[time-nuts] test mailing from a new member
magnus at rubidium.dyndns.org
Wed Jun 17 01:28:23 UTC 2009
David Bengtson skrev:
> On Tue, Jun 16, 2009 at 6:07 PM, Magnus
> Danielson<magnus at rubidium.dyndns.org> wrote:
>> Hal Murray skrev:
>>> david.bengtson at gmail.com said:
>>>> I've recently gotten an opportunity to work on a GPSDO at work, and
>>>> so I now have a real need to pick up some more information on the
>>>> design tradeoffs and approaches in a GPSDO. I've plowed through a pile
>>>> of old emails, but it's difficult to get a good overview from email
>>>> I would appreciate links to discussions on the tradeoffs inherent in a
>>>> GPSDO. Sampling clock rate, filter bandwidths, jitter requirements
>>> Be sure you know about hanging bridges. Start with the
>>> "Timing for VLBI" slides at http://gpstime.com/
>> The key issue is, what type of specs is needed? What may it cost?
>> What kind of receiver may be considered? What's the intended application?
> Specs.. Hmmmm. Good question. The application is for a frequency
> reference, so frequency stability vs. PPS accuracy is important. Cost
> is perhaps less important than size. The OCXO under control is a
> small, surface mount part with +/- 0.5 PPM over temp, but flexibility
> to use higher accuracy parts would be good. As this is for an internal
> application, I'm not sure how much else I can disclose.
You should consider using a board having a 10 MHz input. Otherwise the
normal route is to use the PPS pulse out of a board (adjusted by any
form of offset value usually available in the bitstream) and make a TI
compare to a divided down variant of the OCXO. The standard approach
used by many is to use a PI-loop where the bandwidth and Q-value is set
with thought. A more complex approach is to go for a Kalman filter,
which has the benefit of being able to more dynamically adapt and tune
it. A Kalman filter for a simple setup like this isn't particularly hard
to design, even if the math may seem a bit complex at first.
Don't use to few bits in the DA.
There are a number of simple projects around. Check if their achieved
level of performance is matching your needs. In that case you know more
or less what you need to stretch for.
This is a topic which can go over the deep end if you let us... so
asking for the required specs is really, what do you need? What can you
settle with? It gives an indication of how deep you need to go.
One issue to keep in mind is what kind of hold-over performance do you
need, if any. That is... how much may it deviate after how long a time
without propper GPS signal.
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