Reflectometers are electronic instruments that use time-domain reflectometry to characterize and locate faults in metallic cables. They can also be used to locate discontinuities in a connector, printed circuit board, or any other electrical path. For optical fibres, an optical time-domain reflectometer is used. TDRs measure reflections along a conductor by transmitting an incident signal onto the conductor and listening for its reflections. If there are any impedance variations, then some of the incident signal will be reflected back to the source.