Time-domain reflectometer - AbsoluteAstronomy.com

A time-domain reflectometer (TDR) is an electronic instrument used to characterize and locate faults in metallic cables (for example, twisted wire pairs

Twisted pair

Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; for instance, electromagnetic radiation from unshielded twisted pair cables, and crosstalk between neighboring pairs...

, coaxial cable

Coaxial cable

Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...

s). It can also be used to locate discontinuities in a connector, printed circuit board

Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...

, or any other electrical path. The equivalent device for optical fiber

Optical fiber

An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...

is an optical time-domain reflectometer.

Description

A TDR transmits a short rise time

Rise time

In electronics, when describing a voltage or current step function, rise time refers to the time required for a signal to change from a specified low value to a specified high value...

pulse

Pulse

In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...

along the conductor. If the conductor is of a uniform impedance

Electrical impedance

Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

and is properly terminated

Electrical termination

Electrical termination of a signal involves providing a terminator at the end of a wire or cable to prevent an RF signal from being reflected back from the end, causing interference...

, the entire transmitted pulse will be absorbed in 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

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

.

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

Signal reflection

Signal reflection occurs when a signal is transmitted along a transmission medium, such as a copper cable or an optical fiber, some of the signal power may be reflected back to its origin rather than being carried all the way along the cable to the far end. This happens because imperfections in the...

that is measured at the output/input

Input/output

In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...

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

Cable

A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...

length.

Because of this sensitivity to impedance variations, a TDR may be used to verify cable impedance characteristics, splice

Fusion splicing

Fusion splicing is the act of joining two optical fibers end-to-end using heat. The goal is to fuse the two fibers together in such a way that light passing through the fibers is not scattered or reflected back by the splice, and so that the splice and the region surrounding it are almost as...

and connector

Electrical connector

An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...

locations and associated losses, and estimate cable lengths.

Example Traces

These traces were produced by the 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 (that is, it is terminated into zero ohms impedance). 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 back 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

Velocity of propagation

The velocity factor , also called wave propagation speed or velocity of propagation , of a transmission medium is the speed at which a wavefront passes through the medium, relative to the...

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 (terminated into an infinite impedance). 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 (open circuit) to -1 (short circuit). 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

Preventive maintenance

Preventive maintenance has the following meanings:#The care and servicing by personnel for the purpose of maintaining equipment and facilities in satisfactory operating condition by providing for systematic inspection, detection, and correction of incipient failures either before they occur or...

of telecommunication

Telecommunication

Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...

lines, as they can reveal growing resistance levels on joints and connectors

Connectors

Connectors are said by author Malcolm Gladwell to be people in a community who know large numbers of people and who are in the habit of making introductions. A connector is essentially the social equivalent of a computer network hub...

as they corrode

Corrosion

Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen...

, and increasing insulation

Electrical insulation

thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal 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

Technical Surveillance Counter-Measures

TSCM is the original United States Federal government abbreviation denoting the process of bug-sweeping or electronic countersurveillance...

, where they help determine the existence and location of wire tap

Telephone tapping

Telephone tapping is the monitoring of telephone and Internet conversations by a third party, often by covert means. The wire tap received its name because, historically, the monitoring connection was an actual electrical tap on the telephone line...

s. 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 whose signal traces are carefully crafted to emulate transmission line

Transmission line

In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

s. By observing reflections, any unsoldered pins of a ball grid array

Ball grid array

A ball grid array is a type of surface-mount packaging used for integrated circuits.- Description :The BGA is descended from the pin grid array , which is a package with one face covered with pins in a grid pattern. These pins conduct electrical signals from the integrated circuit to the printed...

device can be detected. Additionally, 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

Integrated circuit

An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

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, a low-energy electromagnetic impulse generated by the sensor’s circuitry is propagated along a thin wave guide (also referred to as a probe) – usually a metal rod or a steel cable. When this impulse hits the surface of the medium to be measured, part of the impulse energy is reflected back up the probe to the circuitry which then calculates the fluid level from the time difference between the impulse sent and the impulse reflected (in nanoseconds). 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 most cases, this can be corrected for without undue difficulty. However, in complex environments, such as in boiling and/or high temperature environments, this can be a significant signal processing dilemma. In particular, determining the froth(foam) height and true 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. (CEATI), a consortium of electrical power organizations, has applied Spread-spectrum time-domain reflectometry

Spread-spectrum time-domain reflectometry

Spread-spectrum time-domain reflectometry is a measurement technique to identify faults, usually in electrical wires, by observing reflected spread spectrum signals. SSTDR is a type of time-domain reflectometry that can be advantageous to other systems due to the ability to use SSTDR in high-noise...

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

TDR is used to determine moisture content in soil and porous media, where over the last two decades substantial advances have been made; including in soils, grains and food stuffs, and in sediments. The key to TDR’s success is its ability to accurately determine the permittivity (dielectric constant) of a material from wave propagation, and the fact that there is a 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). Recent reviews and reference work on the subject include, Topp and Reynolds (1998), Noborio (2001), Pettinellia et al. (2002), Topp and Ferre (2002) and Robinson et al. (2003). The TDR method is a transmission line technique, and determines an apparent TDR permittivity (Ka) from the travel time of an electromagnetic wave that propagates along a transmission line, usually two or more parallel metal rods embedded in a soil or sediment. TDR probes are usually between 10 and 30 cm in length and connected to the TDR via a 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 (Dowding & O'Connor, 1984, 2000a, 2000b; Kane & Beck, 1999). 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 (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.

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 measurment 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 (d) is a known factor and the other is the time that takes the drop of water to reach the that depth (t), therefore the speed of water Infiltration (hydrology)

Infiltration (hydrology)

Infiltration is the process by which water on the ground surface enters the soil. Infiltration rate in soil science is a measure of the rate at which soil is able to absorb rainfall or irrigation. It is measured in inches per hour or millimeters per hour. The rate decreases as the soil becomes...

(v) can be determined. This a good method to assess the effectiveness of Best Management Practices (BMPs) in reducing stormwater

Stormwater

Stormwater is water that originates during precipitation events. It may also be used to apply to water that originates with snowmelt that enters the stormwater system...

Surface runoff

Surface runoff is the water flow that occurs when soil is infiltrated to full capacity and excess water from rain, meltwater, or other sources flows over the land. This is a major component of the water cycle. Runoff that occurs on surfaces before reaching a channel is also called a nonpoint source...

TDR in semiconductor device analysis

Time domain reflectometry is used in semiconductor failure analysis

Failure analysis

Failure analysis is the process of collecting and analyzing data to determine the cause of a failure. It is an important discipline in many branches of manufacturing industry, such as the electronics industry, where it is a vital tool used in the development of new products and for the improvement...

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 for aviation wiring for both preventative maintenance and intermittent fault location. The 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 being considering for live aviation monitoring as the spread spectrum reflectometry works on a live wire.

Utah State conducted research on use of time domain reflectometry for identifying chafing

Chafing

Chafing when used as a nautical term describes the process of wear on a line or sail caused by constant rubbing against a hard, usually metallic, surface. Various methods are used to prevent chafing. Chafing of lines that rest on a choke on a boat can be prevented by putting a protecting material...

of electrical wires in aircraft. This chafing is known to cause electrical failures on aircraft so the ability to identify potential problems prior to a failure that has life-ending implications.

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