What is time domain reflectometry?


The ability for handheld electronics diagnostics tools like the T3 Innovation Cable Prowler to identify the state of a wire simply by plugging into it is an immensely powerful technological advancement that has sped up the growth in size and complexity of networks. Without these useful devices, finding a shorted wire in a maze of cables connecting thousands of computers would seem nearly impossible. They do this through the use of Time Domain Reflectometry (TDR), or the measurement of electrical “reflections” along the wire.

When Does a Conductor Reflect a Signal?

If a conductor is connected to a circuit and has a continuous level of resistance throughout its length, then any electrical signal sent down it should dissipate at the end without generating any feedback. When there is a problem with the wire, such as an open or a short, the variation in current flow causes a reactionary waveform to be reflected back along its length. Noise in the wire can interfere with the reflected signal depending on the voltages being used.

How Does a Time Domain Reflectometer Work?

There are three major components necessary for measuring TDR. The first and foremost is a conductor that will be tested, whether it’s a coaxial cable or a Cat5e. The second item is an electrical source that can generate signal pulses for observation. Lastly, an oscilloscope needs to be placed down the line from the voltage source so it can capture both the primary and reflected waveforms. The exact operations of a diagnostic device’s TDR measurement function may vary, but the same underlying principle of having a conductor, a signal, and an observation method hold true.

How Do We Use the Properties to Our Advantage?

By sending just a small pulse (instead of a steady supply) of current down the conductor and then monitoring the electrical activity of the conductor through an oscilloscope, we can see the shape of both the primary signal and any reflections. By comparing the two, we can make assumptions about the state of the conductor. For example, a reflected waveform that has a lower amplitude and reversed sign of the original signal indicates a short circuit, whereas an amplitude reduction on its own would point towards an open circuit.

The minor variations in the signal tell the operator (or the program analysing the waveform) what sort of condition the wire is in, and the overall length of the function can be used to determine its length. One of the more interesting waveforms is the minor reflection caused by a wiretap on a phone line, but that’s one you shouldn’t come across too often. More commonly, TDR measurement is used for buried and inaccessible wires or extremely complicated and physically inconvenient systems like aeronautics equipment.

An Essential Part of the Technician’s Toolkit

Although TDR measurement is now one of the more mundane features on advanced tools such as the Cable Prowler, it is no less vital to their operational goal of quickly and easily ascertaining the state of the network, both physically and digitally. And now you know the basic method of creating a Time Domain Reflectometer if you are locked away in the basement of a mad scientist’s castle and need to find and replace a shorted wire to escape! You will probably just have a similarly amusing daydream while using the convenient function on your Cable Prowler, though.