A time-domain reflectometer is an electronic instrument that uses time-domain reflectometry
to characterize and locate faults in metallic cables. It can also be used to locate discontinuities
in a connector, printed circuit board, or any other electrical path. The equivalent
device for optical fiber is an optical time-domain reflectometer.
Description A TDR transmits a short rise time pulse along
the conductor. If the conductor is of a uniform impedance and is properly terminated, the
entire transmitted pulse will be absorbed by 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.
Increases in the impedance create a reflection that reinforces the original pulse whilst
decreases in the impedance create a reflection that opposes the original pulse.
The resulting reflected pulse 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 almost
constant for a given transmission medium, can be read as a function of cable length.
Because of its sensitivity to impedance variations, a TDR may be used to verify cable impedance
characteristics, splice and connector locations and associated losses, and estimate cable
lengths. Example traces
These traces were produced by a time-domain reflectometer made from common lab equipment
connected to approximately 100 feet of 50 ohm coaxial cable. The propagation velocity
of this cable is approximately 66% of the speed of light in a vacuum.
Explanation Consider the case where the far end of the
cable is shorted. When the rising edge of the pulse is launched down the cable, the
voltage at the launching point "steps up" to a given value instantly and the pulse begins
propagating down the cable towards the short. When the pulse hits the short, no energy is
absorbed at the far end. Instead, an opposing pulse reflects back from the short towards
the launching end. It is only when this opposing reflection finally reaches the launch point
that the voltage at this launching point abruptly drops back to zero, signalling the fact that
there is a short at the end of the cable. That is, the TDR had no indication that there
is a short at the end of the cable until its emitted pulse can travel down the cable at
roughly the speed of light and the echo can return up the cable at the same speed. It
is only after this round-trip delay that the short can be perceived by the TDR. Assuming
that one knows the signal propagation speed in the particular cable-under-test, then in
this way, the distance to the short can be measured.
A similar effect occurs if the far end of the cable is an open circuit. In this case,
though, the reflection from the far end is polarized identically with the original pulse
and adds to it rather than cancelling it out. So after a round-trip delay, the voltage at
the TDR abruptly jumps to twice the originally-applied voltage.
Note that a theoretical perfect termination at the far end of the cable would entirely
absorb the applied pulse without causing any reflection. In this case, it would be impossible
to determine the actual length of the cable. Luckily, perfect terminations are very rare
and some small reflection is nearly always caused.
The magnitude of the reflection is referred to as the reflection coefficient or ρ. The
coefficient ranges from 1 to -1. The value of zero means that there is no reflection.
The reflection coefficient is calculated as follows:
Where Zo is defined as the characteristic impedance of the transmission medium and Zt
is the impedance of the termination at the far end of the transmission line.
Any discontinuity can be viewed as a termination impedance and substituted as Zt. This includes
abrupt changes in the characteristic impedance. As an example, a trace width on a printed
circuit board doubled at its midsection would constitute a discontinuity. Some of the energy
will be reflected back to the driving source; the remaining energy will be transmitted.
This is also known as a scattering junction. Usage
Time domain reflectometers are commonly used for in-place testing of very long cable runs,
where it is impractical to dig up or remove what may be a kilometers-long cable. 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.
Using a TDR, it is possible to pinpoint a fault to within centimetres.
TDRs are also very useful tools for technical surveillance counter-measures, where they
help determine the existence and location of wire taps. The slight change in line impedance
caused by the introduction of a tap or splice will show up on the screen of a TDR when connected
to a phone line. 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. By observing reflections, any unsoldered pins
of a ball grid array device can be detected. Short circuited pins can also be detected
in a similar fashion. The TDR principle is used in industrial settings,
in situations as diverse as the testing of integrated circuit packages to measuring liquid
levels. In the former, the time domain reflectometer is used to isolate failing sites in the same.
The latter is primarily limited to the process industry.
TDR in level measurement In a TDR-based level measurement device, the
device generates an impulse that propagates down a thin waveguide – typically a metal
rod or a steel cable. When this impulse hits the surface of the medium to be measured,
part of the impulse reflects back up the waveguide. The device determines the fluid level by measuring
the time difference between when the impulse was sent and when the reflection returned.
The sensors can output the analyzed level as a continuous analog signal or switch output
signals. In TDR technology, the impulse velocity is primarily affected by the permittivity
of the medium through which the pulse propagates, which can vary greatly by the moisture content
and temperature of the medium. In many cases, this effect can be corrected without undue
difficulty. In some cases, such as in boiling and/or high temperature environments, the
correction can be difficult. In particular, determining the froth height and the collapsed
liquid level in a frothy / boiling medium can be very difficult.
TDR used in anchor cables in dams The Dam Safety Interest Group of CEA Technologies,
Inc., a consortium of electrical power organizations, has applied Spread-spectrum time-domain reflectometry
to identify potential faults in concrete dam anchor cables. The key benefit of Time Domain
reflectometry over other testing methods is the non-destructive method of these tests.
