The MAGIC behind time domain reflectometers

4 August 2014

I hear the word time domain reflectometer (TDR) and for some reason visions of the DeLorean time machine fitted with a flux capacitor comes to mind, you know, needing to generate 1.21 ‘gigawatts’ of electricity. A time domain reflectometer just sounds like something Doc Brown would have used to travel back to the future.

In reality, a TDR has nothing to do with time travel, and has everything to do with the measurement technique used to determine the characteristics of electrical lines by observing reflected wave forms of electricity. In essence, it locates faults in metallic cables like twisted pair wire or coaxial cables.

So, is the TDR magic and mysterious like the flux capacitor? Well, not exactly….The TDR transmits a short rise time pulse along a conductor. If the conductor is of uniform impedance and is properly terminated, the entire transmitted pulse is absorbed by the far end termination and no signal will be reflected back to the TDR. If there is a break in the impedance, this will cause some of the signal to be sent back toward the original source (like radar), alerting the user that there is a problem.

Since the TDR is very sensitive to impedance variations, it is often used to verify cable impedance characteristics, splice connector locations and associated losses as well as estimate cable lengths. TDRs are used for a variety of applications including in place testing for very long cable lines where digging them up or removing them is impractical; preventative maintenance of telecommunication lines as they can detect resistance on joint and connectors as they corrode; technical surveillance counter-measures, where they can determine the existence and location of wire taps; as well as in testing of integrated circuit packages to measure liquid levels.

Maybe Doc Brown could have found a way to use this electronic instrument in his lab….