Time-domain reflectometer
A time-domain reflectometer (TDR) is an electronic instrument used to determine the characteristics of electrical lines by observing reflected pulses.
By analyzing the magnitude, duration and shape of the reflected waveform, the nature of the impedance variation in the transmission system can be determined.
Generally, the reflections will have the same shape as the incident signal, but their sign and magnitude depend on the change in impedance level.
Alternatively, the display can be read as a function of cable length because the speed of signal propagation is almost constant for a given transmission medium.
Narrow pulses can offer good resolution, but they have high frequency signal components that are attenuated in long cables.
For example, spread-spectrum time-domain reflectometry (SSTDR) is used to detect intermittent faults in complex and high-noise systems such as aircraft wiring.
[8] These traces were produced by a time-domain reflectometer made from common lab equipment connected to approximately 100 feet (30 m) of coaxial cable having a characteristic impedance of 50 ohms.
A similar effect occurs if the far end of the cable is an open circuit (terminated into an infinite impedance).
They are indispensable for preventive maintenance of telecommunication lines, as TDRs can detect resistance on joints and connectors as they corrode, and increasing insulation leakage as it degrades and absorbs moisture, long before either leads to catastrophic failures.
TDRs are also very useful tools for technical surveillance counter-measures, where they help determine the existence and location of wire taps.
TDR equipment is also an essential tool in the failure analysis of modern high-frequency printed circuit boards with signal traces crafted to emulate transmission lines.
The TDR principle is used in industrial settings, in situations as diverse as the testing of integrated circuit packages to measuring liquid levels.
The device determines the fluid level by measuring the time difference between when the impulse was sent and when the reflection returned.
In particular, determining the froth (foam) height and the collapsed liquid level in a frothy / boiling medium can be very difficult.
Over the last two decades, substantial advances have been made measuring moisture in soil, grain, food stuff, and sediment.
The key to TDR's success is its ability to accurately determine the permittivity (dielectric constant) of a material from wave propagation, due to the strong relationship between the permittivity of a material and its water content, as demonstrated in the pioneering works of Hoekstra and Delaney (1974) and Topp et al. (1980).
Time domain reflectometry has also been utilized to monitor slope movement in a variety of geotechnical settings, including highway cuts, rail beds, and open pit mines (Dowding & O'Connor, 1984, 2000a, 2000b; Kane & Beck, 1999).
In TDR stability monitoring applications, a coaxial cable is installed in a vertical borehole passing through the region of concern.
Until recently, the technique was relatively insensitive to small slope movements and could not be automated because it relied on human detection of changes in the reflectance trace over time.
Farrington and Sargand (2004) developed a simple signal processing technique using numerical derivatives to extract reliable indications of slope movement from the TDR data much earlier than by conventional interpretation.
This is a good method to assess the effectiveness of Best Management Practices (BMPs) in reducing stormwater surface runoff.
The TDR provides an electrical signature of individual conductive traces in the device package, and is useful for determining the location of opens and shorts.
Additionally, this technology is worth considering for real time aviation monitoring, as spread spectrum reflectometry can be employed on live wires.
[12] Multi carrier time domain reflectometry (MCTDR) has also been identified as a promising method for embedded EWIS diagnosis or troubleshooting tools.
Based on the injection of a multicarrier signal (respecting EMC and harmless for the wires), this smart technology provides information for the detection, localization and characterization of electrical defects (or mechanical defects having electrical consequences) in the wiring systems.
Hard fault (short, open circuit) or intermittent defects can be detected very quickly increasing the reliability of wiring systems and improving their maintenance.
