On Tue, Jun 16, 2009 at 9:28 PM, Magnus Danielson<[email protected]> wrote: > David Bengtson skrev: >> >> On Tue, Jun 16, 2009 at 6:07 PM, Magnus >> Danielson<[email protected]> wrote: >>> >>> Hal Murray skrev: >>>> >>>> [email protected] 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 > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
