Time Domain Reflectometer Tutorial

- summary or tutorial about the basics of time domain reflectometry techniques and the time domain reflectometer, a test instrument used for locating faults in long cables.

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The time domain reflectometer is an item of test equipment used to locate faults in inaccessible cables.

The time domain reflectometer is typically used for long runs of cables such as twisted pairs, coaxial cables and the like.

However the test instrument may also be used for applications including the location of discontinuities in connectors and even printed circuit boards where failures may not always be obvious or easy to trace.

The time domain reflectometer is widely used in areas such as telecommunications where long cable runs are often used. However these are not the only areas where they are used.

Time domain reflectometer basics

The basis is time domain reflectometry is to treat a cable as a transmission line and look at its properties in this manner.

Although it is possible to use instruments such as network analysers and the like to check the integrity of cables this way, these test instruments are very expensive and not easy to use. A much better approach for many applications is to use time domain reflectometry techniques and a specific test instrument. This considerably simplifies the operation as well as reducing the cost of the test instrument. Also many time domain reflectometers are specifically made for portable operation, enabling them to be used far more easily in the scenarios where they are required, i.e. for telecommunications cables that may be running under roads, paths, etc.

The time domain reflectometer operates by sending a short pulse along the line in question. With the far end terminated in the required impedance, i.e. that of the line, if there are no problems with the line, then all the energy in the pulse will travel along the line at the propagation velocity and be dissipated in the load and no reflection will be observed.

Time domain reflectometer functional block diagram
Time domain reflectometer functional block diagram

From this it can be seen that the time domain reflectometer consist of a pulse generator and a sampler. The sampler could be an oscilloscope that displays the waveforms on the line. In reality a little more signal processing is often included to help locate problems and issues with the line

However if there is a discontinuity in the line, energy will be reflected back to the reflectometer where it is detected.

Within the reflectometer it is possible to analyse the returned pulse assuming that the voltage of the outgoing pulse level is Ei, and the reflected pulse has a level Er.

Time domain reflectometer waveforms
Time domain reflectometer waveforms

It can be seen that the outgoing pulse registers on the sampler screen. It then takes a finite time for the pulse to travel along the line. If all the power from the pulse is absorbed, then nothing will be returned and the display on the sampler will not show any change. However if power is returned it will alter the overall shape of the waveform seen at the test instrument.

The power return may occur for a variety of reasons from a break somewhere in the cable to a poor match at the remote end. The time delay, T will be twice that for the wave to travel to the mismatch point, i.e. out and return time together.

The sampler will be able to detect not only the level change and be able to calculate the mismatch, but also the time difference from which the distance along the line where the discontinuity exists can be calculated.

Locating cable faults

One of the key points of a time domain reflectometer is that it is able to locate failures within a cable. This is a key issue where the cable may be sealed as in the case of coaxial cable and it may not be possible to see inside the cable. Also where cables are buried under ground any failures can be located and the required holes dug to locate the area where the cable problem has occurred.

Time domain reflectometry equation

    D = distance in metres
    νρ = velocity of propagation in metres per second
    T = transit time from the monitoring point to the mismatch in seconds.

This is a straightforward calculation to make and is normally made within the time domain reflectometer, giving the user a good indication of where the fault may be located.

The main issue is the propagation velocity within the cable. This can be determined by testing a known length of the cable under test and leaving the remote end open.

Nature of mismatch

Not only is it possible for the time domain reflectometer to discover where the fault or problem has occurred along the cable, it is also possible to discover much about the nature of it as well. The reflected pulse enables the test instrument to see both the nature and magnitude of the mismatch.

Time domain reflectometry equation

    ρ = reflection coefficient
    ZL = load impedance in ohms
    Z0 = line impedance in ohms

From a knowledge of the reflection as well as either Z0 or ZL, either ZL or Z0 can be determined.

ZL can be determined for any tests by placing a known load at the end of a good line, e.g. a spare length of coax, etc., and the cable impedance can be determined from this. With a knowledge of the cable impedance it is then possible to apply this to the cable under test.

By Ian Poole

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