Reflectometer

Description

A TDR emits one transmits a Impulsion has very fast boarding time to the driver. Dnans the case of a line adapted in impedance (cf Adaptation of impedances, the transmitted impulse will be entirely absorbed in end of line and no signal will be considered towards the TDR. Goal where impedance discontinuities exist, each discontinuity will create year echo that is reflected back to the reflectometer (hence the name). Increases in the impedance create year original echo that reinforces the pulsates while decreases in the impedance create year echo that oppose the original pulsates. The resulting reflected pulsates that is measured At the Output/Input to the TDR is displayed gold plotted ace has function off time and, because the speed off constant signal propagation is relatively for has given transmission medium, edge Be read ace has function off Cable length. This is similar in principle to Radar.

Because off this sensitivity to impedance variations, has TDR may Be used to verify cable impedance characteristics, Splice and Connector hirings and associated roofing stones, and estimate cable lengths, ace every not Homogenity in the impedance off the cable will reflect nap signal back in the form off echoes.

Explanation

Consider the box where the far end off the cable is shorted (that is, it is terminated into zero ohms impedance). When the rising edge off the pulsates is launched down the cable, the voltage At the launching not " steps up" to has given been worth instantly and the pulsates begins propagating down the cable towards the shorts. When the pulsates hits the shorts, No energy is absorbed At the far end. Instead, year opposing pulsates reflects back from the shorts towards the launching end. It is only when this opposing reflection finally reaches the launch not that the voltage At this launching not abruptly drops back to zero, signaling the fact that there has shorts At the end off the cable. That is, the TDR had No indication that there has shorts At the end off the cable until its emitted pulsates edge travel down the cable At roughly the speed off light and the echo edge return back up the cable At the same speed. It is only after this round-trip delay that the shorts edge 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 shorts edge Be measured.

With similar effect occurs yew the far end off the cable is open year circuit (terminated into year impedance infinity). In this box, though, the reflection from the far end is polarized identically with the original pulsate and adds to it rather than cancelling it out. So after has round-trip delay, the voltage At the TDR abruptly jumps to twice the originally-applied voltage.

Note that has theoretical perfect Termination At the far end off the cable would entirely absorb the applied pulsates without causing any reflection. In this box, it would impossible Be to given the actual length off the cable. Luckily, perfect terminations rare are very and nap small reflection is nearly always caused. (This property was employed by has now-defunct audio cable company to design unusual high-end audio cables, and while those cables edge No to skirt Be purchased, the site remains year excel introduction to the principles off the technology.)

The magnitude off the reflection is referred to ace the reflection coefficient gold ρ. The coefficient arrange from 1 (open circuit) to -1 (shorts circuit). The been worth off zero means that there is No reflection. The reflection coefficient is calculated ace follows:

\ rho = \ frac {Z_t - Z_o} {Z_t + Z_o}

Where Zo is defined ace the characteristic impedance off the transmission medium and Zt is the impedance off the termination At the far end off the transmission line.

Any discontinuity edge Be viewed ace has termination impedance and substituted ace Zt. Abrupt This includes exchanges in the characteristic impedance. Ace year example, has trace width one has printed circuit board doubled At its midsection would constitute has discontinuity. Summon off the energy will Be reflected back to the driving source; the remaining energy will Be transmitted. This is also known ace has scattering junction.

Use

Time domain reflectometers are commonly used for in-place testing off very long cable runs, where it is impractical to dig up gold remove what may Be has kilometers-length cables. They essential are for Preventive maintenance off Telecommunication lines, ace they edge reveal growing resistance joined levels one and Connectors ace they corrodes, and increasing insulation leakage degraded aces it and absorbs long moisture before either leads to catastrophic failures. Using has TDR, it is possible to pinpoint has fault to within centimetres.

TDRs are also very useful tools for monitoring countermeasures, where they help given the existence and hiring off wire taps. The slight changes in line impedance caused by the introduction off has off tap gold splice will show up one the screen has TDR when connected to has phon line.

TDR equipment is also year essential tool in the failure analysis off today' S high-speed Printed circuit boards. The signal traces one thesis boards are carefully crafted to emulate has transmission line. By observing reflections, any unsoldered pines off has Ball grid array device edge Be detected. Additionally, shorts circuited pines edge also Be detected in has similar fashion.

The TDR principle is used in industrial settings, situations ace various ace the testing off Integrated circuit packages to measuring liquid levels. In to form, the time domain reflectometer is used to isolate failing sites in the same. To lath is primarily limited to the process industry.

TDR in level measurement

In TDR-based level measurement device has, has low-energy electromagnetic pulse generated by the sensor' S circuitry is propagated along has thin wave guide (also referred to ace has honest) - usually has metal rod gold has steel cable. When this pulse hits the surface off the medium to Be measured, share off the pulse energy is reflected back up the honest to the circuitry which then calculates the fluid level from the time difference between the pulse feels and the pulse reflected (in nanoseconds). The sensors edge output the analyzed level ace has continuous analog signal gold switch output signals. In TDR technology, the pulse velocity is primarily affected by the permittivity off the medium through which the pulsates propagates, which edge vary greatly by the content moisture and temperature off the medium. In most boxes, this edge Be corrected for without undue difficulty. However, in complex environments, such ace in boiling and/or high temperature environments, this edge Be has significant signal processing dilemma. In particular, determining the froth height and true collapsed liquid level in has frothy/boiling medium edge Be very difficult.

