Transmission line reflection mystery - Page 1
Forum section: electronics / beginners

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 05, 2012, 12:57:59 am
content: Hi,Thanks to advice from this forum, my pulse generator is working and I've been looking at pulse reflections in transmission lines. (The idea to try this came from watching w2aew's YouTube video on this subject.)As you'd expect, when I send a pulse down a piece of unterminated coax, I'm getting a reflection back a short time later. (I'm splitting the signal between the scope and the coax at the BNC input of the scope.) The time between the pulse and its reflection is 40.8ns. The coax measures 15.5 feet, so I suppose this piece of RG11/U has a velocity of propagation of 77%.My question is about the subsequent reflections, which, since they are all 40.8ns apart, must be echos of the pulse between the two ends of the coax. Although the first reflection is positive (same as the original pulse), the second reflection is negative. Then, the third is positive, the fourth negative, and so on. You can see this in the attached image.What I can't understand is why the even-ordered reflections are negative. If reflection itself inverted the signal, then the first one should be negative, and the second one should be positive. But it seems that the first reflection is special--it doesn't invert the pulse--but that bothers me and makes me believe that I am missing something very basic.Can someone who understands this clue me in?Thanks!- Ken

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: July 05, 2012, 01:38:34 am
content: Interesting question. I wish I had the answer.for velocity I got this figure.RG-11/U Foam HDPE (Beld. 9292) 84.0So it seems that as you say each pulse is very roughly a 30ft round trip, so all trips must be reflections of the previous reflection.What does the input look like? Is it a step or a pulse?How is the split done what are the lengths? Is your scope effectively high impedance if it is in the middle? Otherwise this will have an effect.

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 05, 2012, 02:10:48 am
content: Quote from: HackedFridgeMagnet on July 05, 2012, 01:38:34 amThe input is a pulse, with a width of around 10ns. The pulses were generated at around 50kHz, so what you are seeing here are definitely the artifacts associated with a single pulse, and not other pulses coming from the pulse generator. If I remove the coax from the scenario, the single first pulse stands alone; all of the reflections disappear.The splitter is a "T" mounted on the BNC jack of the scope. Between it and the scope I have a 50-ohm pass-through terminator. I tried removing it, and of course levels increased, but when scaled back down the overall impression is the same. I'm connecting the pulse generator and the "T" splitter with 50-ohm coax about 6 feet long.

author: Rufus
author_rank: Super Contributor
timestamp: July 05, 2012, 02:30:44 am
content: Quote from: KenSchwarz on July 05, 2012, 12:57:59 amTermination impedances higher that the characteristic impedance of the cable produce a +ve reflection, termination impedances lower produce -ve reflections (try shorting the open end of the cable).Your source impedance is less than the cable impedance so the first +ve reflection from the open end of the cable gets reflected back from the driving end -ve and you see the still -ve reflection from the open end.If your source impedance was a perfect match for the cable you would only see one reflected pulse.The generator should be a 50 ohm source (and termination) you should not have another terminator at your measuring point. The trace is consistent with you having one making the impedance at that point 25 ohms, each reflection reduces in amplitude by about 50% because of that 25 ohm half impedance.

author: notsob
author_rank: Frequent Contributor
timestamp: July 05, 2012, 02:33:34 am
content: Watch this

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 05, 2012, 03:37:31 am
content: Quote from: Rufus on July 05, 2012, 02:30:44 amGot it! Mystery solved, thanks!- Ken

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 05, 2012, 03:39:05 am
content: Quote from: notsob on July 05, 2012, 02:33:34 amHadn't seen this one. And there are more from this same author that look useful. Thanks for your help!- Ken

author: Jeff1946
author_rank: Regular Contributor
timestamp: July 05, 2012, 08:38:21 pm
content: Reflections from a coax depend upon the termination. No termination, positive reflection. Shorted, negative termination. Ideal (50 ohms for RG-58) no reflection. A nice animation for this effect is shown for a mechanical system at the following web site:http://www.acs.psu.edu/drussell/Demos/reflect/reflect.htmlLots of other nice stuff here too, poke around some.

author: JuiceKing
author_rank: Regular Contributor
timestamp: July 05, 2012, 08:49:25 pm
content: Quote from: Jeff1946 on July 05, 2012, 08:38:21 pmThank you, that link added a lot to my understanding.- Ken