There are synchronous free space optical gigabit ethernet links available, it shouldn't take too much to modify one for White Rabbit. Bruce
On Saturday, 30 April 2016 10:13 PM, Magnus Danielson <mag...@rubidium.dyndns.org> wrote: Hi, On 04/29/2016 11:45 PM, Michael Wouters wrote: > On Sat, Apr 30, 2016 at 6:14 AM, Magnus Danielson > <mag...@rubidium.dyndns.org> wrote: >> Well, giving the conditions mentioned, doing ranging codes such as those >> used by GPS is very easy and cheap. Doing this in bidirectional isn't too >> hard. Doing a suitably high chip-rate should cost very little. > > I've done two-way time-transfer over optical fibre using exactly this > technique. The TDEV is about 1 ps for tau>1s. Not so cheap, about 25K > euro per node (20K signal processing - NI FPGA, 2K laser and power > supplies, 1K detector, 1K RF electronics) in my setup, but that cost > could be greatly reduced since a $100 OEM FPGA could do the signal > processing (I've already done work on this but currently looking for > motivation to finish it off) and a simple, intensity-modulated laser > would probably be fine. A 2K euro budget would be a challenge though. FPGA-wise, you need a very little FPGA resources. If you consider the RedPitaya (200 USD) for instance, it is way beyond what is needed. >> The two-way time-transfer is relatively easy, but you will need to do some >> calibration to get the precision needed. >> > > At first glance, I would think that you should be able to define the > optical RX/TX path to within 10 cm without any trouble and that gives > you 300 ps accuracy. Even on fibre links, I don't think anyone would > claim an accuracy of better than a few hundred ps. With a bit of calibration you can remove each nodes systematic asymmetry. For optical fibers many does not even bother to do a pseudorandom rangning. A repeating pattern suffice, such as that of SDH frames. Cheers, Magnus _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.