#37: Use a scope to measure the length and impedance of coax
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- เผยแพร่เมื่อ 7 มี.ค. 2012
- This video shows one way to use a scope and function generator to measure the length of a piece of coax transmission line as well as estimate its impedance. It uses a "poor man's TDR" type of measurement by launching a pulse into the coax and measuring how long it takes to return after being reflected by the open circuit end. This same technique can be used to determine the distance to a fault (open or short). A simple method for determining the impedance of the line is also shown.
This video touches briefly on transmission line and reflection theory, but is definitely not intended to dive deep into these topics. There are literally books written about this topic - so that won't be covered here.
I recently did a video to show how to do this with a simple, low cost square-wave generator instead of using the function generator shown in this video. It can be found here:
• #88: Cheap and simple ... - วิทยาศาสตร์และเทคโนโลยี
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This was a great demo, especially the use of the variable resistor to show termination resistance. Very cool to see that.
One of my jobs as a broadcast tv engineer was sorting out which section of camera triax cable between outside broadcast vehicle and the camera position was open or shorted on long runs where lengths of 100, 200 or 400 metres were combined to make up the desired length.
A TDR would have made the identification of the bad leg so much easier, and often thought about building one.
Instead I made a a simple continuity tester which could detect open and shorted conductors on each cable before installation thereby saving the long march - usually in the rain on a golf course - with a camera testing back to the OB truck length by length.
Whatever the method of fault detection it still required an outdoor repair with a gas soldering iron.
I am addicted. I watched a large number of your Oscilloscope videos and learned, or re-learned something in every one of them. Great work.
Me too
I built a TDR circuit that was published in QST magazine decades ago, and soon after, my boss bought a commercial unit for the shop!
I still have the unit! And I have coax to test as well, so, yes, I still use it!
Many thanks! Using the technique to finding a fault in one of the coax in my house! thanks again for making the lockdown very productive!!
I just want to thank you for your wonderful style. You've managed to clear a lot of cobwebs. And, it's always a bonus to watch tutorials with the same scope I have! Makes things so easy!
Excellent! I have two little Boxes, one with a 470 Ohm Linear Pot with an approximate scale, the other with Switched Resistors at 50, 75, 93, 100, 150, 300 and 600 Ohms - both are so useful when dealing with known lengths of unknown Co-Ax or Twin.
For viewers out there looking for information on how to use your test equipment, like a Oscilloscope and a RF Generator. There's a old book published by Tab Books in 1977 by George Leon which is still available through Amazon Market Place. I bought it back in college 20 years ago. All kinds of useful test jig circuits to make measurements with a AF or RF Signal Generator, Highly recommended!!
Thank you for publishing this! What a great set of insightful tips that de-mystify the concepts around transmission line theory!
Thank you so much sir! I am an engineer and couldn’t grasp this concept after reading through documents on this until I saw you demonstrate on the scope. Very clear now!
Back in the 80s, when I worked for KUAT in Tucson, we used that method to find a dent in a length of 3 1/4" rigid line, which was several hundred feet long. But we didn't use a jury-rigged scope/pulse generator! No, it happened that the station was owned by the University of AZ. So we borrowed an honest-to-goodness time domain reflectometer from the physics department. About that time, the physics dept. also bought us a spankin' new 50 KW dummy load, so one of their grad students could use it for his PhD dissertation project.
How big was it and was it oil cooled ?
As usual, top quality video. Like I said before, this gentleman should be a textbook example of a lecturer/tutor at universities.
Built the 74AC14 generator and tried it out. Works like a champ. Great little project. Thanks very much!
Great tutorial! I love the speed at which and the depth to which you explain things. I'm going to build one of those Schmidt TDRs just for the heck of it (the parts are all floating around the table somewhere.)
Man I just found this video today. I figured I could do this kind of thing, I just hadn't sat down to work it out. I'm using a Siglent 2202X-E scope, and learning the ins and out (my first scope). Outstanding video brother!
