#88: Cheap and simple TDR using an oscilloscope and 74AC14 Schmitt Trigger Inverter

This video is a follow on from my previous video that discussed how to measure the length and impedance of coax using an oscilloscope and an pulse generator. This previous video goes into more of the theory and the different measurements you can make. It can be found here:
However, this video showed the use of a pulse or function generator as the signal source. The problem is that most inexpensive generators won't have a fast enough risetime to be able to resolve shorter cables or finer precision in the measured length. This video shows a very inexpensive circuit that you can build that has a very fast rising/falling edge speed, making it easy to more precisely measure the cables.

It is based on a 74AC14 Schmitt Trigger Inverter chip and a handful of passive components. One of the six inverters is used as an oscillator which drives the remaining 5 inverters in parallel to drive the coax under test.

More detail regarding the Schmitt Trigger circuit can be found in the video uploaded immediately prior to this video:

This video is shot in HD

[jeffreyyoung]

I built the TDR circuit from QST years ago, and I used it at work with the better scopes to check coaxes on the reel, and the boss was so impressed, he went out and spent a ton of money on a box that had a microprocessor that did the math for you. But it never worked better, nor was it built under $5.00!

[Vintage_USA_Tech]

in the late 70s I couldn't wait to crack open my new Popular Electronics magazine just to build the next new project in what seemed to be a magical time for electronics hobbyist and home inventors. And I have to say everyone of your videos brings that magic back X10 in a way we could only have dreamed of in the 70s. Thanks a million for all the great videos.

[richardneel]

Makes my heart happy to see someone using an RPN calculator. I get the video is almost 10 years old, but had to comment on the lost art. :)

Had to comment that this technology is also used in optical telecommunications networks, but is called OTDR (O for optical - of course). We use it every day to find optical fiber cuts/damage.

[Lskaggs]

Great idea and excellent circuit!  When I worked for the Navy I used Pulse Generators that cost hundreds of thousands of dollars, your circuit is outstanding, I have got to build one for myself. Thanks!

[billjones]

Suddenly my 40-meter yagi had a high SWR. I used one of these little TDR circuits and determined the exact location of the problem. I'm sold. Thanks! 73 de K8CU

[christheother]

Man, the precision of that setup is remarkable.

[BillySugger]

Love it! I’ve built a few 80:20 instruments like this to get at least 80% of the functionality at way less than 20% of the cost and complexity of a commercial instrument.

[PeterWMeek]

Great way to determine the impedance of an unknown bit of coax. (video #37)

You can also calculate the VF for an unknown bit of coax. Take a measured length and back-calculate what the VF *must* be to get the measured delay from a known length.

Thumbs up for bothe the #37 and #88 videos. (this comment posted on both videos)

[norbs]

Man, you are a genius! I just build the circuit and it works great, I measured a 3m cable and it gave a result of 3.003m. I used the CD74AC14M96, SOIC-14 version from texas.

[scottgilson]

Thanks for a great video! I built it to find a break in a Romex power cable. Got 1.2ns/foot. Used a 74LS14 as that was all I had available. Needed to change the 6.8 kOhm to 1 kOhm to get it to oscillate.

[MaqsoodAlamShafiq]

It is indeed a pleasure watching you explaining tricks of electronics trade the conventional way using vintage tools. Love it.

[stevejagger]

This is a subject I have had an interest in for some long time.
Thank you for the clear explanation.

[zekim]

Great idea, thanks for posting! There should really be a hex inverter cookbook!

Teachable suggestions: in addition to open-end, you could also show terminating the coax properly, and then shorting it. Many students find the result counterintuitive.

Even cooler, show the far end voltage (with a low-C probe) on your second scope channel!

[nemanjatodorovic]

You have the best tutorials I've ever seen! honestly! thank you!

[ronaldlijs]

There is excellence in how you explain/present your videos, PLEASE PLEASE keep them coming, this is great stuff!

[fabiotrevisan]

Great video Allan!
I`m just re watching your video on this TDR pulse generator, which I first watched maybe 2 years ago, which inspired me to build it re purposed for something else.
Two years ago I was looking for something that I could use as a fixture to calibrate a 60Mhz oscilloscope's step response. A "poor men's" approach for someone who didn't have a fast edge pulse generator (<1ns) and this one is still the only one I have.
It just crossed my mind to thank you and report back that the quality of the step is fairly decent and for this sort of bandwidth and up to about 100Mhz, it's good enough to calibrate the scope to the specs (less than 3% overshoot / ringing.
In fact, the output of the AC14 ganged buffers is quite sharp and flat (almost no overshoot at all) and just a little pre-shoot bump (some told me it's due to the AC14's ground bonding wire, inside the package).
I actually build it a little bit smaller than yours, with an SMD AC14 soldered (just the power pins) to a dual sided blank PCB heavily decoupled, and with 0603 smd resistors soldered directly to the AC14's pins for the output.
It works like a charm.

[johnbrandolini]

Alan, thanks for the great video. It prompted me to build my own 9 volt powered time base generator based on a 10 MHz clock oscillator. I wanted something a little more versatile so I I'm using 2 74HCT74 DFlops to give me a range from 10 MHz to 625 KHz. I tried it at the lowest frequency with an ~ 5' unterminated cable and lo and behold it worked! I got a 17 nanosec delay which after calculating the length equaled 66.1 inches which was pretty close to the length of the cable I made. Isn't math and physics great!

[TzOk]

It works not only for coax, I've just used this method to find a cut wire on a 40m extension cord. It was spot on.

[lingcod]

We had a manufactured TDR which was invaluable for figuring the length of cable on a spool. Very good video showing a low cost way of doing the same thing. Not as fast, but you're not selling to a client either.

[Eduardo-Brasil]

FANTASTIC content, congratulations!!! Based on the project in this video, it is possible to develop a UTP network cable tester that can detect the distance of each pair of cables and thus check whether the quality of the cable and connectors are good, perhaps using a microcontroller to transform the information timing in metric measurements???