high speed time domain reflectometry - Page 1
Forum section: electronics / projects

author: qwaarjet
author_rank: Regular Contributor
timestamp: October 28, 2016, 02:48:09 am
content: I have an application where I want to measure the length of wave guide via TDR, cant talk about the why. The trick is I want to do this very fast(>10Khz) and record the length/ time for every pulse sent. My go to would be to use some National Instruments digitizer (pxie-5162?) and signal gen but is there a some COTS solution out there to do this kind of measurement and record it. A VNA can certainly do the measurement but are there any that can do it that kind of rate?

author: tggzzz
author_rank: Super Contributor
timestamp: October 28, 2016, 10:58:08 am
content: Quote from: qwaarjet on October 28, 2016, 02:48:09 amMore info would be helpful:min/max length?resolution?accuracy?dielectric or velocity factoris any averaging allowed?max allowable applied signals?any other signal present when measurement is being made?max rate of change of length?

author: qwaarjet
author_rank: Regular Contributor
timestamp: October 28, 2016, 12:33:37 pm
content: ~300m length<1nS resaccuracy <1%VF TBD ~.6no averageno practical limit to V levelno other signalunknown rate of change

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2016, 03:43:11 am
content: By the way, do you know the velocity factor as accurately as you want to know the length?Waveguide is dispersive, i.e., the velocity factor varies with frequency.Which also screws with the fundamentals of TDR -- you can only measure the length as accurately as the wavelet can be resolved. If it spreads out too much along the way, you can't tell very well anymore.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: October 29, 2016, 08:54:34 am
content: Quote from: qwaarjet on October 28, 2016, 12:33:37 pmSo, you want to be able to spot 0.3m and larger changes in length of a 300m waveguide (assuming the velocity factor is 1) and that can happen in 0.1ms.Strange waveguide.If you want/need a COTS solution, a short-range radar might be a starting point.(BTW, it is 1ns, not nS; 1nS is 1Gohm)

author: LaserSteve
author_rank: Super Contributor
timestamp: October 30, 2016, 09:09:15 pm
content: 300 meters sounds like a UHF TV tower???Sounds like You need a VNA with the ability to locate a discontinuity, not a TDR. They do exist,I watched one being used to locate a bullet hole in a TV transmitting site. Hunters like to try to shoot the anti-collision lights out. Which leads to bullet holes and water leaks in the large waveguide.STEVE

author: Someone
author_rank: Super Contributor
timestamp: October 30, 2016, 11:24:16 pm
content: Quote from: tggzzz on October 29, 2016, 08:54:34 amNext specification you discover is that its not a waveguide but a pipe of some sort with a liquid filling.... but must be done cheap!

author: vk6zgo
author_rank: Super Contributor
timestamp: October 31, 2016, 06:28:55 am
content: Quote from: LaserSteve on October 30, 2016, 09:09:15 pmThat would probably have been a coax line,not a waveguide---the HELIAX® versions of both look very similar.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 31, 2016, 03:58:27 pm
content: WR-1150 is NOT Heliax. For waveguide you use FDR instead of TDR.For the OP, take a look at this thesis:Nanosecond R-F pulses for Waveguide Fault-FindingShoemaker, William BruceMonterey, California. Naval Postgraduate SchoolSteve