Many of the Chinese E-bay GPSDO's are frequency-locked, not phase-locked. I believe yours in question, is frequency locked.
The seller's description certainly shows that his only goal was frequency and not phase. Sometimes I joke to Tom that maybe we should call this list "phase nuts" and not "time nuts" :-) Tim N3QE On Wed, Feb 5, 2020 at 10:07 AM 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.
