TDR

Introduction

Because the propagation of electromagnetic waves in cables is transverse electromagnetic waves, the propagation speed of which is affected by the medium surrounding the conductor, so many researchers have used this feature to carry out various applied researches. The hydrate saturation in the sediment is mainly determined by the porosity and water content of the sample. At present, the methods of measuring the water content of sediments mainly include gravimetric method, radioactive method (such as neutron scattering method, gamma ray method), electrical resistance method, ground penetrating radar technology and time domain reflectometry (TDR).

These methods have their own advantages and disadvantages, and they are suitable for water content measurement research of different scales. The gravimetric method can accurately measure the water content of sediment samples, but its disadvantage is that the sediment needs to be sampled and destroyed, which is not suitable for the water content measurement of hydrate sediment samples; the radioactive method can accurately measure the sediment in situ However, specific test samples need to be calibrated separately, and radioactivity must be prevented from causing physical damage to testers; resistance method also requires specific test samples to be calibrated in order to obtain more accurate measurement values; ground penetrating radar technology It is suitable for in-situ surveys of large-scale water distribution, and uses TimeDomain Reflectometry (TDR) in the water content test of small sample scale, which has non-destructive detection, high accuracy, small calculation amount, and flexibility The advantages of large, convenient real-time field measurement and simultaneous detection of sediment water content and salinity, etc., are more favored by people.

In the hydrate experiment, the TDR technology can be used to test the water content of the sediment in real time, and back-calculate the saturation of the hydrate to achieve the purpose of real-time testing of the saturation.

Time Domain Reflectometer

As early as the 1960s, Time Domain Reflectometry (TDR) technology was produced. This technique involves generating a time step voltage that propagates along the transmission line. Use an oscilloscope to detect the reflection from the impedance and measure the ratio of the input voltage to the reflected voltage to calculate the discontinuous impedance.

In the 1970s, it was learned that the Fourier transform of the network reflection coefficient as a function of frequency is the reflection coefficient as a function of time. The data measured by the network analyzer in the frequency domain can be used to calculate and display the network step and impulse response of the network as a function of time. The traditional TDR capability in reflection and transmission increases the potential for measurement in band-limited networks.

In the reflection mode, the network analyzer measures the reflection coefficient as a function of frequency. The reflection coefficient can be regarded as the transfer function of the incident voltage and the reflected voltage. The inverse transformation converts the reflection coefficient into a function of time (shock response). The convolution of the reflection coefficient and the input step or pulse can be used to calculate the step and impulse response. In transmission mode. The network analyzer measures the transfer function of the two-port device as a function of frequency. The inverse transformation transforms the transfer function into the impulse response of the two-port device. The convolution of the impulse response and the input step or pulse is used to calculate the step and impulse response.

Explanation

TDR measures the reflection along the conductor. To measure these reflections, TDR transmits the incident signal to the conductor and monitors its reflection. If the conductor has a uniform impedance and is properly terminated, there will be no reflections and the remaining incident signal will be absorbed at the far end through the termination. Conversely, if there is a change in impedance, some of the incident signal will be reflected back to the source. TDR is similar to radar in principle.

Reflections

Generally, reflections will have the same shape as the incident signal, but their sign and amplitude depend on the change in impedance level. If there is a step increase in impedance, then the reflection will have the same sign as the incident signal; if the impedance is gradually decreasing, the reflection will have the opposite sign. The magnitude of the reflection depends not only on the amount of impedance change, but also on the loss of the conductor.

The reflection is measured at the output/input of the TDR and displayed or plotted as a function of time. Alternatively, the display can be read based on the cable length, because for a given transmission medium, the speed of signal propagation is almost constant.

Because of its sensitivity to impedance changes, TDR can be used to verify cable impedance characteristics, joint and connector locations, and related losses, and estimate cable length.

Event signal

TDR uses different event signals. Some TDRs transmit pulses along a conductor; the resolution of these instruments is usually the width of the pulse. Narrow pulses can provide good resolution, but they have high-frequency signal components that are attenuated in long cables. The shape of the pulse is usually a half-period sine curve. For longer cables, use wider pulse widths.

