Time Domain Reflectometry (TDR) and Time Gating

Time Domain Reflectometry (TDR) is a technique that measures and displays the impedance of a network (cable, filter etc) over time. Traditionally this is done with a device that generates very fast pulses and injects them on the network, and measures the time and amplitude of reflected pulses.

This gives information about the impedance as a function of the time it takes to reflect. And, since the signals travel with (nearly) the speed of light, the time can be translated to a distance through a cable.

We can do the same kind of measurement with a VNA but in a roundabout way. A VNA measures the (reflected) impedance over frequency. We can use a Fourier transform to convert this into a series of impedances over time, just as a TDR instrument produces.

With TDR functions you can analyze a coax cable or circuit impedance as a function of the distance from the measurement port. This gives information about the physical geometry of a network and possible problems.

One application is Distance to Fault (DTF) analysis, to find the place of a damaged cable.

TDR also makes it possible to apply Time Gating, to filter out part of the Gain response in time.