Understanding DTF or Distance To Fault, using a TDR
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- Опубліковано 20 сер 2014
- This Video explains how to find the Distance To Fault in or DTF, using a Time Domain Reflectometer or "TDR." Use this to locate problems with your coax cable.
- Наука та технологія
Working in the irrigation industry, I have used a TDR to locate bad splices, buried decoders/valves boxes and open/cut wires. I have found that in a 2-wire system the VoP can change from 73 on a straight piece of wire laying on the ground to 70 on a piece that is coiled on a spool to 57 when it is buried in the ground which all of my situations are. I have to make the VoP change in my TDR to get the proper measurement of the amount of wire left on a spool. After watching your video I’m surprised that it would change that much. I had very limited knowledge of how and why a TDR worked but now I am more understanding of the tool that makes my life of diagnosing problems in a large scale irrigation system work.
Excellent video on TDR DTF, thanks for posting!
Well done, a seemingly complex subject, but simply explained, easy to follow. Thank you
Great job Paul on your explanation of the velocity and time.
Thanks Chuck!
Excellent, I learned lots, thank you Mr. Carlson.
Great Video, I was going to ask what a TDR was... in your scope video, but you explain here. Learning is FUN! Ha ha.
Handy for measuring big rolls of coax, or finding shorts in a long run...... Or calibrating an oscilloscope.
awesome videos and great explanations keep going !!!
amine mohamed Thanks amine!
Could you explain, or cite a reference, as to how the TDR head generates the short sharp pulse, given its 300 VDC supply?
Great video!
A couple of tips.
1) Work out also the measurement units in the equations: it helps proving that the operations are correct eg. (in/ns) * ns = in.
2) Use parenthesis when doing sequences of divisions would help clarify the order in which de divisions are done and help understanding the measurement units.
Keep in mind for the tolerance on measuring length that not every foot of cable is made the same. There are tolerances along the length of a spool of cable.
Math teachers!
Look in for a demonstration of "HOW IT IS DONE".
Introduce a problem with no obvious solution.
State the available facts and where they can be sourced.
Use the facts to get to the destination, missing NO steps and assuming NOTHING of the knowledge of the pupil.
Thank you Mr Carlson. The Uni lecturer I needed, but not the one I deserved.
thank you that makes great sence
It wasn't your fault it was the squirrels fault :)
That was a great explanation. Thank you. I would consider that maybe instead of the velocity factor being off, the scope measurement might not be precise enough. Without actually doing the math your time measurement is off by about 200 pico seconds. That is hard to measure with your average scope. I still appreciated how you explained the way to correct it.
your videos are really awesome and very informative. it's just a bit confusing for me that you don't use metrics (but feet and inches) in your calculations.
anyway, i really enjoy all the clear and precise infos.
Ivan Engler metric to imperial conversions are even easier than the math presented in the video. If you can do the math he's explaining you are more than capable of doing the simple conversions.
What is the best way: To send one short pulse (about 100ns) or send a step? I assume the short pulse would get more attenuated when travelling through the cable as it is made up of higher frequencies? And the step would travel longer as it is DC after the step rising edge.
Interesting that the VF changes over time! Coax cables employed in radio altimetry systems on aircraft are of critical length. They cannot be repaired/re-terminated and must be replaced if damaged. I suspect the changes in VF are so small that they do not affect the measurement to a great extent.
seeing imperial units used in circuit analysis gave me a weird feeling. is that science or what :)
Just a quick question, do you have to take the air space in the cable fault into consideration as it would be different between a shorted cable to a broken cable and the speed between open air and direct short contact in the return travel time of the cable or would it not make a big difference?
+Wayne Turner Not sure if I understand your question exactly, but let me try to help. In my experience, anything that changes the impedance of coaxial cable at a particular point will cause a change in the flat line display normally following the TDR pulse. The size of the deviation is determined by the change in impedance, while the vertical direction of the trace movement indicates the polarity of the impedance change. As shown in the video, an open circuit is indicated by an upward spike in the trace. Conversely, a short in the coax will be indicated by a downward spike, or deflection. In either case, the type of fault should have no effect upon the round trip time from/to the TDR; the distance between TDR pulse and returned reflection from fault varies with distance x velocity factor. If you are testing a series of coaxial cable segments connected by quick disconnects, such as BNC connectors, you can sometimes spot a small variation in the reflected signal at the length corresponding to the distance to each connector. It is difficult to obtain a perfect impedance through a bulkhead connector and a quick disconnect on each side, for instance, so a variation of the display line large enough to be seen may not indicate that you have found a fault, but just indicate a small mismatch of impedance at each part of the connector assembly. On the other hand, a kinked or squashed coax may be causing problems and not give a clear indication of short or open, but some kind of recognizable disturbance of the trace will usually be seen at the fault. If you have not used a TDR before, I think you will find them very easy to use and invaluable in locating faults in conductors buried underground or inside an airplane's wire bundles.
Is it possible to detect vibration in the cable with TDR?
Good!
Just the cable construction variations would leave your determined 0.6% VF increase a bit of good luck. That does indeed translate to 1.3 inches on your length. RS (CT100pro) specifies its dielectric diameter within +/-0.15mm which would be around 6.5% variation. The conductor diameter is guaranteed to be within 2% and so on. I would keep in mind a 10% tolerance on printed specs about right.
Is a similar technique used for twisted part telephone/network cable?
Hi Glen, You bet. These things are even used for moisture content evaluation in soil.
Decide beforehand how much precision you want in your measurement. Then make sure your initial values and any measurements have enough significant digits to get you the precision you want. For example, the speed of light is 11.80282752... in/ns. Of course you are limited by the least precise measurement.
This is late but isn't BNC Bayonet Neill-Concelman?
what is 11.8" ns meanning?
How about instead of an open, it tests a pinched or dented place along the way to a good antenna. I wanna see that video.
It will show impedance variations along a pinched coax. You can calculate the distance to the pinch.
This is quite an old video, so I don't know if this is helpful at this point... in case it is, here's a humble request to employ Dimensional Analysis (a.k.a. the factor-label method, the unit-factor method, or the unity bracket method -- see wikipedia's Conversion_of_units#Factor-label_method) when doing this math... if only so we know what units you're talking about, when. Thanks!
Second is small 's'. Capital 'S' is Siemens, unit of electrical conductance.
I get that you said speed of electricity to avoid possible confusion, but it's worth pointing out that it's actually the speed of light.
Well, not always though. "velocity factor"
Yeah, I should have clarified as you did that it's only for the ideal case. Also the velocity factor seems very analogous to the refractive index of materials (light slowing down), hell they might be the same thing.
@@MrCarlsonsLab The number with the velocity factor applied is still the speed of light. It's the speed of light in that cable.
Your excellent video could be more excellent with a correct name for the BNC which is NOT Bayonet Navy Connector but rather this:
en.wikipedia.org/wiki/BNC_connector
Are you sure about that. www.te.com/content/dam/te/global/english/industries/enterprise-network-solutions/knowledge-center/documents/enterprise-white-paper-design-fundamentals-for-high-bandwidth-bnc-connectors-106088ae.pdf
Mr Carlson's Lab Point taken....I rely on Wikipedia since it is never wrong. :)
Jim Blakely It's a mysterious abbreviation kind of like HAM radio. Glad you enjoyed the Video.
Great, General Knowledge!
looks like an egg candler. its still done to find bloody eggs