Thanks for the response!. Somehow I figured there would be an "it depends" in there, such is life! It does sound promising though.
Some of the stuff is easy to find. Shields like to break near connectors, or if the cable passes through trees I can pretty much bet good money a squirrel has gotten my coax mixed up with his acorns. Never knew aluminum was so tasty.
The worst for me inevitably turns out to be a couple shotgun pellets shorting out the cable. Not exactly the easiest thing to find and they NEVER radiate when you're looking for them!
What would be the maximin rise time that would be preferred in a function generator for this?
(I don't own a fc or I would have tried to answer those questions myself!)
- Yes, you can determine the distance to a short (only the first short!). You can see this at the end of the video where I dialed the pot down to zero. You can measure the width of the pulse that's shown, and that will be the roundtrip delay from the scope to the short and back. Calculate the distance the same way as I did for the open.
For those that missed it, here is the video that WBB is referring to:
Use a scope to measure the length and impedance of coax
Ideally, you'd want the risetime to be a few times faster than the propagation delay through the cable of interest, so that you get a clean "plateau" between the incident edge and the reflected edge. For typical coax with a 0.66 velocity factor, the speed of a signal is 7.79 inches per nanosecond. Thus, a 10ns rising edge of the pulse will be spread out over about 6.5 feet of cable (i.e. the voltage 6.5 feet from the generator is just starting to move at the same time that the rising edge has finished moving at the generator). If the pulse's risetime is slower than the roundtrip delay, then the reflection will be seen during the edge, making it much harder to accurately measure the delay.
For basic coaxial work like was shown in the video - for 10, 20, 30 feet or more of cable, a 10ns risetime is good enough.
You might check the risetime of calibrator signal on your scope - it might be fast enough to use for this application.
IIRC is was AN47, but doesn't appear to be on the Linear web site anymore.
IIRC is was AN47, but doesn't appear to be on the Linear web site anymore.If I go to linear.com and search for AN47, the first link is a PDF of the correct appnote. The later circuit with a charge line, as described in LT AN79, 94 and 122, might be better, since it generates something approaching a step, as opposed to a semi-Dirac pulse.
Great tutorial video as always W2aew, thank you Sir !
This video inspires me to learn & tinker with this poor man TDR thingy, and since you did mention high speed rise signal as a reference in that video, I assume I can also use my vintage Tek 2901 time mark generator that generates those really sharp spikes for this purpose too right ?
Example of the time mark signals generated by that 2901 at various speed at my Tek 2465B.
That looks like a long length of coax!
Aw heck! I wrote down the 400ns in the above post from notes on another piece of Coax.
You're absolutely right, the return pulse is coming at 240ns.
That looks like a long length of coax!
I'm guessing it's about 50'... 400ns / 7.79ns per foot = 51.3
Another question, not that I haven't already veered this far enough off topic from oscilloscopes to function generators, sorry. Would it be possible to use the TTL signal for short distances and switch to the normal square wave type signal for longer distances? I really like the older equipment (both for budget as well as nostalgia) but looking up the specs for the older models isn't the fastest thing in the world. Better than it used to be though! Anyway, it seems pretty common to find TTL rise times of <50ns but the square wave rise times seem to hover around the 100ns range. So I thought the TTL signal would be decent for short distances and the normal square wave would work fine once cable length exceeded rise time delay. Or am I completely off base?
Thanks again!
Wow - impressive photos! You'll have to tell me how you get such nice photos of analog scope screens like that! No reflections of the camera, etc. Nice. Are you using some sort of a hood like the old polaroid style scope cameras?
Now, go grab some coax and let's see what those TDR reflections look like!
I've posted a few videos in the recent weeks, the most recent was today...