15:52 +0000 (UTC), (David
Combs) wrote:

In article ,
Neon John www.johngsbbq.com wrote:
On Mon, 19 Sep 2005 05:55:54 +0000 (UTC), (David
Combs) wrote:


He said there was some kind of device they could attach
to *one* end and it could tell you how many feet down
the wire the break was.

It's called a Time Domain Reflectometer or TDR.
snippity
Man, that's pretty cool!

(Uh, what's such a thing *cost*? (just wondering))

Wide range. Little handheld units that display only footage and
designed for a specific application might run $500. A top of the
line Tektronix scope-based TDR might run $4k or more. Then there's
the "poor man's TDR". A fast risetime pulse generator and a suitable
oscilloscope with a delayed timebase or a digital scope. If that kind
of equipment is already on hand, the cost is little more than some
interconnect cables and the time involved.

One launches the pulse, times the return on the scope screen and then
computes the distance manually. In the good old days, with hand
prepared lookup tables or slide rule. Nowadays with a programmable
calculator or handheld computer like a Palm.

A scope-based TDR can convey all sorts of info beyond an open or short
and distance. Any impedance discontinuity causes a reflection. A
good unit like the tek can see the impedance upset in even a good
connector like a Type N. It can see coax bent too sharply or a staple
put in too tightly. It easily sees splices. One can even get a
pretty good idea of an antenna's match (SWR) by looking at how little
of the pulse is returned.

Stuff that's come along "since my time" :-) includes checking the
transmission line characteristics of digital buses on PCBs and the
like. Here's some good reading from the Tek site if you're
interested:

http://www.tek.com/Measurement/appli...lysis/tdr.html

Looks like they're building the TDR function into the higher end
digital scopes now. Slick. I still like my old analog Tek TDR
though. The eye is still mightier than the DSP for some things.

John
---
John De Armond
See my website for my current email address
http://www.johngsbbq.com
Cleveland, Occupied TN