TDR used in the earth and agricultural sciences
A TDR is used to determine moisture content in soil and porous media. 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
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
and Topp et al.. Recent reviews and reference work on the subject include, Topp and Reynolds,
Noborio, Pettinellia et al., Topp and Ferre and Robinson et al.. The TDR method is a transmission
line technique, and determines apparent permittivity from the travel time of an electromagnetic
wave that propagates along a transmission line, usually two or more parallel metal rods
embedded in soil or sediment. The probes are typically between 10 and 30 cm long and connected
to the TDR via coaxial cable. TDR in geotechnical usage
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. In stability
monitoring applications using TDR, a coaxial cable is installed in a vertical borehole
passing through the region of concern. The electrical impedance at any point along a
coaxial cable changes with deformation of the insulator between the conductors. A brittle
grout surrounds the cable to translate earth movement into an abrupt cable deformation
that shows up as a detectable peak in the reflectance trace. 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 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. Another application of TDRs in geotechnical
engineering is to determine the soil moisture content. This can done by placing the TDRs
in different soil layers and measurement of the time of start of precipitation and the
time that TDR indicate an increase in the soil moisture content. The depth of the TDR
is a known factor and the other is the time it takes the drop of water to reach that depth;
therefore the speed of water Infiltration can be determined. This is a good method to
assess the effectiveness of Best Management Practices in reducing stormwater Surface runoff.
TDR in semiconductor device analysis Time domain reflectometry is used in semiconductor
failure analysis as a non-destructive method for the location of defects in semiconductor
device packages. 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.
TDR in aviation wiring maintenance Time domain reflectometry, specifically spread-spectrum
time-domain reflectometry is used on aviation wiring for both preventative maintenance and
fault location. Spread spectrum time domain reflectometry has the advantage of precisely
locating the fault location within thousands of miles of aviation wiring. Additionally,
this technology is worth considering for real time aviation monitoring, as spread spectrum
reflectometry can be employed on live wires. This method has been shown to be useful to
locating intermittent electrical faults. See also
Frequency domain sensor Murray loop bridge
Noise-Domain Reflectometry Return loss
Standing wave ratio References
Further reading Hoekstra, P. and A. Delaney, 1974. "Dielectric
properties of soils at UHF and microwave frequencies". Journal of Geophysical Research 79:1699–1708.
Smith, P., C. Furse, and J. Gunther, 2005. "Analysis of spread spectrum time domain reflectometry
for wire fault location". IEEE Sensors Journal 5:1469–1478.
Waddoups, B., C. Furse and M. Schmidt. "Analysis of Reflectometry for Detection of Chafed Aircraft
Wiring Insulation". Department of Electrical and Computer Engineering. Utah State University.
Noborio K. 2001. "Measurement of soil water content and electrical conductivity by time
domain reflectometry: A review". Computers and Electronics in Agriculture 31:213–237.
Pettinelli E., A. Cereti, A. Galli, and F. Bella, 2002. "Time domain reflectometry: Calibration
techniques for accurate measurement of the dielectric properties of various materials".
Review of Scientific Instruments 73:3553–3562. Robinson D.A., S.B. Jones, J.M. Wraith, D.
Or and S.P. Friedman, 2003 "A review of advances in dielectric and electrical conductivity
measurements in soils using time domain reflectometry". Vadose Zone Journal 2: 444–475.
Robinson, D. A., C. S. Campbell, J. W. Hopmans, B. K. Hornbuckle, Scott B. Jones, R. Knight,
F. Ogden, J. Selker, and O. Wendroth, 2008. "Soil moisture measurement for ecological
and hydrological watershed-scale observatories: A review." Vadose Zone Journal 7: 358-389.
Topp G.C., J.L. Davis and A.P. Annan, 1980. "Electromagnetic determination of soil water
content: measurements in coaxial transmission lines". Water Resources Research 16:574–582.
Topp G.C. and W.D. Reynolds, 1998. "Time domain reflectometry: a seminal technique for measuring
mass and energy in soil". Soil Tillage Research 47:125–132.
Topp, G.C. and T.P.A. Ferre, 2002. "Water content", in Methods of Soil Analysis. Part
4., SSSA Book Series No. 5. Soil Science Society of America, Madison WI.
Dowding, C.H. & O'Connor, K.M. 2000a. "Comparison of TDR and Inclinometers for Slope Monitoring".
Geotechnical Measurements—Proceedings of Geo-Denver2000: 80–81. Denver, CO.
Dowding, C.H. & O'Connor, K.M. 2000b. "Real Time Monitoring of Infrastructure using TDR
Technology". Structural Materials Technology NDT Conference 2000
Kane, W.F. & Beck, T.J. 1999. "Advances in Slope Instrumentation: TDR and Remote Data
Acquisition Systems". Field Measurements in Geomechanics, 5th International Symposium
on Field Measurements in Geomechanics: 101–105. Singapore.
Farrington, S.P. and Sargand, S.M., "Advanced Processing of Time Domain Reflectometry for
Improved Slope Stability Monitoring", Proceedings of the Eleventh Annual Conference on Tailings
and Mine Waste, October, 2004. Smolyansky, D.. "Electronic Package Fault
Isolation Using TDR". Microelectronics Failure Analysis. ASM International. pp. 289–302.
ISBN 0-87170-804-3. External links
Radiodetection Extended Training - ABC's of TDR's
Work begins to repair severed net AN-1304-2: Time Domain Reflectometry Theory
AN201 Step verses Pulse TDR Technology