Messing about with a DIY TDR

[John Rumm]

This probably needs a better title (and possibly more or less detail depending on what you want from it!)

Suggestions / improvements etc?

[Cursitor Doom]

Why bother building one? I got one of these from Leo Bodnar. Only 60 quid!

[John Rumm]

Erm, you do know what the DIY stands for in the group title? :-)

How about for fun, entertainment, experimentation, discovery, curiosity etc?

Also you are perhaps missing one of the points of the article, since it is also about techniques for finding where a cable might be damaged using a number of different methods including TDR.

Although perhaps it would be better as two separate articles, and/or expand the applications... perhaps include things like finding waterlogged coax. You might even be able to identify which sockets on a ring have spurs just from the cable ends at the CU.

(It might be worth gating off the output to convert it into a true pulse gen rather than and edge for some of these applications though)

Mine worked out about £13 in parts if I had to go buy them all (and that includes the case, switch and PSU). Factor in some time for making it, probably still a bit cheaper.

However if I needed commercially, then £60 is probably fair. (although splashing £1000 on a Megger TDR500/3 would probably be a more useful purchase for site work).

[williamwright]

In the 'nail through TV cable' scenario, it's possible (perhaps surprisingly) to get a good estimate of the position of the damage by measuring the resistance across the inner core and screen at both ends. I used a normal (£70 approx) multimeter for this many times when I was installing RF systems using the builder's built-in cables. Bill

[Cursitor Doom]

It's a fair cop, guv, get the bracelets on I'll come quietly.

Depends how handy you are with a soldering iron I guess. I'm not up to much when it comes to fine motor skills. Plus at my time of life I can't be faffing around any more so buying ready-made makes more sense. Might wake up dead tomorrow!

[John Rumm]

Yup resistance readings with enough resolution can find quite a number of faults IME.

I just tried an experiment with a length of old "low loss" co-ax. I was wondering if there would be enough detail to see the effects of a cable kink or crushed section... Apparently not appears to be the answer.

[Cursitor Doom]

A kink would create a zone of impedance which differs from that of the characteristic impedance of the coax. You won't detect that with a resistance check. You'd have to somehow marry up a TDR with a VNA for that. Not something I personally would want to have to devise.

[The Natural Philosopher]

Pulse generator and a two channel scope will see the reflections if fast enough

delay is a ns for every 8" of cable

[The Natural Philosopher]

how to build and use one

[John Rumm]

I was not doing a resistance check, I was looking at the reflection characteristic with the pulse injector.

[Andy Bennet]

Second picture up from bottom text at end contains confusing exponentiation...

"So if we multiply c by 12.9-9 Sec and by a velocity factor of 0.6"

12.9e-9 makes more sense, or 12x10^-9.

Andy

[John Rumm]

I stared at that for a while thinking "that exponent is being rendered ok as a superscript on my browser"... completely missing the missing base! :-)

Thanks, fixed.

[Andy Burns]

Shouldn't it be 1.2x10^-10 ?

[Robin]

engineering notation?

Despite my quondam physics ambitions I think there's much to be said for it in contexts like this. I cd still blind read the former as 12 ns without hesitation. I'd pause over the latter.

[Andy Bennet]

Measuring things in dns (deca nano seconds) sounds just plain weird engineering wise in the SI system!

[John Rumm]

Probably 1.29x10^-10 if one were being pedantic :-)

[The Natural Philosopher]

Try 1.2 x 10^-8, if you must be pedantic,and try and get it right..

[Cursitor Doom]

Not a recognised format in electronics, though. The power has gotta be

-3, -6, -9, -12 and so on for milli, micro, nano, pico etc.

[SH]

I would read it as 1.2 angstrom seconds :-D