My Time Domain Reflectometer (a la w2aew) - Page 1
Forum section: electronics / projects

author: android
author_rank: Regular Contributor
timestamp: July 22, 2015, 02:15:44 pm
content: Thanks to w2aew's excellent YouTube videos on TDR, I've finally got around to building my own version.I was going to use through hole, but ended up using SMT parts so it looks pretty clean. I tested it with a 5m length of RG59 and measured a 41 ns round trip which seems about right (although by my calculation that would make the velocity factor about 0.81 ...I don't know whether that's normal or not).

author: AF6LJ
author_rank: Unknown
timestamp: July 22, 2015, 02:25:32 pm
content: Nice;I have been meaning to build one of those.

author: mtdoc
author_rank: Super Contributor
timestamp: July 22, 2015, 07:02:14 pm
content: Awesome! I like the googly eyes.I made one a while back - dead bug style. A fun and usefull project. Kudos to Alan!

author: tggzzz
author_rank: Super Contributor
timestamp: July 23, 2015, 12:16:05 am
content: I'm currently playing around with a homebrew TDR that can measure stub lengths with a resolution of ~40mm. Since it is dirt cheap (£40 IIRC) it does have some limitations, but might be useful nonetheless.I'm slowly documenting it, and will publish it in the not too distant future, unless higher priority interrupts occur.

author: android
author_rank: Regular Contributor
timestamp: July 23, 2015, 09:17:11 am
content: Quote from: mtdoc on July 22, 2015, 07:02:14 pm...best practice is to keep an eye on cable terminationsQuote from: tggzzz on July 23, 2015, 12:16:05 am...sounds good. I'd be interested to see the outcome. How do you get 40 mm resolution? That's about a 165 ps interval for my RG59 cable.While I was shopping for SN74C14 chips I stumbled across this interesting chip:LTC6991 TimerBlox: Resettable, LowFrequency Oscillator. It has a 1 ns rise time (compared to the 74C14's 4 ns - but I don't know how important that really is) and a really simple circuit. Only 20 mA drive current though.

author: tarantula
author_rank: Contributor
timestamp: July 27, 2015, 08:38:55 pm
content: Why not Jim Williams' avalanche pulse generator?http://www.jensign.com/avalanchepulsegenerator/index.html

author: w2aew
author_rank: Super Contributor
timestamp: July 27, 2015, 08:59:02 pm
content: Looks good guys - but you should really add a power supply filter cap (like 100nF or so) directly across the supply pins of the chip - especially since you don't have a ground plane....and it does cause me pain to see the trace shown on the Siglent scope, while the 300MHz Tek 2465 is still just below it, unpowered

author: tautech
author_rank: Super Contributor
timestamp: July 27, 2015, 09:06:57 pm
content: Quote from: w2aew on July 27, 2015, 08:59:02 pmMaybe it's

author: android
author_rank: Regular Contributor
timestamp: July 27, 2015, 09:22:36 pm
content: Quote from: tarantula on July 27, 2015, 08:38:55 pm...didn't know about it. Might give it a try though, thanks.Quote from: w2aew on July 27, 2015, 08:59:02 pm...it's there alright (1206 ceramic)....plus a larger 4.7uF ceramic too.Quote from: w2aew on July 27, 2015, 08:59:02 pm...I did test it on the Tek too but I don't really trust it because it's uncalibrated since I fixed it (dead attenuater and U800). Still has some strange behaviour but I've already spent too much on it to bother opening ì up again. At least I don't need to worry about the EEPROM fading...it doesn't have the type that causes all the pain.

author: Warhawk
author_rank: Frequent Contributor
timestamp: July 27, 2015, 09:43:33 pm
content: Quote from: android on July 27, 2015, 09:22:36 pmHi, I use my version of Jim's avalanche pulse generator and it's a great thing for TDR. You can even recognize reflections on all T adapters and so on. Very educational. Build yourself a one

author: tggzzz
author_rank: Super Contributor
timestamp: July 28, 2015, 04:02:27 pm
content: Quote from: tggzzz on July 23, 2015, 12:16:05 amFor more information, seehttps://entertaininghacks.wordpress.com/2015/07/27/poor-mans-homebrew-tdr-with-4cm-resolution-part-1/Here's a taster of its capabilities: a cable with a tee junction at 3m with an open stub. The two impulses are for a 19cm open-circuit stub and a 29sm open-circuit stub. The different lengths are clearly visible.

author: android
author_rank: Regular Contributor
timestamp: July 29, 2015, 08:54:10 am
content: Quote from: tggzzz on July 23, 2015, 12:16:05 amYou've lost me already...I know it must be really simple, but what is meant by "open-circuit stub"?I've just been measuring unterminated lengths of coax.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 29, 2015, 10:04:52 am
content: Quote from: android on July 29, 2015, 08:54:10 am"Open circuit Stub" is RF "slang" for sections of open circuit transmission line connected across the main line.---they are commonly used for filtering of unwanted signals.For instance,a section of open ended line connected in such a manner will look like a short circuit across the main line at the frequency at which it is a 1/4 wavelength.(normally,the unwanted frequency)To the wanted frequency,it will look like an open circuit,& will effectively be invisible to it.(you can have "short circuit stubs",too,& combinations of both)The TDR,being wideband,sees it as an impedance mismatch,& will indicate how far down the cable it is.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: July 29, 2015, 02:00:34 pm
content: Short Answer:Open circuit stub = unterminated lengths of coax. In the context of this thread, they are exactly the same.More Blah Blah Blah:In other contexts, an open circuit stubimpliesan unterminated length of coax whose electrical length is 1/4 wavelength (or multiple of that) of some frequency. A stub that is 1/4 wavelength at 7.2 MHz, for example, will greatly attenuate signal components at that frequency and also multiples of that frequency to a lesser degree when it is connected to a transmission line with a tee adapter.W2AEW has a bunch ofreally enlighteningYouTube videos that touch on the subject.Oops. As the OP, you are obviously already aware of W2AEW.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 30, 2015, 05:30:33 am
content: Quote from: Melt-O-Tronic on July 29, 2015, 02:00:34 pmOr any other type of transmission line.A "stub"usually implies an additional section of line connected across the main transmission line.Thus,a single piece of line attached to the TDR for the purpose of testing that line would not normally be termed a"stub".QuoteAgain"or any other type of transmission line"applies.Of course,anylength of line is a 1/4 wavelength atsomefrequency!QuoteOnly atoddmultiples!!At even multiples of the frequency it is 1/4 wavelength at,an open circuit "stub" will look like "just that"--an open circuit!!At other,unrelated frequencies,it will look like an impedance which is more reactive than anything else,tending towards XLor Xc,depending on its length

author: Gavin Melville
author_rank: Contributor
timestamp: August 02, 2015, 10:57:46 am
content: Once you get your head around it, a step (leading edge of longish pulse) works just as well as a short pulse, and much easier to make.