Time domain reflectometry

Updated July 12, 2010

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New for July 2010! Yet another topic that has been missing too long in the Microwaves101 index, as recently pointed out by Poul-Henning.

TDR measurements provide a means toward fault isolation in systems where transmission lines are inaccessible, like buried fiber optic.

For now, we'll merely paste in Poul's email and hopefully get back to creating content on this topic soon.

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.