[time-nuts] Allan Deviation

Brian Kirby kilodelta4foxmike at gmail.com
Wed Jun 16 02:56:00 UTC 2010

Robert Benward wrote:
> Hi All,
> How does one measure allan deviation?  How does my computer measure the 
> stability of an oscillator?  Is this coming from the Z3801A?
> Thanks,
> Bob
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Several methods can be used.  Basic is time interval between a frequency 
source and a oscillator/device under test.  Usually a computer is used 
to record the data from the time interval counter and to then to post 
process the data.  Three programs come to mind that can process the 
variances - Alavar, Plotter and Stable32.

You can measure the Allan deviation of the Z3801A, but you have to 
compare it to another frequency source better than it.

As an example, you can use a divider to make a 1 pulse per second 
signal  from the Z3801A  (actually it has a divider built in, it is 
available on the DB25 connector).  Then use say compare it to the 1 PPS 
of a cesium beam standard.  One PPS signal is feed to the start input of 
the time interval counter and the other PPS signal is feed to the stop 

Since the data is coming in at 1 second periods, the program that 
process the variances will calculate the deviation at 1 second, then say 
10 seconds, then 100 seconds, etc. and then generate a plot. 

This is a very basic explanation, start some research and you'll end up 
crazy like the rest of us.....!.


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