Time Domain Reflectometers and What You Can Do With Them

Time domain reflectometers, or TDRs for short, have existed since the 1960s. These useful devices are increasingly more relevant as the world uses greater numbers of electronic cabling.

How Does a TDR Work?

A time domain reflectometer works a bit like radar. It sends a short rise time pulse through a metallic cable, no matter its length. It cannot be used on optical fiber cables.

  • Detecting Inconsistencies: If there are no problems in the line, all the power from the pulse is absorbed, and there are no changes to the waveform displayed on the device. If there are problems in the cable, parts of the signal bounce back and cause a change in the waveform.
  • Making Measurements: If there is an increase in resistance, such as what happens with an open cable pair, the reflection reinforces the original pulse. In the opposite case, such as with a solid short, the reflection opposes the original pulse. This is then plotted as a function of time. The average impedance of the line being measured is taken as the base line.

What Are TDRs Used For?

There are numerous applications for TDRs in various industries. The device has been used in telecommunications for over four decades and also has applications in fields as diverse as aviation and agricultural sciences. 

  • Fault Detection in Cables: Reflectometers are sensitive to changes in impedance and can therefore identify faults in cables that cause resistance mismatches. These include open cable pairs, grounded lines, or even the presence of water in air core insulated cables. This is especially useful when parts of the line are not easily accessible, or when the line is very long. The instrument can also pre-locate the distance to the detected fault quite accurately. 
  • Measuring Cables: Singe the speed at which the TDR signal travels in a particular cable pair is constant, the time taken for the signal to return to the device is an indicator of cable length. This is useful in the measurement of lines that may be many miles long.
  • Security Surveillance: TDRs can detect the presence of wire taps and locate them on landline telephone lines.
  • Preventative Maintenance: The instrument can detect resistance in connectors in telephone landlines as they corrode. It can also detect the presence of increased moisture as insulation degrades. This means failing lines can be replaced before they fail. TDRs are also used in the aviation industry, where engineers have to deal with thousands of miles of wiring. Here, a special type of time domain reflectometry called spread-spectrum time-domain reflectometry is sometimes used as it can monitor live wires in real time.