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

[EWKehren at aol.com]

With the exception of the HP 5065 where the Rb cell actually is the ADEV  
contributor as Corby has repeatedly demonstrated with tests and if you check 
the  filter time constant.  Most Rb's  will be well served with a  cleanup 
OCXO. 
The PRS 10 has good performance as is,  but is limited by DAC  resolution 
to 1 E-12 in frequency resolution/accuracy.
In the popular FE5680 and 5650 we use a separate cleanup circuit using a  
Morion and on FRK's use the modified internal loop with HP 10811, M 1000  and 
Oscilloquartz 8600 deactivating the internal OCXO.
Going the extra mile gives you the best of both and you do not have to  
compromize.
Bert Kehren
 
 
In a message dated 12/9/2014 7:40:56 A.M. Eastern Standard Time,  
kb8tq at n1k.org writes:

Hi

>  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

> 
>> Bob
> 
>  Thanks,
> 
> Mike
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