[time-nuts] Did a member of time-nuts buy this?

Bob Camp kb8tq at n1k.org
Tue Dec 9 07:40:32 EST 2014


> On Dec 9, 2014, at 6:10 AM, Mike Monett <timenuts at binsamp.e4ward.com> wrote:
> Hi Bob, I have been having problems posting this. If other versions
> show up, please disregard.
> I have some questions for you.
>> Hi
>> If you toss a Rb into the GPSDO "mix" things can get quite good.
>> The Rb *should* be better than an OCXO in the > 1,000 second
>> range.
>> It's crossover with the GPS ADEV will be further out than the
>> OCXO's. The gotcha with both the OCXO and Rb is their temperature
>> dependance. Some / many / all of the lower cost Rb's are not much
>> better than a good double oven OCXO in terms of raw temperature
>> performance. The approach they use to "correct" this does not help
>> their ADEV at all. Yes, you can disable the correction and put the
>> whole thing in a temperature controlled environment.
> What does the temperature controller do that degrades the ADEV? Can
> anything be done to modify it to improve the performance?

On the lightweight Rb’s they feed the correction into a DDS. The DDS steps are big enough to show up above the ADEV. You get a “hump” in the ADEV as a result. The solution is to disconnect the temperature sensor that feeds the correction circuit. 

>> Lots of details to take care of. If you get them all right, you'll
>> beat any / all of the older Cs standards.
> Is there a list anywhere of all the details that need to be
> addressed?

Well, there’s this list :)

The things that need to be done are a “that depends” based on the approach you take. A full listing of all you might do down all of the roads you could take would be a wall full of books.

> How about words or phrases that can be used to search
> google and the archives?

GPSDO is a good place to start.

>> One very cute addition would be to pull down the NIST GPS data and
>> use it to correct your system on an hourly / daily basis. If you
>> do that with common view satellites, you most certainly will beat
>> a surplus grade Cs standard.
> How can we do this?

First step is to be able to extract timing data from individual sat’s. Not all GPS modules do this correctly. 

> The NIST archives state
> "The archive is only updated once every 24 hours, so data are not
> available for today's date.

Thus the “daily” statement. There has been discussion on the list that interchange with other list members at a more rapid rate might make sense. Given the floor of most setups, daily updates are not a bad thing. 

> Data from the previous day are added to
> the archive at about 1600 UTC."
> http://www.nist.gov/pml/div688/grp40/gpsarchive.cfm
> Is there another page that has current data? If so, how do we
> incorporate it into the GPSDO?

1) Measure your local time offset by GPS sat number every hour and save the data.

2) When NIST posts there data, pull it down.

3) Compare your data to theirs

4) Do a fit

5) Feed that into your control loop equation. 

The missing element is the per sat data….


>> Bob
> Thanks,
> Mike
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