What is a Time Domain Reflectometer, TDR

Time domain reflectometers are used for testing cables like twisted pairs, coaxial cable, etc., where they can locate the position of faults.

Time domain reflectometer, TDR, includes:
TDR basics Optical TDR

Time domain reflectometers, TDRs are used for testing cable systems and other forms of feeder where they are able to detect and pinpoint issues.

As a result, time domain reflectometers, TDRs are widely used in any area where there may be long or inaccessible lengths of cable that may need to be tested, or they may have faults. Time domain reflectometry can also be used on printed circuit boards to locate issues that can arise there as well.

TDR applications

Time domain reflectometers, TDRs are used in a variety of applications, some obvious, but others less so.

Some of the TDR applications include:

Telecommunications cable landlines: TDRs are an invaluable tool for telecommunications field engineers who need to repair telephone and broadband landlines. The can be used for testing of very long cable runs, where it is impractical to dig up or remove what may be a kilometres-long cable. If a break occurs, a TDR is able to locate the position of the break with considerable accuracy.
Landline preventative maintenance: TDRs are used for preventive maintenance of telecommunication lines. They can detect resistance on joints and connectors as they corrode. TDRs can also detect increasing insulation leakage as it degrades and absorbs moisture. Ultimately this can lead to catastrophic failure, but the TDR is able to detect this before this point is reached.
Landline security surveillance : Time domain reflectometers can detect the existence and location of wire taps. The wire tap introduces a slight change in line impedance and this can be seen on the TDR when connected to a phone line.
Circuit board testing: Specialised time domain reflectometers can be used for the failure detection of modern high-frequency printed circuit boards, especially on tracks designed to emulate transmission lines. The reflections seen by the TDR reveal any unsoldered pins of a ball grid array device or short circuited pins, etc..
Industrial applications: Time domain reflectometry is used in a variety of industrial applications, including the testing of integrated circuit packages where failing areas of an IC can be detected. TDR technology can even be used for measuring liquid levels, etc..

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.

Basic block diagram of a time domain reflectometer, TDR

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.

Idealised waveforms seen by a time domain reflectometer, TDR

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.

Distance = ν_{ρ} (\frac{T}{2})

Where:
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.

ρ = \frac{E_{r}}{E_{i}} = \frac{Z_{L} - Z_{0}}{Z_{L} + Z_{0}}

Where:
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.

Although it may appear to be a specialist test instrument, the TDR is widely used in a variety of industries, but particularly within the telecommunications industry where it is an invaluable tool. Without the time domain reflectometer, locating problems with long inaccessible lines would be very difficult and costly.