[time-nuts] DOCXO vs. Rubidium medium-term stability

[WB6BNQ]

Dave,

Something is not making sense to me here.  As GPS is generally available around
the globe and obviously to your reference stations; how is it that the mobile
will be able to find an area where the GPS is not available ?

As to the mobile, if it is not going to utilize the GPS for a reference, you then
need to determine the worst case error you can have over the course of time that
the mobile is away from its GPS capability.  That factor will dictate the kind of
on-board reference you will need.  It could be that a very good quality crystal
oscillator will suffice.

Bill....WB6BNQ


Dave Jabson wrote:

> Greetings,
>
> I just discovered this mailing list, this is my first submission. Glad to
> find a group of folks who are into this kind of stuff!
>
> I'm working on an data acquisition application for my company that will
> require a very stable oscillator. Without going into too many specifics,
> there will be some reference stations spaced 100's of kilometers apart from
> each other and 1 mobile station that will be operated in an area where GPS
> is not available. I need to be able to collect data at all the stations and
> have the time synchronization be extremely good between the stations,
> including the one without GPS.
>
> For the reference stations it will be sufficient to have the timing of each
> one driven by a good GPSDO. Clocks will be sync'ed to UTC via the NMEA
> string and 1pps edge and all of the digital electronics will use the GPSDO
> 10MHz as their timebase. Periodic re-synchronization to the 1pps edge can be
> done as needed.
>
> The mobile station can be synchronized to GPS initially to synchronize its
> clock as described above but will then have to rely on a free-running
> oscillator. The stability of this oscillator will dictate how much drift the
> mobile station's clock will experience relative to the reference stations.
> Keeping this drift as low as possible is my goal.
>
> I had assumed that a Rubidium oscillator would give me the best stability
> over the course of 8-12 hours. Obviously a Cesium would be better but those
> are impractical due to cost and power constraints. I've begun evaluation of
> a Rb oscillator but now I'm being told by some people that a good DOCXO is
> likely to give me similar medium term stability (with obviously better
> short-term stability).
>
> Anyone here have thoughts on this? Obviously I'd rather go with a DOCXO for
> cost and power reasons if there's no performance benefit to be had using a
> Rb osc. The Rb unit I'm testing (http://www.thinksrs.com/products/PRS10.htm)
> seems to perform well but I am interested in hearing others' thoughts.
>
> Thanks,
>
> Dave
>
> ----------------------
>
> Dave Jabson
> Systems Engineering Manager
> Quasar Federal Systems
> 5754 Pacific Center Blvd, Suite 203
> San Diego, CA 92121
> 858-412-1706
> www.quasarfs.com
>
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