[time-nuts] GPSDO that isn't GPS locked for large time scales?

[Tim Shoppa]

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) <James.Schatzman at ac0xu.com>
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 -- time-nuts at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>