Understanding DTF or Distance To Fault, using a TDR

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.

[lawnboy]

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.

[basildouglas]

Well done, a seemingly complex subject, but simply explained, easy to follow. Thank you

[CharlieTechie]

Great job Paul on your explanation of the velocity and time.

[daveys]

Excellent video on TDR DTF, thanks for posting!

[marcocosentino]

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.

[jeromewhelan]

Could you explain, or cite a reference, as to how the TDR head generates the short sharp pulse, given its 300 VDC supply?

[cranegantry]

Excellent, I learned lots, thank you Mr. Carlson.

[digitalradiohacker]

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.

[gmcjetpilot]

Great Video, I was going to ask what a TDR was... in your scope video, but you explain here. Learning is FUN! Ha ha.

[Subgunman]

I found the cure to squirrels. It either 1/2 or 7/8 inch heliax, a bit costly but efficient. Also an HPA rifle works if you live in the country. Ma Bell does not like to bury long runs, it’s usually airreal run on poles. Seldom have issues on electric drops coming in from above.
This is a project I need to build as long as I can obtain a suitable O scope.

[IvanEngler]

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.

[chuckreed]

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.

[reedjasonf]

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.

[charlesaskings]

It wasn't your fault it was the squirrels fault :)

[MrApaHotel]

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.

[mrmatymoo]

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.

[jmcbike]

Great video. Maybe an Arduino TDR circuit with LCD to show the results?

[globin]

Is a similar technique used for twisted part telephone/network cable?

[priestblood]

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?

[aminemo]

awesome videos and great explanations keep going !!!