The fast rise time step is also used. Instead of looking for the reflection of the complete pulse, the instrument focuses on the rising edge, which can be very fast. The technical TDR of the 1970s used a step size with a rise time of 25ps.

There are other TDRs that use related technologies to transmit complex signals and detect reflections. See Spread Spectrum Time Domain Reflectometer.

Research overview

Topp et al. first applied the TDR technology to study the relationship between the effective dielectric constant of the soil and the soil moisture content, and proved that the dielectric constant has a good relationship with the moisture content of many types of soil And put forward a calculation formula for estimating water content.

Dalton et al. used the same probe to measure soil conductivity, and proposed the conductivity measurement method of TDR technology.

Nissen et al. used TDR technology to conduct a series of studies on soil conductivity testing. They first studied the unbalanced relationship between the spatial sensitivity of the dual-probe probe and the sample volume. After that, they conducted a measurement study of ion mobility. Through research, it is found that the small probe is simple, cheap, stable and reliable in the conductivity test.

Wright et al. used a time domain reflectometer to detect the formation and decomposition of methane hydrate and achieved satisfactory results. They used the time domain reflectometer to test the characteristics of the dielectric constant of the medium, and through the relationship between the dielectric constant and the volumetric water content in the medium, they conducted some theoretical studies on methane hydrate. In the experiment, they believe that after the formation of hydrate, its dielectric constant is similar to that of ice. The dielectric constant of ice is significantly different from that of water and is close to that of air. In the study of unfrozen water in the tundra, many researchers used the apparently different dielectric constants of ice and water to measure the water content of the unfrozen water in the tundra. Wright et al. In the 1990s, TDR technology was applied and researched in my country.

Gong Yuanshi et al. measured the soil moisture in farmland, studied the relationship between the growth process of crops and soil moisture content, and estimated the evapotranspiration of farmland soil moisture. Research on the spatial variability of farmland moisture has found that TDR technology has the characteristics of rapid, accurate, automatic and continuous in the measurement of farmland soil moisture, which provides a strong basis for agricultural production. It is proposed that TDR technology is most suitable for coarse and light soils with low conductivity. For organic matter and kind of clay or saline-alkaline soil, the probe should be improved or corrected.

Wang Shaoling and others used time-domain reflectometry to monitor the water distribution and changes over time in the tundra. They took advantage of the distinctly different dielectric constants of frozen water and unfrozen water to measure the content of unfrozen water in the tundra. According to the changes in the distribution of unfrozen water at different times and at different depths on the Qinghai-Tibet Plateau, it is found that the distribution of water and the pattern of water migration are also different in different regions. In the freezing process of the seasonal frozen layer, the direction of water distribution and migration is the same as the direction of heat flow in the soil, which is from bottom to top. In the study of the seasonal melting layer, it was found that the way of water replenishment affects the migration of water.

Ren Tusheng et al. used thermal pulse-time-domain reflectometry to measure soil hydrothermal dynamics and physical properties. Ye Yuguang et al. applied TDR technology to the real-time determination of hydrate saturation in sediments, and Diao Shaobo et al. used thermal TDR Technical measurement of the thermophysical parameters of hydrates in porous media and other studies have achieved satisfactory results. With the continuous development of TDR technology, its application fields are becoming more and more extensive.

Working principle

The TDR detector is basically composed of the following parts: transmitter, receiver, transmitting and receiving system, signal processor and display. When used as a cable detector, it is directly connected to the cable under test.

In water content, conductivity and other applications, it can be connected with a special probe according to the needs of the test. The basic working principle is shown in Figure 1.

Signal transmission

When the pulse signal sent by the transmitter propagates in a homogeneous medium, its propagation speed is constant, the propagation speed V, the distance L and the transmission wave propagation The relational formula of t_R for the reflection wave to return to the emission point after reaching the reflection point is as follows:

V=2L/t_R

TDR uses coaxial cable as the transmission line. Coaxial cables are easy to manufacture and have good shielding properties. Transverse electromagnetic waves (TEM), transverse electromagnetic waves (TE), and transverse magnetic waves (TM) are transmitted in coaxial cables, but transverse electromagnetic waves are the most commonly used, and other waveforms need to be suppressed. The electromagnetic waves emitted by TDR are transverse electromagnetic waves. Transverse electromagnetic waves only have a horizontal electric field and a horizontal magnetic field in the transmission. There is no electric field along the axis, and there is no electric field and magnetic field in the coaxial cable.

Figure 2 shows the distribution of electric and magnetic fields in a coaxial cable, which are uniform and symmetrical. The transmission of electromagnetic waves can also be transmitted by dual wires, but due to the greater attenuation of electromagnetic waves in dual wire transmission, dual wires are generally not used for long-distance transmission. The waveguide is also an ideal conductor for transmitting electromagnetic waves, but due to its large size, the length of the waveguide must match the wavelength of the electromagnetic wave. Therefore, in the transmission of high-power electromagnetic waves, waveguides are used to transmit electromagnetic waves.

In addition, the use of flat wires can also transmit electromagnetic waves. In fact, it is a modification of coaxial cables. It not only has the shielding properties of coaxial cables, but also is easy to manufacture and low in cost.

Probe structure

In the water content test, the probe structure is different for different tests. Campbell and Hemovaara proposed a seven-electrode probe to test soil and liquid The dielectric constant.

Many researchers have also designed probes with many structures. Zegelin proposed a two-electrode probe for testing soil layers. When multi-electrode electrodes measure the dielectric constant, the value of the test may be mixed because of the different dielectric constants of different layers. The probe structure commonly used for soil testing today is a three-electrode probe. Robinson and Friedman proposed flat double electrodes, which can detect more effectively. Wright et al. used coaxial probes in the study of natural gas hydrates.

In the study of natural gas hydrate, we use an asymmetric coaxial probe according to the actual situation. The electric and magnetic field distributions of probes with different structures are shown in Fig. 3. From the electric field and magnetic field distributions of the double-electrode probe in Fig. 3, the symmetry of the distribution of magnetic field and electric field is the worst, while the symmetry of the electric field and magnetic field of the coaxial probe is the most. it is good. Of course, when testing the dielectric constant of a liquid, the coaxial probe is the best.

However, when measuring the dielectric constant of the soil, the two-electrode probe is the easiest to embed, while the coaxial probe is difficult to embed. From the point of view of effectiveness, the detection effect of the flat two-electrode probe is better than that of the three-electrode probe, but the three-electrode probe is better than the flat electrode in terms of the resolution of measuring the dielectric constant of the soil layer. Of course, in the test, the electrode with a round rod between the two flat electrodes may be superior to the flat two-electrode probe. Because it also has a certain shielding effect, and the embedding is the same as the flat two-electrode probe. In addition, a heating device and a temperature measuring device are added to the new probe structure to detect the thermal characteristics of the soil.

Applications

Time domain reflectometers are usually used for in-situ testing of very long cable lines, where it is impractical to dig or remove cables that may be kilometers long. They are essential for preventive maintenance of communication lines, because TDR can detect the resistance of joints and connectors when they corrode, and they can reduce insulation leakage and absorb moisture long before they cause catastrophic failure. Using TDR, the fault can be accurately located within centimeters.

TDR is also a very useful tool for technical supervision countermeasures. They help determine the presence and location of wire connectors. When connected to a telephone line, slight changes in line impedance due to the introduction of taps or connectors will be displayed on the TDR screen.

TDR equipment is also an indispensable tool in modern high-frequency printed circuit board failure analysis, and its signal routing can simulate transmission lines. By observing the reflection, any unsoldered pins of the ball grid array device can be inspected. The short-circuit pin can also be detected in a similar manner.

The TDR principle is used in industrial environments, in various situations, from integrated circuit packaging and testing to liquid level measurement. In the former, a time domain reflectometer is used to isolate the same faulty site. The latter is mainly limited to the processing industry.

Measure at level

In a TDR-based liquid level measurement device, the device generates pulses that propagate along a thin waveguide (called a probe)-usually a metal rod or steel cable . When the pulse hits the surface of the medium to be measured, part of the pulse is reflected back to the waveguide. The device determines the liquid level by measuring the time difference between the transmitted pulse and the reflected return. The sensor can output the analyzed level as a continuous analog signal or switch output signal. In TDR technology, the pulse speed is mainly affected by the dielectric constant of the pulse propagation medium. The dielectric constant of the medium can vary greatly according to the moisture content and temperature of the medium. In many cases, this effect can be corrected without difficulty. In some cases, such as in boiling and/or high temperature environments, calibration may be difficult. In particular, it can be very difficult to determine the foam (foam) height and the collapsed liquid level in the foam/boiling medium.