Thank you for all your time in doing these very helpful videos.
Sir, you are a teacher equal to my all-time-favourite Richard Feynman. Thank you.
WOW! Thank you!
@@w2aew you are actually like him in our minds. Thanks a lot it was a great help from you.
The pot on the end of the cable blew my mind.. thanks for posting this video! Gosh, I want a scope now...
Great video! TDR in network analyzers is super expensive, I like this method simply to get an estimate. Great job!
Thanks w2aew, that's really made transmission lines a whole lot clearer. And as smeuse wrote Having test gear is great, knowing how to use it effectively is better!
I have just brought a DSO-2090 USB PC-Oscilloscope, and will follow your tutorial step by step
Thanks mate.
I keep going back to your videos for information. Your doing us all a favor. Thanks, I've learned a lot from your videos.
A great explanation in a short time which cleared my mind from lot of misunderstanding about cable impedance. Thanks a lot. I will keep on watching your other videos.
Another excellent video! Thanks for keeping the tutorials flowing.
Excellent tutorial. Your videos are the most resourceful. Thank you for sharing your professional knowledge and experience. Immediately subscribed to your channel to continue to learn from your fantastic lectures.
Fantastic video. Perfect mix of theory and practical application. Thanks very much.
Thanks for the enlightening video. It helps immensely. Very practical and lucid.
I just realized that my question about testing other types of cables has already been answered down below. :)
Thanks again for all your vids. Keep up the good work.
What amazing practical way to determine the length and the impedance of a coax cable.
I just tried this in the real world, and it works. I found to get a more accurate measurement on smaller cables, first I connected a long length of coax of approx 20m, set cursors, then added the cable under test and measure the amount the 'step' has increased. As we tend to use metres in the UK not feet, the increase in nanoseconds just happens to be close to the number of cm of cable. i.e 100ns equates to 9.9m which is handy.
caveat - assuming both cables same impedance and velocity factor or 2/3
Keep these videos coming! Having test gear is great, knowing how to use it effectively is better!
Excellent vid. Great explanation and clear demonstration. S.I. units would be preferred by most students. Thank you very much.
nS and Ohms ARE SI units. It's only the feet that weren't, and that's trivial to either convert or understand.
Thanks Alan for your video. I used this technique to determine the velocity factor of some coax I used for a phasing harness for stacking two loop antennas. I referenced a link to your video in the description section of one of my videos: 2 Meter Halo Antenna Part 12 -- "Phasing Harness understanding & calculations"
An on screen comment is also show at 1:13 in the video.
73,
Mike N6TWW
Performing TDR Using a Pulse Gen and Scope was a crowning moment in my shop. I always considered a scope to be the most important and useful instrument in any electronics shop but TDR added another dimension. It seemed like breaking through a barrier of sorts. Or rather having a revelation of just how damn indispensable a scope is for any true ElectronHead! When seeing a reflected wave for the first time I damn near came in my pants! I was having a discussion with a generation Z (EE) that commented in one of the many "Don't Blow Up Your Scope" videos. He lamented that the scope's GND topology was a pain in the ass. He complained that they're not a Differential Input. He further lamented that using the algebraic ADD function of a multi-trace scope eats up available inputs. I told him that this generation is spoiled and they've never known a life without cell phones. A public telephone is something seen in old movies.
I have no such complaints about the input topology of a modern scope. Think about it... A 100 Mhz scope can accurately measure
I've even used this technique to measure speaker wire - see this video: th-cam.com/video/z6UJPqQYzNc/w-d-xo.html
@@stevejagger8602 I totally concur.😉
This video is the reason I got my Extra ham license! There is a question on the current test about adjusting impedance to get a perfect square wave. Thank you.
Wow, never thought of using an o-scope for this. so cool
Thank you. The impedence-matching pot was a nice touch.
