Poor man\'s TDR...

[Jasen Betts]

On 2023-04-27, Martin Brown <\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

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)

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.

--
Jasen.
🇺🇦 Слава Україні

 

[Jasen Betts]

On 2023-04-27, Martin Brown <\'\'\'newspam\'\'\'@nonad.co.uk> wrote:

On 27/04/2023 11:19, Arie de Muijnck wrote:
On 2023-04-27 11:05, Martin Brown wrote:
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.

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.

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.

--
Jasen.
🇺🇦 Слава Україні

 

[John Walliker]

On Friday, 28 April 2023 at 13:00:53 UTC+1, Jasen Betts wrote:

On 2023-04-27, Martin Brown <\'\'\'newspam\'\'\'@nonad.co.uk> wrote:
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)
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.

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