Are these permanent installations or portable? If portable, how quickly do they need to lock up to within sub-nanoseconds?
In a permanent (or semi-permanent) installation, it's hard to beat GPS-steered cesium, with a loop-tau of DAYS to eliminate all the GPS jitter, ionosphere effects, etc. -RL ------------------------------------------------------------ Robert Lutwak, Senior Scientist Symmetricom - Technology Realization Center 34 Tozer Rd. Beverly, MA 01915 (978) 232-1461 Voice [EMAIL PROTECTED] (Business) (978) 927-4099 FAX [EMAIL PROTECTED] (Personal) (339) 927-7896 Mobile ----- Original Message ----- From: "Stephan Sandenbergh" <[EMAIL PROTECTED]> To: <time-nuts@febo.com> Sent: Tuesday, July 04, 2006 8:49 AM Subject: [time-nuts] Dithering vs. locking all the clocks to the OCXO? > Hi All, > > Earlier, I explained that my application require very good relative > stability between various GPSDOs. > > A rough estimate of my requirements is: > > -Baselines of 100s of meters to 10s of kilometres. > > -Sub-nanosecond relative stability (this I forgot to mention earlier - > thanks to TvB for reminding me). > -Time scales of maybe 100s of seconds to 10s of minutes. > > -The lower limit on my stability requirement is maybe the 200ps of jitter > that the FPGA will add to the processed data. > > My question is this: > > At this stage I'm not sure what all the various causes are for the error > in > the 1PPS output of a GPS receiver. (I sure I will find the answer to this > in > all that references TvB and Magnus gave me). > > However, a quick guess would be the delay caused by atmospheric effects (I > don't think thermal noise would play a big role since the antenna is > looking > straight up) Also, there will be errors higher up in the food chain, such > as > changes in satellite orbits etc. I guess these errors are fairly > systematic. > Lower down in the food chain, I presume the M12+T adds further errors to > the > signal (viz. the antenna, LNA, TCXO jitter, etc). I presume these errors > would be on faster time scales, smaller and much more stochastic in > nature. > > If the resolution of my phase comparator is about 100ps, and keeping in > mind > that I want relative stability, wouldn't it make sense to lock the M12+T's > on-board TCXO to the OCXO (probably not straight forward to do)? I realise > that one would lose the advantage of any dithering effect which would > quickly average any zero mean effect. I guess this will depend on the > nature > of the errors introduced due to clock jitter: Is it Gaussian and zero > mean? > I guess one will have to investigate what happens at down-conversion to IF > etc. And, that ultimately it will depend on the size and nature of noise > caused by the TCXO. > > If one could closely follow the drifts in atmospheric effects (which would > be the same for short baselines) one will have very good relative > stability. > > Regards, > > Stephan > > > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts