Dead Simple Time-Domain Reflectometry With Just A Battery And An Oscilloscope

“Time-domain reflectometry” sure sounds like something that needs racks of expensive equipment to accomplish. In reality, TDR is just measuring the time between injecting a pulse into a cable and receiving its echo, either from the other end of the cable or from some fault or defect along the way. It’s a useful technique, and as [Allen Wolke (W2AEW)] shows us, it can be accomplished with little more than a battery, a resistor, and an oscilloscope. And a little math, of course.

There are, of course, dedicated time-domain reflectometers, but all of them are really just elaborations of the basic principles [W2AEW] demonstrates with his simple setup. The oscilloscope is set up with a tee connector on one channel; one side of the tee is connected to the cable under test, while the shield conductor of the other side is connected to the negative terminal of a 9V battery. A resistor connected to the center conductor is used to complete the circuit, which sends a brief pulse down the test cable. The scope is set up to capture the outgoing pulse as well as the return pulse, allowing the time between the two to be measured. Some simple math gives the length of the cable, the distance to a fault, or with a little rearrangement, the velocity factor of the cable.

The video below shows the simple method at work on coax and Cat 5e Ethernet cable. It even worked on a roll of zip cable, which was a little surprising. If this technique is too simple, you can always elaborate a bit and roll your own TDR tester. Googly eyes optional, of course, but recommended.

22 thoughts on “Dead Simple Time-Domain Reflectometry With Just A Battery And An Oscilloscope

  1. That is the best TDR video I’ve ever seen, though I’m rather embarrassed I didn’t think of doing that. Just not digital enough in my idea of a scope.

    A mercury wetted relay reed would give a more reliable switch transition and be *almost* as simple, but that was not the purpose of this video. And the search for a mercury wetted relay prevented my doing it for many years.

    Here are some examples of TDR using a Tek 11801 & various 20 GHz sampling heads.

    It’s a great way to test RF connectors. It more than justifies building or buying a fast edge pulser. You’ll need a DSO which is higher than the maximum frequency of interest. But for HF QRP work a 100-200 MHz DSO is more than enough.

    With modestly better front end and FW designs we could do 4 GHz on cheap HW using sampling scope technology (you can’t escape the need for a very fast sample and hold circuit. Not easy!)

  2. Great, practical go thru. Thanks for including the FAQ regarding single conductors too. I love that there are so many high quality videos to explain stuff like this to my colleagues so I don’t have to setup the lab to show them. Thanks again!

  3. i’m still novice about this sort of stuff and i thought to myself that i understood it but really it just raises more questions!

    my first thought is that the cable basically acts like a capacitor, it conducts an infinite current (dead short) for an instant because the whole thing starts out discharged, which pulls the voltage down because the resistor only lets a small current through. then the wavefront hits the end of the capacitor and now the whole thing has some charge on it and its current draw decreases rapidly.

    but what i don’t understand is, wouldn’t the capacitance of the cable have an effect separate from the length of it? once the wavefront reaches the end, there is no longer any part of the capacitor that is infinitely conducting but all of it would still be somewhat conducting? i haven’t even attempted any of the math, is the difference between the capacitor charge curve and the duration of the short just so significant that it looks like a discrete transition instead of a curve?

    i just kind of have this intuitive expectation that after the wavefront reaches the end of the cable, it will switch from the low state to a capacitor-charging RC curve. i guess the RC constant is so short that it looks like a discontinuity??

    1. This is a wave phenomena. Don’t think of it as charging a capacitor but more as an empty water channel in which you inject a sudden flow of water. The water front moves to the end of the channel where it bounces (reflects). As there is still water coming in, it has no other way then to travel back *on top* of the inflowing water. So if you take continuous level measurements at the beginning of the channel you get first a steep rise as the flow starts, followed by a time where the level stays the same. Then suddenly the level will rise to twice its initial steady level as the first water is coming back at you (at which point it is time to duck).

  4. With your hand on the longish wire, the rise time can’t be wonderful, which will really limit the length of cable you can test. You need harmonics to overcome the distributed reactance of a long cable.

    Or do multi-kilobuck TDR pulse generators use SMA connectors and ultra-short conducting paths just as a reason to charge multi-kilobucks?

    Still, a cute and educational hack!

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.