Hi More or less what the seller is doing is a simple FLL. It appears he does not have an integrator on it so it will hit an ADEV floor and never get any better than that.
Bob > On Feb 4, 2020, at 7:23 PM, AC0XU (Jim) <[email protected]> wrote: > > Time Nuts- > > I recently purchased (Ebay) a GPSDO from an overseas manufacturer. This is a > new item and the Ebay description seemed quite encouraging. Having had one in > the lab for several weeks what I observe is that: > > 1) ADEV for short to medium durations (Tau <1,000 sec) is comparable to or > better than other GPSDOs in my lab. Not as good as the best undisciplined > oscillators in my collection, as expected, but no problem here. > > 2) ADEV for longer durations (Tau>10,000 sec) is considerably worse than > other GPSDOs and Cesiums in my lab. I measure well over 1E-11 at tau=100,000 > sec for the new unit. This is my problem with it. As far as I can see, it > behaves like it is not actually locked to GPS. > > I queried the vendor about the apparent lack of synchronization with GPS and > this was his response. > > "I understand everything. When constructing this device, I was guided by the > need to get a good 100 MHz and 10 MHz signal, which will be maintained with > an accuracy better than 0.1 ppb throughout its life. I used the GPS signal to > correct the frequency of the main OCXO as the elements ages. I tried to > ensure that the frequent loss of GPS signal and its quality did not affect > the parameters of OCXO itself. For this reason, the adaptive algorithm is > very delicate in the final phase. > The frequency correction program causes it to stop regulating OCXO in some > cases. > This happens when: > > 1. OCXO will achieve accuracy of 0.01 ppb. > 2. No GPS signal. > > The control system is temporarily frozen when the frequency accuracy of 0.01 > ppb is reached. Therefore, ADEV for T> 1000 seconds resembles the typical > behavior of OCXO during free operation. These parameters do not change when > the GPS signal appears suddenly after a long absence. When constructing the > device, it was assumed that all stability will be dominated by OCXO, and a > value better than 0.01 ppb is not needed in telecommunications applications. > I would add that the value of 1E-13 has never been a goal to achieve, > especially due to the additional costs. Maybe if I gave up the 100 MHz > section, I could do it for the price." > > It surprised and dismayed me that large-tau behavior is so poor in a GPSDO. > The several other GPSDOs in my lab all have ADEV around 1E-13 at 100,000 secs > (based on a mass assessment of the GPSDOs and cesium oscillators). It makes > me wonder what the point was in adding the GPS receiver to this device. Is > there something that I am not understanding? > > Thanks in advance for your comments - Jim > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