I'm just learning these things as a returning ham after a 30yr lay off. So this was very useful, thank-you.
Beautifully explained. Well done and thank you.
Fantastic video, probably the most I've learned on youtube in 10 minutes.
Great way to determine the impedance of an unknown bit of coax.
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)
+Peter W. Meek I did a video on that too (measuring velocity factor), exactly as you've stated.
th-cam.com/video/TpIIftvQPFM/w-d-xo.html
N
Excellent video, it is much easier seeing it done on a video than reading about how to do it. Thanks.
Awesome video! I learned two more practical scope/function generator applications today! :D
Excellent! That is exactly the type of presentation that burns a valuable light bulb moment into my brain.
Thanks, Keep them coming!
brillant.... thanks... I will do these tricks here with a few coaxes for the sake of learning and trying to completelly understand the concept of impedance in a coax... For some reason It's quite a complex topic for my mind to understand... :/
I keep coming back to your videos; thanks for the education you give us. Please Alan, use metric 😀
Metric, smetric! I like stating things like velocity, km/hr? No, how about fl/fn? Furlongs per fortnight not kilometers per hour!
I tried a similar setup to yours and I used a 2V p-p square wave at 1.43MHz (700nS) with 50% duty cycle based on the settings on your scope screen (video 4:30). My 50 Ohm RG58 coax cable measured about 7 inches shorter than the calculation which is pretty good. I found that if you measure the single pulse when the end of the cable is shorted (video 9:30) it is possible to get an estimate of the cable length as well, not the same figures though.
Fascinating. Inspiring. Insightful.
Thank you for this video.
Hi Al .. Mentioned you in my video .. referenced this video .. My video was like an supplemental to yours. I was just showing that the energy coming back down the coax was truly reflected energy and a way to display that on the o'scope. Was just something to do .. No "reflection" intended LOL Thanks for all the nice videos
Nice video. Really helped me a lot!
I once heard an analogy somewhere regarding reflections in transmission lines. It had to do with the fact that the signal was traveling through a medium with certain properties and then it hits a medium with different properties. It's like when you look out the window sometimes and you can see what's outside but at the same time you kind of see your reflection in the glass. Some of the light coming from inside the house is traveling through air and all of a sudden it hits the glass (different medium) and it doesn't like that so it bounces back. The same can be said about sound waves when an echo occurs. It all boils down to the fact that signals of whatever kind don't like changes in properties of the medium. Another example could be a prism which divides the spectrum of light (that is a refraction actually but this also applies to signals in transmission lines i.e. they are attenuated by a factor related to the refraction angle I think).
VERY NICE EXPLANATION!!!! I was searching around trying to find a good VISUAL aid for reflected power and impedance for my little guy to understand... I cant wait to demo this for him! (a young ham to be) 73! Ke8cu
Respect... Thanks for putting things together and sharing.
I'm amazed with ths explanation!
👏🏻👏🏻👏🏻👏🏻👏🏻👏🏻👏🏻
Awesome video! I had never heard of this technique before, and didn't even know it was possible. The calculations went by a bit too fast for me, but I'll watch it again and see if I can catch if better. A question: This method should be possible not only with coax, but with other cables as well, like stereo cables (RCA) for example, correct?
Thanks for this video. I have made a spreadsheet to calculate length based on time and velocity factor, and velocity factor based on time and lenght. Is there a formula to calcualate the cables characteristic impedance based on these measurements?
This was really nice, thanks for sharing.
Thank you so much for this, I see my scopes in a whole new light now!
Alan thanks for the explanation!!!
thank you for this simple impedance measuring method!!!
Hi, is it possible to do something like this but with fiber optics lines? so what kind of set up could be used instead of using an OTDR?
That is very helpful for me!!!! Thank you very much!
Thanks, Since you get the amp currents how do you calculate the wire gauge? Is there a Chart that tells you the wire gauge for the current amps?
