Time Domain Reflectometry

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Updated April 2021

Time domain reflectometry covers a wide swath of techniques. You can generate time domain data from frequency data using fast Fourier transforms, or you can measure reflections directly in the time domain. This page is about the latter case; we are building a page on time domain analysis from frequency data here. 

Let's start this discussion with two short videos on TDR that we found useful.  The first one shows how TDR's work, from the MyConnector101's YouTube channel. Here you will see how to create a TDR response using a pulse generator and an oscilloscope. Did you ever wonder why you see a 2 volt pulse coming from a 1 volt signal on a pulse generator when you connect it to an unterminated oscilloscope input?  Really makes you think, as Walter, Owen's Grandp would tweet.

How a TDR works

TDR measurements provide a means toward fault isolation in systems where transmission lines are inaccessible.  Part of the skill of looking at TDR plots is being able to read the tea leaves. The second TDR video (below) is from an aircraft technician's point of view, on bounder97's YouTube channel.  It's easy to see where shorts and opens occur, where there are bad splices, and how noise affects the readings. It's Interesting to see how the resistance of the wire increases the Z0 reading, the farther you go down the cable. Bravo to the creators of both videos!

How to use a TDR

Bravo to the creators of both videos!

Below, we pasted content from Poul-Henning's email a while ago, which we used to start the TDR discussion.  Thanks again!

I was quite amused by the entry titled "Can you measure Z0 with a ohmmeter?" and would like to point out that such ohmmeters in fact do exist and have been commercially available for almost 50 years.

Not only will these instruments measure Zo of your cable, they will also neatly present the result graphically as function of the distance along the cable.

HP Journal Sept. 1963 v15/n1
introduces the instrument and HP Journal Feb. 1964 v15/n6
explains in detail and with many illustrative examples how useful this instrument is.

With a moderately good digital scope, and a good sharp squarewave, readily available from many members of the 74xx familiy, you can cook up your own TDR analyzer in no time. This is also a great demonstration of the (nearly) speed of light in coax cables.

I am actually surprised that Google is unable to find a little smart TDR construction for radioamateurs, it is an incredibly useful way to locate antenna and cable trouble.

The standalone TDR analyzer has been replaced today with professional network analyzers, which will present a TDR view by transforming a frequency sweep to the time domain with a FFT.

But in fibre optical transmission, TDR-meters still rule: That is how you find out where the ship or backhoe tore your fiber cable.


Author : Poul-Henning