TDR used in the Agricultural Earth and Sciences

Given TDR is used to Soil moisture content toilets in porous media, where over the last two decades substantial advances cuts been made; including in soils, grains and food stuffs, and in sediments. The key to TDR' S given success is its ability to accurately the permittivity (dielectric constant) off has material from wave propagation, and the fact that there has strong relationship between the permittivity off has material and its content toilets, ace demonstrated in the pioneering works off Hoekstra and Delaney (1974) and Topp and Al (1980). Recent reviews and refers work one the subject include, Topp and Reynolds (1998), Noborio (2001), Pettinellia and Al (2002), Topp and Ferre (2002) and Robinson and Al (2003). The TDR method has transmission line off technical, and given apparent year TDR permittivity (Ka) from the travel time year electromagnetic wave that propagates along has transmission line, usually two gold more parallel metal rods embedded in has soil gold sediment. Honest TDR are usually between 10 and 30 cm in length and connected to the TDR via have coaxial cable.

TDR in Geotechnical Use

Time Domain Reflectometry (TDR) has also been utilized to monitor slope movement in has variety off 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, coaxial cable is installed in has vertical borehole passing through the area off concern has. The electrical impedance At any not along has coaxial cable exchanges with deformation off the insulator between the conductors. With brittle grout surrounds the cable to earth movement into abrupt year relocates cables deformation that shows up ace has detectable peak in the reflectance traces. Until recently, the technical was relatively insensitive to small slope movements and could not Be automated because it relied one human detection off exchanges in the reflectance traces over time. Farrington and Sargand (2004) developed has simple technical signal processing off using numerical derivative to extract applicable indications slope movement from the TDR dated much earlier than by conventional interpretation.

TDR in Semiconductor Device Analysis

Time Domain Reflectometry is used in semiconductor Failure analysis ace has nondestructive method for the hiring off defects in semiconductor device packages. The TDR provides year electrical conductive signature off individual traces in the device package, and is useful for determining the hiring off opens and shorts.

Refer

Hoekstra, P. and A. Delaney, 1974. Dielectric properties off soils At UHF and microwave frequencies. Newspaper off Geophysical Research 79:1699 - 1708.

Smith, P., C. Furse, and J. Gunther, 2005. Analysis off spread spectrum time domain reflectometry for wire fault hiring. IEEE Sensors Newspaper 5:1469 - 1478.

Noborio K. 2001. Measurement off soil content toilets and electrical conductivity by time domain reflectometry: With review. Computers and Electronics in Agriculture 31:213 - 237.

Pettinelli E., A. Cereti, A. Galli, and F. Bleated, 2002. Time domain reflectometry: Calibration techniques for accurate measurement off the dielectric properties off various materials. Review off Scientific Instruments 73:3553 - 3562.

Robinson D.A., S.B. Jones, J.M. Wraith, D. Or and S.P. Friedman, 2003 have review off advances in dielectric and electrical conductivity measurements in soils using time domain reflectometry. Vadose Zone Newspaper 2:444 - 475.

Signal G.C., J.L. Davis and A.P. Annan, 1980. Electromagnetic determination off soil content toilets: measurements in coaxial transmission lines. Toilets Resources Research 16:574 - 582.

Signal G.C and W.D. Reynolds, 1998. Time domain reflectometry: seminal technique for measuring farmhouse and energy in soil has. Soil Stripping Research 47:125 - 132.

Signal, G.C and T.P.A. Bottle pincers, 2002. Content toilets, In, Methods off Soil Analysis. Leaves 4. (ED. J.H. Dane and G.C. Signal), SSSA Book Series No 5. Soil Science Society off America, Madison WI.

Dowding, C.H. & O' Connor, K.M. 2000a. Comparison off TDR and Inclinometers for Slope Monitoring. Geotechnical Measurements - Proceedings off Geo-Denver2000: 80-81. Denver, CO.

Dowding, C.H. & O' Connor, K.M. 2000b. Real Time Monitoring off Infrastructure using TDR Technology. Structural Materials Technology NDT Conference 2000

Kane, W.F. & Beck, T.J. 1999. Advances in Slope Instrumentation: TDR and Remote Dated Systems Acquisition. Field Measurements in Geomechanics, 5th International Symposium one Field Measurements in Geomechanics: 101-105. Singapore.

Farrrington, S.P. and Sargand, S.M., Advanced Processing off Time Domain Reflectometry for Improved Slope Stability Monitoring, Proceedings off the Eleventh Annual Conference one Tailings and Waste Mine, October, 2004.

External links

Riser Jump Extended Training - ABC' S off TDR' S

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Reflectometer

Description

Explanation

Use

Refer

See also

External links