[time-nuts] Data Collection for Allan Deviation

Bob Camp kb8tq at n1k.org
Sat Aug 1 08:10:10 EDT 2015


Ok, first a step back:

At one second your GPS ADEV will be in the 1x10^-9 to 1x10^-7 range depending on the unit you have. 

At one second your Rb will be somewhere in the 1x10^-12 to 1x10^-9 range depending on the unit you have. 

If you compare them against each other, the number you get will be the one for the worse of the two. 

Assuming you have a Telcom Rb and not something very unusual. It probably runs at 2x10^-11 at one second and
improves by square root of tau. At 100 seconds, it will be 2x10^-12. 

If your GPS starts out at 1x10^-8, it improves as tau so at 100 seconds it may be at 1x10^-10. 

Since these are rough rules of thumb, they do eventually fall apart. Other error sources get into the mix. The Rb probably 
will not get to 2x10^-13 at 10,000 seconds. The GPS may not be at 1x10^-12 at 10,000 seconds.


So what does this all mean? What you really will be doing is measuring the ADEV of the GPS using the Rb as a 
reference that is good enough that it can be ignored. That is a questionable assumption at 100,000 seconds, but you
need at least 1,000,000 seconds of data to make even a low accuracy estimate there. 

If the objective is simply looking at the GPS, a 53132 counter will probably do a fine job. Run the 1 pps out of the
GPS and a 1 pps out of the Rb. Take the time interval between them at a 0.2 ns level once a second. If the 
Rb does not have a 1 pps, you can do some tricks with the 10 MHz. It will take some effort to “unfold” the data
you get. ( = use 1 pps to 1 pps if you have never done this before)

You can grab a copy of TimeLab and it will do most of the heavy lifting for you. I would still start with a 1 pps to 1 pps
comparison ...


> On Jul 31, 2015, at 11:44 PM, zzsilicon at post.com wrote:
> If I have a GPS receiver with output pin of both 1pps & 10KHz, a Rubidium clock of 10MHz, and a signal generator. How can I determine their Allan Deviation? I know the math formula, but my problem is the data collection.
> I am looking through internet so far. It seems the standard way to determine Allan Deviation is to use a reference clock with a few electronic devices such as frequency counter. What if I don't have any frequency reference or any frequency counter. If I have a Rubidium clock with 10MHz output to an o-scope, what values should I record for the time series data, in order to determine the Allan variation?
> Thanks for the help!
> ZZ
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