Question about replacing coax delay line wit thin film differential delay line - Page 1
Forum section: electronics / projects

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 03, 2015, 09:34:15 pm
content: I need to delay an analog signal by 8ns. It is a pulse with raise time from 1 to 9-ish ns. I used a piece of RG405 coax which turned out to be 1.9m long for the delay that I want. Almost 2m is is a lot of coax even if it is smaller diameter like RG405. So I looked around and I found these things (datasheet attached) - they called "thin film differential delay lines", I got two GL2L5MS400D. When I get them I am not sure what is the correct way of using them. I spent an evening trying to find any examples but I ended up getting more questions and feeling kind of dumb now.I understand that coax line or transmission line can be modeled with lump elements which are series of inductors with capacitors and this is how, I assume these differential delay lines are implemented. Datasheet is not very helpful - equivalent circuit seems to be missing the capacitors and sort of make sense suggesting that pins 1,3, ... are taps with 500ps step but does not say that explicitly.Have anyone used these before?Am I wrong by trying to use this instead of coax?What do I do with unconnected pins? Terminate? With what - 50ohm every one of them?

author: TimFox
author_rank: Super Contributor
timestamp: July 03, 2015, 10:22:21 pm
content: According to the data sheet, the time delay is fixed. The parts are available with delays from 0.1 ns to 3.0 ns in steps of 0.1 ns and 3.5 to 4.5 ns in steps of 0.5 ns. The "400" in your part number means 4.0 ns delay.Note that the part is differential: designed to go between push-pull outputs and inputs (such as ECL). It is equivalent to two matched lengths of coax. They also make a single-ended version (starts with GL1L).The pins 1,3,4, etc. are multiple ground pins for each of two 50 ohm delay lines, and each line requires a 50 ohm termination (pin 7 and pin 10).

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 04, 2015, 12:32:49 am
content: Thanks I think now I got it. I assume that these delay lines are analog so the fact that they are designed for PECL circuits just suggests that they are fast and good for 5V circuits. I have -5V negative pulses and it seems to work OK after I grounded all the pins like on schematics below. I guess missing capacitors on "equivalent" schematics is what confused me. So to adjust the delay I need to ground pins 1, 3 ... or leave them not connected.Doh ... how hard was it to give some clear examples and explain it in a datasheet. Now I wonder if positions of unconnected peens matter? I assume that unconnected ones should be last.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2015, 01:58:23 am
content: Datasheet is incompletely specified. No spec on common or normal mode impedance. So you can't drive it single ended and expect clean performance, you'll get reflections.I'd also like to see a loss vs. freq. curve (and maybe xx dB of dispersion if applicable?) too.Doesn't matter anyway, they're "Call" at DigikeyWhat are you doing that needs a small delay? What bandwidth? Can you do it with a PLL or LC chain? Have you looked at these, why not?...Tim

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 04, 2015, 03:01:30 am
content: I am processing an analog pulse from Avalanche Photo diode so I need a delay line in range of 3 to 8, maybe 9 ns and signal has to stay close to original or be scaled version of it amplitude wise - it is going into a comparator - in my case it is AD96687. It is all a part of Constant Fraction Discriminator for laser range meter. I am looking for an alternative to 1.9 meters of coax - as I mentioned before 2 meters of even smaller diameter RG405 makes quite a big roll! LC chain should work too but it hard to make one with discrete elements, I think. Isn't what GL2L5MS400D is basically?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2015, 03:39:59 am
content: Some idea,http://sprott.physics.wisc.edu/technote/plp/plp354.pdfSeems like you'd need around 30 stages (say 10nH and 1pF each). It might not be too bad if the inductors are planar (thin traces between caps), but it's still a lot of parts. Better than 2m of coax though.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: July 04, 2015, 09:46:29 am
content: Quote from: Alexei.Polkhanov on July 04, 2015, 03:01:30 amCould you put the comparator right next to the APD, and simply transmit the digitised version? That way the APD's output wouldn't be filtered by any delay line.If you could put a time-to-digital converter at that end, you would get the best achievable time resolution and only need a relatively simple digital link.Alternatively, if you are comparing the time of arrival of APD pulse relative to a digital clock, would it be possible to delay the clock signal instead? ICs used in ATE give stable variable digital delay with steps in the ps range and delays in the ns region.