[ntp:questions] Allan deviation survey

unruh unruh at wormhole.physics.ubc.ca
Fri Sep 10 16:05:29 UTC 2010


On 2010-09-10, Miroslav Lichvar <mlichvar at redhat.com> wrote:
> Hi,
>
> I'm trying to find out how a typical computer clock oscillator
> performs in normal conditions without temperature stabilization or a
> stable CPU load and how far it is from the ideal case which includes
> only a random-walk frequency noise.
>
> A very useful statistics is the Allan deviation. It can be used to
> compare performance of oscillators, to make a guess of the optimal
> polling interval, whether enabling ntpd daemon loop to use FLL will
> help, how much better chrony will be than ntpd, etc.
>
> If you have a PPS device and would be willing to run the machine
> unsynchronized for a day, I'd like to ask you to measure the Allan
> deviation and send it to me.
>
> I wrote a small ncurses program that can be used with LinuxPPS to
> capture the PPS samples and create an Allan deviation plot. An
> overview is displayed and continuously updated while samples are
> collected. Data which can be used to make an accurate graph (e.g. in
> gnuplot) are written to the file specified by -p option when the
> program is ended or when the 'w' key is pressed.
>
> Available at:
> http://mlichvar.fedorapeople.org/ppsallan-0.1.tar.gz
>
> Obligatory screenshot :-)
>            Allan deviation plot (span 11:09:55, skew +0.0)
> 1e-05???                                        
>      ???	      
>      +
>      ???     
>      ???  + +
> 1e-06???      +     
>      ???       +++
>      ???          ++ 
>      ???            +++ 
>      ???               ++  
> 1e-07???                 +++
>      ???                    ++ 
>      ???                      +++
>      ???                         ++                ++++++
>      ???                           +++         ++++      
> 1e-08???                              +++++++++         
>      ???                                              
>      ???                                             
>      ???
>      ???
> 1e-09???????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????
>    1e+00       1e+01	   1e+02       1e+03	   1e+04       1e+05
> w:Write   q:Quit   r:Reset   1:Skew 0.0   2:Skew +1.0   3:Skew -0.5
>
>
> To make a good plot:
> 1. disable everything that could make system clock adjustments
> 2. start ./ppsallan -p adev.plot /sys/devices/virtual/pps/pps0/assert
>    (change the sys file as appropriate)
> 3. let it collect the PPS samples for at least one day
> 4. hit q and send me the adev.plot file
>
> Thanks,
>

Except yo uknow that the typical computer clock is driven mostly by
temperature variations, and those are time of day dependent. People tend
to work during the day thus their computer works during the day, and
does not at night. Ie, there is a very strong daily cycle in the temp of
the computer. That is NOT within the Allan model, and the Allan
variation and minimum are really irrelevant with this highly
non-stochastic noise model.




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