# [time-nuts] Characterising frequency standards

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Apr 8 12:11:17 UTC 2009

```Steve

It cant, it must be a matter of interpretation.
Perhaps it means something like:

1 tau means tau = 1x the interval between consecutive measurements.
2 tau means tau = 2x the interval between consecutive measurements

100000 tau means  tau = 100,000 x the interval between  consecutive
measurements

Bruce

Steve Rooke wrote:
> Bruce,
>
> But how does that explain the output of Tom's adev1 program which
> still seems to give a a good measurement at tau = 1s?
>
> 73,
> Steve
>
> 2009/4/8 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
>
>> Steve
>>
>> If you delete every second measurement then your effective minimum
>> sampling time is now 2s and you can no longer calculate ADEV for tau< 2s.
>> You can still calculate ADEV for tau = 100,000 sec.
>>
>> If you delete all but the first 200,000 lines then you can calculated
>> ADEV for tau=1sec and up to tau= 25,000 sec with reasonable accuracy.
>>
>> You shouldn't lose sight of the fact that ADEV and OADEV are both
>> estimates of the Allan deviation.
>>
>>
>> Bruce
>>
>> Steve Rooke wrote:
>>
>>> Tom,
>>>
>>> I understand fully the points that you have made but I have obviously
>>> not made my point clear to all and i apologise for my poor
>>> communication skills.
>>>
>>> This is what I'm getting at:
>>>
>>> and processing various forms of gps.dat from
>>> http://www.leapsecond.com/pages/gpsdo-sim/gps.dat.gz.
>>>
>>> C:\Documents and Settings\Steve Rooke\Desktop>adev1.exe 1 <gps.dat
>>>
>>> ** Sampling period: 1 s
>>> ** Phase data scale factor: 1.000e+000
>>> ** Total phase samples: 400000
>>> ** Normal and Overlapping Allan deviation:
>>>
>>>
>>> So far, so good. Now I delete every even line in the file which leaves
>>> me with 200000 lines of data (400000 lines in original gps.dat file).
>>> (awk 'and(NR, 1) == 0 {print}' <gps.dat >gps1.dat)
>>>
>>> C:\Documents and Settings\Steve Rooke\Desktop>adev1.exe 1 <gps1.dat
>>>
>>> ** Sampling period: 1 s
>>> ** Phase data scale factor: 1.000e+000
>>> ** Total phase samples: 200000
>>> ** Normal and Overlapping Allan deviation:
>>>
>>>
>>> Obviously we don't have enough data now for a measurement of 100000
>>> tau but the results for the other tau are quite close, especially when
>>> there are sufficient data points. Now this is discontinuous data,
>>> exactly what I was trying to allude to.
>>>
>>> OK, so now I take only the top 200000 lines of the gps.dat file (head
>>> -200000 gps.dat >gps2.dat)
>>>
>>> C:\Documents and Settings\Steve Rooke\Desktop>adev1.exe 1 <gps2.dat
>>>
>>> ** Sampling period: 1 s
>>> ** Phase data scale factor: 1.000e+000
>>> ** Total phase samples: 200000
>>> ** Normal and Overlapping Allan deviation:
>>>
>>>
>>> Is there any Linux tools for calculating adev as I'm having to run
>>> Windows in a VMware session?
>>>
>>> 73,
>>> Steve
>>>
>>> 2009/4/8 Tom Van Baak <tvb at leapsecond.com>:
>>>
>>>
>>>> Steve,
>>>>
>>>>
>>>> It is true that if one were only interested in the performance
>>>> of a pendulum (or quartz or atomic) clock for averaging times
>>>> of one day that all you would need is a series of time error
>>>> a day (doesn't have to be that exact). After one week, you'd
>>>> have 7 error measurements (=6 frequency =5 stability points)
>>>> and this is adequate to calculate the ADEV for tau 1 day.
>>>> This alone allows you to rank your clock among all the other
>>>> pendulum clocks out there. Note also you get time error and
>>>> rate error from these few data points too.
>>>>
>>>> As another example, suppose you have a nice HP 10811A
>>>> oscillator and want to measure its drift rate. In this case you
>>>> could spend just 100 seconds and measure its frequency
>>>> once a day, or even once every couple of days. Do this for
>>>> a month and you'd have several dozen points. If you plot
>>>> these frequency measurements you will likely see that they
>>>> approximately fall on a line; the slope of the is the frequency
>>>> drift rate of the 10811. The general shape of the points, or
>>>> the fit of the line is a rough indication of how consistent the
>>>> drift rate is or if it's increasing or decreasing.
>>>>
>>>> Neither of these examples require a lot of data. Both of these
>>>> are real-world examples.
>>>>
>>>> OK so far?
>>>>
>>>> /tvb
>>>>
>>>>
>>>>
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>>>>
>>>>
>>>>
>>>
>>>
>>>
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>
>
>
>

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