Anchor cables used in dams

CEA Technologies (CEATI)’s Dam Safety Interest Group is a consortium of power organizations that has applied spread spectrum time domain reflectometry to identify Potential failures in the anchor cables of concrete dams. Compared with other test methods, the main advantage of the time domain reflectometer is the non-destructive method of these tests.

For Earth and Agricultural Sciences

TDR is used to determine the moisture content in soil and porous media. In the past two decades, substantial progress has been made in measuring moisture in soil, grains, food, and sediments. The key to the success of TDR lies in the ability to accurately determine the material's dielectric constant (dielectric constant), because there is a strong relationship between the material's dielectric constant and its water content, as demonstrated by the pioneering work of Hoekstra and Delaney. (1974) and Topp et al. (in 1980). Recent reviews and reference work on this topic include Topp and Reynolds (1998), Noborio (2001), Pettinellia, etc. (2002), Topp and Ferre (2002) and Robinson et al. (Year 2003). The TDR method is a transmission line technology, and the apparent dielectric constant (Ka) is determined based on the propagation time of electromagnetic waves propagating along the transmission line, usually two or more parallel metal rods embedded in soil or sediment. The probe length is usually between 10 and 30 cm and is connected to the TDR via a coaxial cable.

Use in geotechnical engineering

Time domain reflectometers are also used to monitor slope motion in various geotechnical settings, including highway cutting, railway subgrades, and open-pit mines (Dowding&O'Connor, 1984, 2000a, 2000b; Kane& Beck, 1999). In stability monitoring applications using TDR, the coaxial cable is installed in a vertical borehole that passes through the area of ​​interest. The electrical impedance at any point along the coaxial cable changes with the deformation of the insulator between the conductors. There is a brittle grout around the cable, which converts earth motion into sudden cable deformation, which is shown as a detectable peak in the deformation trace. Until recently, this technique was relatively insensitive to small slope movements and could not be automated because it relied on humans to detect changes in reflection traces over time. Farrington and Sargand (2004) developed a simple signal processing technique that uses numerical derivatives to extract reliable slope motion indications from TDR data, earlier than traditional interpretation.

Another application of TDR in geotechnical engineering is to determine the soil moisture content. This can be done by placing the TDR in different soil layers and measuring the time when the precipitation starts and the time when the TDR indicates an increase in soil water content. The depth of TDR (d) is a known factor, and the other is the time (t) it takes for the water droplet to reach that depth; therefore, the water penetration velocity (v) can be determined. This is a good way to evaluate the effectiveness of best management practices (BMP) in reducing stormwater surface runoff.

Analysis of semiconductor devices

Time domain reflectometry is used as a non-destructive method for defect location in semiconductor device packaging in semiconductor failure analysis. TDR provides electrical characteristics for each conductive trace in the device package, which can be used to determine the location of open and short circuits.

Maintenance in aviation wiring

Time domain reflectometers, especially spread spectrum time domain reflectometers are used in aviation wiring for preventive maintenance and fault location. Spread-spectrum time-domain reflectometers have the advantage of accurately locating fault locations within thousands of miles of aviation wiring. In addition, this technology is worth considering for real-time aerial monitoring, because spread spectrum reflectometers can be used in the line of fire.

It has been proven that this method can be used to locate intermittent electrical faults.

Multi-carrier time domain reflectometry (MCTDR) is also considered a promising method for embedded EWIS diagnostic or troubleshooting tools. This intelligent technology is based on the injection of multi-carrier signals (respect for EMC and harmless to wires) to provide information for the detection, location and characterization of electrical defects (or mechanical defects with electrical consequences) in the wiring system. Hard faults (short circuits, open circuits) or intermittent defects can be detected very quickly, thereby increasing the reliability of the wiring system and improving its maintenance.