A TDR measurement is done by generating a very fast, clean voltage step signal that is monitored by a high-bandwidth digitizer. A National Instruments high-speed digitizer board combined with signal processing techniques are the basic tools necessary to perform a TDR analysis.
One possible test setup is to use a signal source (such as a NI Digital Pattern Instrument, Waveform Generator, Pulse Generator, etc.) and high-speed NI Scope/Digitizer to perform TDR. In this example, a step signal is generated by the signal source and sent down a transmission line (i.e. coaxial cable) with impedance Z_0. As the step propagates along the transmission line, impedance discontinuities cause some signal energy to be reflected back to the source. These reflections are measured by the oscilloscope and then can be analyzed to yield information about the nonideality of the medium.