An Introduction to the Time Domain Reflectometer

We have been using Time Domain Reflectometers (TDRs) on telephone cables since the late 1960s, but only the best technicians know how to truly capitalize on the advanced capabilities offered by the TDR. This one piece of equipment can triple your efficiency and reduce call backs. But you have to know more than the basics: you really need to maximize on this tool’s capabilities.

How Does the TDR Function?
A TDR is a test set used to characterize and locate faults in metallic cables. It transmits a short rise time pulse along the cable pair. If the cable pair is of a uniform admittance and properly terminated, the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected toward the TDR. Any impedance discontinuities will cause some of the incident signal to be sent back towards the source. This is similar in principle to radar.

An increase in the impedance (an open cable pair) creates a reflection that reinforces the original pulse. A decrease in the impedance (a solid short) creates a reflection that opposes the original pulse.

The resulting reflected pulse that is measured at the output input to the TDR is displayed or plotted as a function of time, and because the speed of signal propagation is relatively constant on a cable pair, the total travel time of the pulse down and back can be read as a function of cable length.

Because of this sensitivity to impedance variations, a TDR may be used to identify cable impedance mismatch characteristics, such as: an open cable pair, a shorted cable pair, a grounded cable pair, a splice, a load coil including a smart coil, a capacitive build out in a section of cable, the beginning of a lateral or bridged tap, the end of a lateral or bridged tap, series resistance, water in a section of air core Plastic Insulated Conductor (PIC) cable, and to estimate cable lengths.

The average impedance of a telephone cable pair is 100 ohms. When using a TDR designed for telephone cables the base line is 100 ohms. Some TDRs indicate the impedance by matching the display line to a reference on the screen.

Using the TDR
When connecting a TDR to a cable pair properly terminated at the far end there would be an input or launch pulse on the left of the screen and then a flat line would show to the termination.

Anything that shows on the screen that is not on the base line is an impedance mismatch including the launch pulse. The launch pulse is where the clips are connected to the cable pair.

An open cable pair shows more impedance than the cable pair so the trace goes up.

A short or a ground has less impedance is less than the pair so the trace goes down.

A splice reflects very little of the energy back and usually looks like an S lying on its side.

A load coil allows some of the energy to be absorbed and the face of the trace is not as sharp as an open and the back of the trace is longer.

A lateral or bridged tap has less impedance at the beginning, and if the end of the bridged tap is open its portion of the trace will be up. If the end of the cable is open its trace will also be up.

Series resistance reflects back less energy, and because its impedance is greater than the cable pair and enough gain is used on the TDR it will be similar to an open cable pair.

These are basic pictures of TDR traces. When you are using a TDR on a cable pair the pictures can be clouded and mixed in with clutter. With distance the twist of the pair will cause reflections or ripple. In some instances the filter will mitigate these ripples.

Your TDR can be used as a standalone unit for major reflections and as a companion meter along with an open meter and a resistance bridge to do thorough section analysis of faulted cable sections.

In order to become proficient with your TDR, practice in a work center with known lengths of cables and drops. Make splices, opens, shorted and grounded cable pairs, splits, and other faults that will be encountered in the field.

Signing off
Was this helpful? Let me know if it was, and if you have questions or advice and tips on using a TDR. Also, I will be at OSP EXPO 2009 September 2-3 in Minneapolis, Minn., and I hope you will, too. This year I will again host an "Ask the OSP Expert" session. Send me questions you would like answered during this session. We will also take questions at the time of the event. Call me, email me, just don’t ignore me! www.mccartyinc.com. [email protected]. 831.818.3930.