WarrenS wrote: > I am having trouble keeping my responses from starting new threads, > If anyone could shed some light on what I'm doing wrong it would be a > great help to me. It will be interesting to see what thread this post ends up > in. > > (Now back to the original subject; I've cleaned up things and added my > responses in the text below) > > > Re: [time-nuts] GPSDO using 100Hz > from Tom Van Baak > Mon, 24 Nov 2008 17:17:34 -0800 > > > by WarrenS > >>> Besides asking if anyone is using the 100Hz output, >>> I would like to know why it is that the generally available >>> GPSDO don't use the 100Hz. I have found that this can >>> give about 1 ns of certainly with a simple PLL and analog >>> RC filter, whereas the 1 PPS has more like 100 ns of uncorrected >>> uncertainty in it, and for the most part the 1PPS needs a processor >>> to use it instead of simple Phase Lock Loops that I am able to >>> be used on the 100 PPS >>> > > by /tvb > >> Where do you get your "1 ns" and "100 ns" figures? >> Have you actually measured these values? Also, >> over what time average (tau) are you assuming >> this level of resolution? >> > > > by WarrenS > The 1ns and 100ns are measured peak values of the > nominal phase Jitter over a few seconds time as > displayed on a scope and averaged from a time interval meter. > The averaged time has to be long enough to let the GPS Phase > noise cycle thru a few of the Oncore's sawtooth cycles, > and short enough so as not to include the added received GSP > signal errors. > The GPS signal errors that I see are around 10 ns over a 10 plus > seconds period, with short excursions of an additional 30 ns > every few minutes generally lasting no longer more than a few seconds. > These Phase noise values I see are pretty much independent of time, > as long as the viewed average time is not too long or too short. > > There is also another nominal 50 ns phase error that comes along maybe > every hour on the 1 PPS and once or twice a day on the 100 PPS both > lasting from 1 to 10 minutes or so. This happens when the various > different frequencies involved sync up for a while. > > > by WarrenS > >>> On the LeapSecond.com site >>> http://www.leapsecond.com/pages/gpsdo/ >>> It would seem that much of the noise what they have >>> plotted is the results of NOT setting the GPSDO tracking >>> TC slow enough. The GPS is adding noise, to the low noise >>> oscillator. >>> This is why the Allan Deviation increases by more than 3 db in >>> the tracking mode at mid averaging times below a 1 >>> 1000 seconds, (15 minutes) >>> For a low noise Rubidium Oscillator like the SRS PRS10 >>> > > >>> Bruce early said, "where the ADEV for various GPSDO remains >>> below 1E-11 over the Tau range of [0.1s, 100,000s]. >>> I said: That Tau has nothing to do with the tracking time >>> constant that the GPSDO has been is set to, which is usually >>> recommended to be set for a TC of several hours for optimal results. >>> > ( for low noise Rubidium Oscillators) > > from /tvb > >> For ideal results start with the TC = the tau where the ADEV >> of the GPS engine & phase detector crosses the ADEV of the >> OCXO. However there are other practical considerations. >> > > >> At short averaging intervals when there is a phase difference >> it is easy for the GPSDO to assume the OCXO is stable and >> the GPS engine has noise. So you average more samples. >> > > >> At long averaging intervals when there is a phase difference >> is it easy for the GPSDO to assume the GPS engine is more >> correct and the OCXO has drifted. So you steer using EFC. >> > > >> At the tau of the ideal TC, the GPSDO sees an average phase >> difference but can't totally blame either the OCXO or GPS for >> the error. By definition at this tau, half the noise is due to each >> subsystem. Hence you nearly always see an ADEV hump that >> goes above where you'd really like it. Best case sqrt(2). >> > > >> The ADEV hump itself is not indication of a mistuned TC; it is >> an indication that the GPSDO is working correctly. >> > > >> You can move the hump left and right and distort its shape by >> changing the TC. >> > > >> In practice I suspect most commercial GPSDO have a TC that >> appears too low for your liking. I can discuss why if you wish. >> /tvb >> > > from WarrenS > Thanks, Very Good information to know and remember, and It > gives me a much better understanding of what is happing. > I see that in many of the graphs shown on the Leap Second plots, > the tracking hump is more like 4 to one instead of 1.5 to one. > Is this because the tracking TC is too fast for best noise? > I would love to hear more about it, Please 'Discuss why'. > > Also if you would, I'd like to have a better understand of > what seems like an over obsessions with Low Noise GPSDO. > I do understand the need (or at least the desire) > to have low noise oscillators when using them directly for high > frequency and/or short time scale data taking applications, > and the need for a good stable oscillator where it must maintain > frequency if the GPS signal is lost, > BUT it sounds like this is not what a large percentage of the > second hand GPS trackers discussed here are used for. > It does not seem like it would matter what the noise of the internal > OSC is, if the unit is only being used for averaging things over longer > periods such as looking at phase drift over time when doing freq checks. > Any data taken at a period of say 1000 second to several days is > pretty much limited by the noise of the GPS and not the Oscillator, > so why is there the desire for the low noise oscillators in these cases? > > Warren > ***************** > Often the use of a low noise OCXO is mandated by unstated applications such as:
1) The OCXO output is used to drive the external frequency standard input of a counter. 2) The OCXO output is used calibrate other oscillators to 1E-9 or perhaps a little better and averaging over 1000 seconds or so during adjustments isnt a sensible option. 3) The OCXO output may be used to control the frequency of a microwave transmitter and/or the LO of a receiver which requires low drift and phase noise. 4) Obsession Bruce _______________________________________________ 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.