An instrument for comparing the incident and reflected waves at a device or equipment under test. It is generally used at radio or microwave frequencies in a distributed circuit or transmission line system, and compares the incident and reflected electrical power waves. The comparison can be either magnitude only, yielding a scalar measurement, or of magnitude and phase, a vector measurement. The reflectometer is generally used in a CW mode, but can be used with pulsed waveforms, where the reflected signal is received after a time delay, and provides a measure of the location in space of discontinuities in the network. When used in this manner, the instrument is called a time-domain reflectometer. Such measurement techniques find application in optical fibre systems, where discontinuities and branches in the fibre network can be characterized.