Path: csiph.com!v102.xanadu-bbs.net!xanadu-bbs.net!us.feeder.erje.net!feeder.erje.net!eu.feeder.erje.net!newsfeed.fsmpi.rwth-aachen.de!news-1.dfn.de!news.dfn.de!fu-berlin.de!uni-berlin.de!individual.net!not-for-mail From: Paul E Bennett Newsgroups: comp.arch.embedded,sci.electronics.design,comp.realtime Subject: Re: Comparing phase of physically distant signals Followup-To: comp.arch.embedded Date: Fri, 16 Aug 2013 21:07:47 +0100 Organization: Hidecs Consultancy Lines: 54 Message-ID: References: Mime-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7Bit X-Trace: individual.net kgCwTkyUPZ6Fi1Fzv+BouwDEUXkauXdzoPP6+E+W40gpVLKfrQ Cancel-Lock: sha1:RqyF0dPMZp7kqfmR0zGMtPy072M= User-Agent: KNode/4.10.2 Xref: csiph.com comp.arch.embedded:13121 comp.realtime:345 Don Y wrote: > Hi, > > [posted to S.E.D in the hope that a hardware analog might exist] > > I synchronize the "clocks" on physically distributed processors > such that two or more different machines can have a very finely > defined sense of "synchronized time" between themselves. > > During development, I would measure this time skew (among other > factors) by locating these devices side-by-side on a workbench > interconnected by "unquantified" cable. Then, measuring the > time difference between to "pulse outputs" that I artificially > generate on each board. > > So, I could introduce a disturbance to the system and watch to > see how quickly -- and accurately -- the "clocks" (think FLL and > PLL) come back into sync. > > How do I practically do this when the devices are *deployed* > and physically distant (vs. "electrically distant" as in my > test case)? > > Two ideas come to mind: > 1) two equal length cables to connect the "pulse outputs" > from their respective originating devices to the test gear. > 2) two *radios* to do the same thing -- after accounting > for different flight times > > [Though I wonder how hard it is to qualify two different > radios to have the same delay, etc. Far easier to trim > two long lengths of wire to the same length!] > > Of course, I would like to minimize the recurring cost of > any solution as it is just present for system qualification > (and troubleshooting) and offers no run-time advantage to > the design. You might find this of interest and useful if you had a couple of fibres in your installed bundle. I was chatting to one of the guys visiting JET from CERN today and he stated that the timing accuracy is 1ps on phase and 1ns on time synchronicity. -- ******************************************************************** Paul E. Bennett IEng MIET..... Forth based HIDECS Consultancy............. Mob: +44 (0)7811-639972 Tel: +44 (0)1235-510979 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************