Information AboutTime-domain Reflectometer |
| CATEGORIES ABOUT TIME-DOMAIN REFLECTOMETER | |
| electronic test equipment | |
| soil physics | |
| semiconductor analysis | |
|
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 they can reveal growing resistance levels 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 feet or inches. TDRs are also very useful tools for Surveillance Countermeasures , 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. 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. The measured time for the pulse to return is compared with a reference value, allowing the actual height of liquid in a vessel to be determined. TDR used in the Earth and Agricultural Sciences TDR is used to determine Soil Moisture water content in 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. References Hoekstra, P. and A. Delaney, 1974. Dielectric properties of soils at UHF and microwave frequencies. Journal of Geophysical Research 79:1699-1708. 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. Pettinellia 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. 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. (Ed. J.H. Dane and G.C. Topp), SSSA Book Series No. 5. Soil Science Society of America, Madison WI. METHOD OF OPERATION A TDR transmits a fast Rise Time Pulse along the conductor. If the conductor is of a uniform Impedance and properly Terminated , the entire transmitted pulse will be absorbed in the far-end termination and no signal will be reflected back to the TDR. But where impedance discontinuities exist, each discontinuity will create an echo that is reflected back to the reflectometer (hence the name). Increases in the impedance create an echo that reinforces the original pulse while decreases in the impedance create an echo that opposes the original pulse. The resulting Reflected Pulse that is 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 relatively constant for a given transmission medium, can be read as a function of Cable length. This is similar in principle to Radar . Because of this 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, as every nonhomogenity in the impedance of the cable will reflect some signal back in the form of echoes. A simple way to think about this is to consider the trivial 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 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. (This property was employed by a now-defunct audio cable company to design unusual high-end audio cables , and while those cables can no longer be purchased, the site remains an excellent introduction to the principles of the technology.) The magnitude of the reflection is referred to as the ''reflection coefficient'' or ''ρ''; this can be related to the ratio of the nominal impedance of the system versus the actual impedance at each discontinuity. SEE ALSO
EXTERNAL LINKS
|
|
|