I just traded a battery charger for a BK Precision 2120.Thanks for the video(S) all of them.You are a good teacher and I'll check out all of the them.I work in a diesel repair shop,and I'm hoping to start testing the electronic injector systems.Do you have anything along those lines? Thanks,Skip J.
Super! Can't wait to try it. Keep it up.
Well explained, thank you.
Very nice demonstration. Thanks.
Any book recommendations for understanding reflection etc in the cables? It's a bit beyond me.
Thanks. The lower the AC power cord capacitance means the capacitance from the shielding braided wire to the White ( Neutral ) and Black ( Hot ) wires? or the capacitance value of the wire insulation that you strip off to see the strained wire?
Excellent explanation! I made my interns watch this one 👍🏻
Wow amazing. Thanks for the video :)
Fantastic tutorial!
Excellent work!
Great video!
I also noticed that the cable between the generator and the scope needs to be the same impendance as the cable under test (50 Ohm) as well to get a clean signal. I repeated the setup using 75 Ohm cable and it worked out similarly. Need to learn more about the internal terminating 50Ohm / 1MOhm (High Z) settings on the scope and generator.
Eugene Khutoryanski has a good animation on his channel that gives a good hand in gaining an intuitive understanding of transmission lines.
:o)
Is it possible to adapt a 75 Ohms cable to a 50Ohms oscilloscope output without distortion?
With the TDR I found Iwas using different resistance cables.
JP
Thank you, great explanation
Very good presentation.
Excellent video, thankyou!
Wow, that made so much sense!
Awesome video! Thanks a lot!
Imperial metrics looks interesting :D .. Great Video
I am struggling to get this to work for me!!
I suspect its the signal generator I'm using ( an old Thandar TG102 unit )as I have measured the rise time on the square wave signal of 100khz as 60-65nS.
Is this too slow for this TDR purpose?
hi. i still have more problems understanding the impedance. really i will have to measure the impedance in earthing system in order to improve electrode performance.. will you advice me or give an example regarding that?? kind regards
What an interesting video! I wondered how can one measure the impedance of a speaker with scope?
w2aew much respect and many thanks for your very competent videos.
Youre filling an important gap of knowledge here which is highly appreciated.
73 oh8xat
I had a rabbit bite through the outer shield of my Wilson cellular antenna in a few places (RG58 cable) but the inner conductor is fine. Is there a simple way to test the cable to see if repairing the bitten portion is justifiable? I don't want to damage the cell phone with any trash coming back from the punctured parts of the shield.
What are the main differences between this test and what a fancy TDR can do?
Can u tell, how to measure attenuation loss of copper cable by TDR
Excellent Video
Awesome video. I guess this is poor man's time domain reflectometry? How constant is VF in a practical sense? Is it useful that you can measure that this way as well?
The imperial units though *shudder*
So what are the differences between this test and what a TDR can do?
Your videos are superb. I think perhaps you should dump the imperial measurements - life is much easier when it's metric!
Do you know what material they use for the outer jacket to be water resistant, because AC cord says its water resistant but the white wire, black wire, green wire sleeves jackets look the same but the outer jacket has some material to protect it from water
Just what I need thank you!
Thanks for the info. Why I was trying to do the reverse....who knows. Many thanks kind regards Spence
Very nice video. Better than guys with toys fumbling with the knobs. Could use an 8112 pulse gen with a very narrow, fast rise time too? Makes for a TDR setup.
Thank you for the Oscilloscope videos. Is there a way to use an Oscilloscope to look at my audio coming into my Ham FM repeater. I'd like to improve the output of the audio coming out of the repeater.
You can look at the audio, but you won't really be able to tell much about its quality (unless it is severely distorted). The problem may be with the deviation settings in the transmitter, which you won't be able to see on a scope.
Is the graticule really that ugly, or did the camera add in that bright red box around the perimeter of the screen due to lighting?
Great video, thanks!
Just dropping by 7 years later.