Time-domain reflectometer

In telecommunication, a time-domain reflectometer (TDR) is an electronic instrument used to characterize and locate faults in metallic cables (e.g., twisted pair, coax).

Note 1: A TDR transmits a fast rise time pulse along the conductor. The resulting reflected pulse is measured at the input as a function of time and displayed on the instrument or plotted, as a function of cable length.

Note 2: A TDR may be used to verify cable impedance characteristics, splice and connector location and associated losses, and estimate cable lengths.

Source: from Federal Standard 1037C