Can anyone think of a cunning quick and dirty way to do TDR on modest length 3 - 10 m cables to look for breaks ideally with resolution 10cm or better and without using an expensive fast sampling scope?
That means return pulse timing accurate to 30ns or better. (in the ballpark of what modern CPU internal timers can do)
Most cable breaks can be found by measuring the capacitance of the wire from each end against all others shorted together.
Arie
Thank you both for the suggestion of capacitance meter. That's easy enough... Even without knowing the capacitance per unit length I can measure from each end and use the ratio to guess where the break is.
Its an XLR mike cable belonging to the group that did a free gig at our village hall that went noisy and I promised to take a look at it. The shield has broken somewhere along its length and I want to find where.
(I was hoping for a broken or dry joint on one of the XLR connectors)
an multimeter on the screen and massaging the cable from end to end might find it
Enhance to binary search and start cutting in half, then quarter, etc ;-)
The idea is to leave them with a lead long enough to be useful...
Connect a high voltage supply across the two shield ends and use your thermal imaging camera to find the arc. Or use a portable radio to find the arc.
[It may sound silly, but I have a cheap eBay HV arc generator PCB and a TIC both within arm's reach.]
Resolving 10 cm needs a roughly 60 ps TDR rise time. The scope would need to have a bandwidth around 5 GHz.
There are cheap VNAs that can do FFT tricks to emulate TDR.
Or as suggested, measure capacitance.
Most cable shields are leaky; something could be done with that. Or mag fields. Or even thermal imaging.
Connect one shield end to a signal generator and slide a scope probe along the outside. The break should be obvious.
Just look for where the smoke is coming out.
That ought to work pretty well - thanks.
>
I tried that first - sadly no joy.
Peculiar thing for me was that despite it only being an earthing shield the sound was badly compromised with a lot of crackling and popping on the two balanced lines. XLR audio gear isn't my thing at all but I can solder a lot better than whoever last put the lead together!
I saw no signs of a short or path to either signal line both were well isolated from the shield >10M.
A sig gen and a cheap scope can measure femtofarads, or even attofarads.
Yes. This is the way the US Phone Companies did it, but for detecting shorts with a Wheatstone bridge.
Capacitance came later. They also could measure distance to wet cable (copper wires, pulp paper insulation, lead cable jacket with small hole allowing water ingress). I don't recall the exact method, but I read it in a book published by AT&T in 1953.
It would be a good bet for a studio cable - they almost always break at a connector.
Joe Gwinn
Breaks normally happen in the 6" at one end, and you can usually tell which end has had a harder life just by looking. You don't always need testgear.
Yes - the NanoVNA does it.
Nano VNC has a TDR display mode (I think it does a fourier tansform on the return function) $80 to $150 depending on model and how genuine.
Drive the ends of the screen from a ends of a centre-tapped secondary, connect the tap to the X scope probe, drive the Y axis from the input signal,
And run a grounded tinfoil sheet along the length of the cable, where the slope changes suddenly there's your break.
I have a network cable tester which includes a simple TDR function that indicates distance to open or short with a resolution of 10cm for cables at least a few metres long. It does continuity testing, mapping and TDR for cables up to 2km long, but seems to need a minimum of a few metres before the TDR functions works. This limitation can be overcome by adding a short cable of maybe 3m between the tester and the cable under test. The accuracy is "good enough" using default calibration data for the usual network cable types including coax. It is variously marketed as Kolsol AT278 or Noyafa NF-8601S. Current price on Amazon is around £124. It is much less direct from China. I have used it far more than I expected to. John
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