TDR?

[Paul Elliott]

If you have a fast squarewave (or pulse) generator, and an oscilloscope, you can make your own poorly-calibrated-but-still-useful TDR. Just put a BNC "Tee" on your scope input, connect the generator with a short length of coax to one side of the tee, and your cable under test to the other side of the tee. Set the generator to a few kHz and observe the waveform on the scope. Experiment with lengths of coax, with the end open / shorted, terminated. It works well enough for basic troubleshooting, sometimes anyway.

The generator and scope need to be fast (>500 MHz) for looking at VHF antennas, but 100 MHz or slower can find opens and shorts pretty well. The TDRs are only suitable for working on coax or paired-wire cables. It won't tell you much if the ground wire is at any distance from the hot wire. Don't use the TDR on powered wires!

[Dockhead]

Originally Posted by Paul Elliott

If you have a fast squarewave (or pulse) generator, and an oscilloscope, you can make your own poorly-calibrated-but-still-useful TDR. Just put a BNC "Tee" on your scope input, connect the generator with a short length of coax to one side of the tee, and your cable under test to the other side of the tee. Set the generator to a few kHz and observe the waveform on the scope. Experiment with lengths of coax, with the end open / shorted, terminated. It works well enough for basic troubleshooting, sometimes anyway.

The generator and scope need to be fast (>500 MHz) for looking at VHF antennas, but 100 MHz or slower can find opens and shorts pretty well. The TDRs are only suitable for working on coax or paired-wire cables. It won't tell you much if the ground wire is at any distance from the hot wire. Don't use the TDR on powered wires!

I wonder how many kilometers of coax, Cat5, twisted pairs, and paired-wire cable I have on board. I think an investment $200 for a purpose-made TDR, and a Fluke no less, is perfectly justifiable on my boat (acquistion of new tools seems to always find some justification or another ). I only wish I had had it last May It would have saved me hours and days of faffing about.

I don't have an oscilloscope and have no (apparent) need for one, so that's out as a substitute.

[socaldmax]

Originally Posted by Dockhead

I wonder how many kilometers of coax, Cat5, twisted pairs, and paired-wire cable I have on board. I think an investment $200 for a purpose-made TDR, and a Fluke no less, is perfectly justifiable on my boat (acquistion of new tools seems to always find some justification or another ). I only wish I had had it last May It would have saved me hours and days of faffing about.

I don't have an oscilloscope and have no (apparent) need for one, so that's out as a substitute.

When I first read your post about buying the Fluke cable tester, I was going to suggest something very similar to what Paul Elliot posted for several reasons.

1. I'm not a huge fan of Fluke equipment. I also don't buy Snap-On or Mac tools, generally speaking I think they're overpriced compared to other quality brands. I have a $35 DMM that is a Chinese copy of a Fluke 87v. A coworker had just bought a Fluke for over $350 and we had to send them both in to the Navy's cal lab for calibration prior to use. Mine came back calibrated, his Fluke which cost 10x more, came back as "unable to cal." Normally I'll spend the extra money to get higher quality, but we often saw that isn't what you get. He did get it replaced, but it had the exact same functions and identical precision and accuracy as mine for 10x the price.

2. The major benefit of a TDR is not so much it's ability to measure the distance to opens/shorts, it gives you a visual indication of the impedance mismatch at the connection, caused by a poor crimp, poor solder joint or corrosion. The Fluke cable tester is like a pair of smudged reading glasses compared to a microscope.

3. My suggestion would be to spend around $200-300 to buy either a used oscope on Ebay (Agilent (formerly HP), Tektronix, BK Precision are all good brands) with at least 200 or 300MHz bandwidth, the higher the better. There are also small portable digital "pocket scopes" in that same price range, but I don't know enough about any of them to recommend any. With an o'scope, you'll be able to isolate and track signals that no DMM can see, troubleshoot to a level far beyond "go/no go."

4. Then build yourself an "octopus" to go along with your o'scope. I built mine for about $20 and it's invaluable. It allows power off in-circuit testing of any component and will tell you if that cap really is holding a charge or filtering that signal, or if that resistor is still 220K ohms. Here's a link to building and using one: Davidson College Instrumentation Specialist - Physics - Octopus


For only $50-$100 more than the Fluke, you'll be getting 10x the capability for a much wider range of uses.

[Lake-Effect]

Nice post, socaldmax. The info on the octopus was a great refresher for me.

I have a soft spot for Fluke - when i ran a small service dept I had a Fluke 77 that was utterly dependable. And I've always coveted their later models. They have brought out some great innovations that others copied. Nonetheless... for my own use I also usually buy the better quality knock-off meters at like 1/4 the price of a comparable Fluke.

I've used oscilloscopes my whole career and personally own a couple for over 20 years... but i don't think i could recommend one to a boat-owner unless they intend to get, or already have serious electronic experience. Their use isn't trivial, and getting results requires knowledge and careful interpretation. Whereas the portable TDRs are optimised for use in the field for fault-finding, and they can be used productively without an electronics degree.

[transmitterdan]

Just a side note about cheap TDRs and antennas. To a pulse type TDR a good antenna and bad antenna look pretty much the same. A good high efficiency antenna looks like a big reflection to a pulse type TDR even though everything is ok. The reason is that antennas are tuned to a specific frequency band but pulse type TDRs are not frequency selective. To get a "cheap" frequency selective TDR useful for antenna circuits the price goes up by about 10X (e.g. Vector Impedance Analyzer (Vector Network Analyzer) - VIA Echo - - AEA Technology Inc.).