[time-nuts] Rb as source for ADEV?

Magnus Danielson magnus at rubidium.dyndns.org
Thu Feb 6 23:03:26 EST 2014


On 07/02/14 04:31, Tom Van Baak wrote:
>> Hi Tom,
>>
>> If I understand you right, the only thing I need from the adev program is the OADEV values and those are the new
>> ADEV? Here's the plot for the past 24 hours using "0" for bins. It takes a few seconds to run, doesn't it?
>
> Yes, just grab the code for calc_adev() in http://leapsecond.com/tools/adev_lib.c and always set ovlp = 1. Either way, it's ADEV; it's just that the overlap method gives better results (smaller error bars).
>
>> http://www.evoria.net/AE6RV/GPSstd_PLL/Plots/ADEV.2.6.png
>
> Suggestion: do not label the x-axis "time in seconds". This gives the impression that the axis is some sort of elapsed time. Better wording might be "integration time", or "sampling interval", or "averaging time", or "tau", or "sampling interval, tau".

Averaging time is unfortunate in several aspects, part for it not being 
averaging being done and part for the fact that averaging is a separate 
processing step that can be done, and you can make a plot orthogonal to 
the ADEV plot with various degrees of averaging-pre-filter.

The MDEV uses a tau averaging for a tau observation time.

The tau is really the observation time, and ADEV is the frequency 
stability for that observation time, that is the RMS relative frequency 
noise when observing it tau seconds later. Similarly the TDEV is the 
time stability in RMS s for the observation time tau.

ADEV also rises for long taus on oscillators, where you expect even 
better averaging to keep it going further down.

The field is confused enough, so I want to avoid this confusion.

Cheers,
Magnus


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