Time domain reflectometry
The Time Domain Reflectometry , also known by the English name Time Domain Reflectometry , short TDR is a method for the identification and analysis of run lengths and reflection characteristics of electromagnetic waves and signals. The process is also known in the German-speaking world as cable radar .
In practice, the area of optical time domain reflectometry is playing an increasingly important role , especially in network technology .
functionality
There are different procedures for the experimental verification:
- A pulse generator generates a sequence of very short square-wave pulses, each about 20 ns duration, which follow at such a great distance that the echoes of all previous pulses have decayed. These are routed to the inner conductor of the coaxial cable via a relatively large resistor , where the first channel of the oscilloscope is also connected with a low-capacitance probe. The other channel of the oscilloscope is connected to the other end of the cable, where the electrical properties of the cable can be checked when it is loaded with different components.
- When the cable end is open, the square-wave pulses oscillate back and forth between both ends and lose energy in the process. It occurred only on pulses of the same polarity.
- Of course, no voltage can be measured at the short-circuited cable end , but the pulses are reflected back to the beginning of the cable with reversed polarity. Since the terminating resistor (R> Z o ) is too high , the pulses are now reflected with the same polarity. The picture is reminiscent of a dampened oscillation .
- If the right end is terminated with the wave impedance of Z o corresponding to the cable (usually 50 ohms) , no pulses are reflected. An infinitely long cable would behave the same way.
- With the aid of a step function generator, a steep signal is generated at one end of the line. The signal edge propagates over the medium and is reflected at the other end or at points of interference. With the help of a suitable evaluation circuit or an oscilloscope , the transmitted signal is then compared with its reflection and information about transit time , amplitude and the capacitive , resistive and inductive characteristics of the reflection are determined. The simple view of the reflections in the oscilloscope makes it possible for the viewer to make an assessment of the reflection behavior even without in-depth specialist knowledge.
history
The first experiences with time domain reflectometry were made by Smith-Rose in the 1930s with the help of radar beams . He was one of the first to recognize the relationship between electrical quantities and the water content of porous materials. Driven by the advances in radar technology during World War II , suitable measuring devices were developed, which then led to the first usable devices in the 1960s. One of the first areas of application here is the location of cable breaks and crushing in electrical engineering . This first use can still be found today in the term cable radar , which has become widely used in German- speaking countries.
The new technology was then used in science towards the end of the 1960s and the beginning of the 1970s in chemistry . It was here that the first scientific insights were gained by researching the interrelationships between the frequency dependence of the dielectric constant of organic molecules and their structure. However, the breakthrough of the new technology did not come until GC Topp in 1980 in the geosciences , when determining the volumetric water content in the soil. Since the measurement accuracy depends very much on the edge steepness, i.e. directly on the level of the frequencies used, the invention of the tunnel diode and high-frequency oscilloscopes led the technology to higher accuracies and thus to new areas of application. Pulse rise times in the picosecond range (10 −12 s) are common today.
Areas of application
Length measurement
One of the first applications of time domain reflectometry was the length measurement of cables in the electrical industry . This measures the time it takes for an emitted pulse to re-arrive after reflection. If you know the speed of propagation in the cable, which depends on the dielectric , you can deduce the length of the cable directly from the measured time. The term cable radar developed from this field of application.
While the oscilloscope was needed for these measurements in the past, there are already ready-made measuring devices that display the length value directly. This technique is widely used in telecommunications and network technology . In the case of new cabling in buildings, the network cable laid is billed according to the measured values of the time domain reflectometry. Due to the ever increasing bandwidth , however, a trend towards optical time domain reflectometry can be seen, in which the medium used is a glass fiber .
Localization of interference sources
The aim of locating sources of interference is to detect broken or pinched cables in underground cables , for example, and to locate their position. Here one makes use of the property of time domain reflectometry to recognize not only total reflections , but every change in the medium. Total reflection only occurs at the end of the cable, a cable break or a short circuit between the inner and outer conductor.
If the impulse propagates along the unchanged medium, the wave impedance in the cable does not change. However, if the pulse wave is squeezed, the impedance changes and there is a partial reflection . From the time of the arrival of the reflection and its nature, conclusions can be drawn about the location and extent of the bruising .
Moisture measurement
A technique frequently used in geology , agriculture and industry to determine humidity is, in addition to capacitive humidity measurement, time domain reflectometry. Here one uses the fact that the dielectric constant of most materials, such as soil , grain or coffee , differs greatly depending on the water content.
The volumetric moisture can be calculated using the transit time of a pulse along two or more parallel conductors (e.g. in the form of rods that are inserted into the material) .
See: Humidity measurement with time domain reflectometry .
Conductivity measurement
Depending on the degree of conductivity , conductive media shorten certain frequencies in parts and lead to attenuation of the other frequencies . Substituting the amplitude values of the transmitted pulse with the amplitude values of the reflected pulse in relation , this conclusions can be on the conductivity of the medium. However, since the maximum amplitudes of the high frequencies are difficult to determine, this is a difficult method, the application of which is to be sought in parts of the moisture measurement in the soil.
Level measurement
In a level measuring device based on TDR, the electronics of the sensor generate a low-energy electromagnetic pulse, which is coupled into a conductor (also called a probe) and guided along this probe - usually a metal rod or a steel cable. If this pulse hits the surface of the medium to be measured, part of the pulse is reflected there and runs back along the probe to the electronics, which then use the time difference between the transmitted and received pulse (in the nanosecond range) Level calculated. The sensor can output the evaluated level as a continuous analog signal or switching signal. One advantage of this relatively complex method is that the measurement result is hardly influenced by the properties of the medium to be measured such as density, conductivity and dielectric constant or by the ambient conditions such as pressure and temperature, and that no moving parts that are susceptible to failure are required.
See also
- Optical time domain reflectometry
- Reflectometer
- Continuous wave radar # Applications of modulated continuous wave radar
literature
- RL Smith-Rose: The electrical properties of soil for alternating currents at radio frequencies . In: Proceedings of the Royal Society of London A 3, 140, May 1933, no. 841, ISSN 0962-8444 , pp. 359-337, online .
- GC Topp, JL Davis, AP Annan: Electromagnetic determination of soil water content: Measurements in coaxial transmission lines . In: Water Resources Research 16, 1980, 3, ISSN 0043-1397 , pp. 574-582.
- M. Stacheder: The Time Domain Reflectometry in Geotechnics. Measurement of water content, electrical conductivity and mass transfer . AGK, Karlsruhe 1996, ( Series of Applied Geology of the University of Karlsruhe 40, ISSN 0933-2510 ).
