[time-nuts] how many seconds out does GPS discipline being to improve Rubidium stability?

Bob Camp kb8tq at n1k.org
Sat Feb 11 09:29:14 EST 2017


Backing up a bit here.

> On Feb 10, 2017, at 7:35 PM, gkk gb <modjklist at comcast.net> wrote:
> Hello experts, I need a Rubidium frequency reference for my company, and wonder if I also need to GPS discipline it.
> I characterize crystal-based OCXOs for ADEV, MTIE, and TDEV, and my longest measurement time is 100,000 seconds (28 hours). 

If your longest measurement is a 100,000 second ADEV, then your measurement time will be out in the 
1,000,000 to 10,000,000 second range. Is that really what you are doing? 

If 100,000 seconds ADEV is your longest measurement, what is the shortest tau you are interested in?
A Rb is not going to be much use for testing a good OCXO at shorter tau. Where the crossover happens
depends a lot on the grade of OCXO you are working with. By the time you get to 1 second 
most OCXO’s will be noticeably better than most Rb’s.

> I'm looking at this graph from SRS for PRS10,
> http://www.thinksrs.com/assets/instr/PRS10/PRS10diag2LG.gif

I would suggest that plot is probably not the best one to depend on for GPS performance.  In a GPSDO setting 
the cut over points are all over the place depending on which design you look at. 

> and thinking that as long as I calibrate a Rubidium source annually, there's no need for a GPS (since it only appears to degrade stability). Is this true in general, or is the graph misleading me because it may be true here, but not always.

The big issue is going to be temperature stability. If you have a Rb that is (say) 5x10^-10 over 0 to 50C, that is likely 1x10^-11 / C (or maybe more). A 2C delta in 
your lab as the HVAC cycles will give you a 2x10^-11 “hump” in your ADEV plot. 

Also consider that if you want an “easy” measurement of the devices you are testing, the reference source probably should be 
5X better than what you expect out of the DUT. You probably will not have that luxury in this case. That gets you into multiple
references and things like three corner hat testing. 

> So my question, is a GPS necessary to discipline a Rubidium standard to characterize the best crystal oscillators for stability, or can I do without it (and just calibrate the Rubidium annually to maintain accuracy) and actually get better stability?
> How many seconds out is a GPS generally needed to improve accuracy from a Rubidium standard?

If you really are running 1,000,000 to 10,000,000 second long tests, you need the GPS.

Lots of variables 


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