Hi Actually it’s a bit worse than you might expect.
The uncorrected sawtooth will give you about 20 ns of wander. At the one day level, GPS without some sort of ionosphere help (like a dual frequency receiver) will add another 10 ns or so to that. Net, your pps is spread over a 30 ns range. The output of the OCXO is at 5 MHz, the Rb is at 10 MHz. Maybe you double the OCXO to 10 MHz. It only has a zero crossing every 100 ns. (200 ns if you don’t double it). You will have a 100 ns “dead zone” in your counter. That assumes it’s synchronous. If it’s a ripple counter, who knows what it will do. Net result, You have 30 ns of error, and a 100 ns resolution. Net is 130 ns. You will hit 1x10^-9 at a bit over 100 seconds. You will get to 1x10^-12 at around 13,000 seconds. Since it’s a dead zone, averaging really does not help you much. In fact, long averages will mess up the ADEV computations. If you have a goal that resolution should be 5X your data, then you get to 1x10^-12 data at around 80,000 seconds. For a good ADEV number, you would like about 100 samples. This gets you out to a 100 day run. Even for a minimalist number, you are running for > 10 days. Any time you have a power interruption, the process re-starts. With things like 5335’s running around for cheap prices, I would suggest doing this with a counter. You are going to spend a lot of days getting very much data. Your time’s got to be worth something …. Bob > On Jan 12, 2015, at 9:10 AM, Attila Kinali <[email protected]> wrote: > > Ciao Andrea, > > On Mon, 12 Jan 2015 11:59:26 +0100 > Andrea Baldoni <[email protected]> wrote: > >> The sampling interval could come from a (long time based on a) sawtooth >> uncorrected PPS from a cheap GPS, a sawtooth corrected from a good one >> (perhaps >> the Lucent GPSDO), or a computer using NTP. > > The GNSS Timing AppNote for the LEA6-T receiver[1] will give you an idea > what jitter you get with GPS. Please be aware that these measurements > were done with an antenna located at a _good_ position (ontop of a 4 story > building with no other high buildings around). Unless you have a simlarly > good location you will have worse performance. > > Said Jackson reported some time ago that he got around 1us of jitter for > a GPS receiver (i presume it was either a LEA5-T or a LEA6-T) behind a > window. After he averaged the position for a long time (several days) > manually and stored that in the receiver he got much better performance > (sorry i cannot find the mail at the moment, you have to look for it in > the archives yourself). > > NTP will give you a jitter in the range of 1-100ms, depending on > your internet connection and its conguestion. On a local network > based NTP system, you can expect jitter in the range of 10-100us IIRC. > > >> Each of these sources should reach a goal stability (say, 1 part in 10^13) >> after averaging them on a different (and very high I suppose) number of >> seconds (averaging them for an infinity number of seconds should give the >> stability of the underlying reference clock, but I'm willing to stop >> sooner...). >> I know there's no reason to go 1E-13 when the Milliren couldn't go that far, >> but the DUT may be also something else like a FE-5680A). > > To get to 1e-13 with GPS (assuming 1-10ns jitter) you need somewhere around > 10k to 100k seconds. At these time scales, the temperature dependent deviation > of your OCXO is likely to dominate your measurement. I would rather do > a two step measurement. If you have a FE-5680A measure its drift with a > tau in the 100ks-200ks region. Then use the FE-5680A as refrence to measure > the drift of the OCXO in 10s-1000s timescales. If you do both continuously, > you can apply some math and get out pretty good numbers (see three cornered > hat method) > > Additional to GPS jitter you also have the deviation of GPS time in > respect to TAI/UTC. This has been in recent years below 5ns (GPS vs > UTC(USNO)). > But because GPS time is steered to be close to UTC it will oscillate slightly > around it. How much, i do not know. (But then the deviation between the > different UTC realizations is larger) [2] > >> The sawtooth uncorrected GPS receiver may never yeld a good stability in the >> short term, but in the long one it should as well because the internal clock >> jitter would average results. > > It would average out if and only if the sawtooth correction would be > completely > independent of anything else. But it isn't. This results in effects where the > cycle to cycle jitter is quite low, but there is a large offset in the > sawtooth > correction. This is know as "hanging bridges" in the GNSS world. > > >> By the way, the stability of the TAI is known or, because it's >> the reference one, it has zero deviation for definition (so you can reach >> its ultimate stability through GPS really only at the infinity...)? > > There is an uncertainty number attached to TAI, but i dont know any numbers > from the top of my head. I'm sure it is mentioned in the BIPM report > somewhere. > > > Attila Kinali > > > [1] > http://www.u-blox.com/images/downloads/Product_Docs/Timing_AppNote_%28GPS.G6-X-11007%29.pdf > > [2] "GPS time and its steering to UTC(USNO)", presentatin by Edward Powers, > 2009 > http://www.gps.gov/multimedia/presentations/2009/09/ICG/powers.pdf > > -- > It is upon moral qualities that a society is ultimately founded. All > the prosperity and technological sophistication in the world is of no > use without that foundation. > -- Miss Matheson, The Diamond Age, Neil Stephenson > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
