[time-nuts] test mailing from a new member

[David Bengtson]

On Tue, Jun 16, 2009 at 9:28 PM, Magnus
Danielson<magnus at rubidium.dyndns.org> wrote:
> 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
>>>>> threads.
>>>>> 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.
>

We have an on-board 10 MHz Oscillator, although that frequency may
change in the future, along with a digital PLL and an off the shelf
GPS receiver. We have a GPSDO  that works, but there are some corner
cases where it doesn't seem to work as expected, so debugging is part
of what I'm going to do.


> 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.

Good suggestion.

>
> 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.
>
Still working on this.
> 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 signa

Hold over and re-acquision time is an issue, something I'm trying to quantify.


Part of the issue I'm running into is a simple unfamiliarity with the
system. Receivers, Transmitters, synthesizers, I know those well, and
when I have questions, I know where to to find the answers. With a
GPSDO, I'm still learning the terminology and the performance
requirements, so I'm not there yet.

Regards

Dave

>
> Cheers,
> Magnus
>
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