[time-nuts] Characterising frequency standards
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Wed Apr 8 11:59:53 UTC 2009
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:
>
> Using your adev1.exe from http://www.leapsecond.com/tools/adev1.htm
> 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:
>
> 1 tau, 3.0127e-009 adev(n=399998), 3.0127e-009 oadev(n=399998)
> 2 tau, 1.5110e-009 adev(n=199998), 1.5119e-009 oadev(n=399996)
> 5 tau, 6.2107e-010 adev(n=79998), 6.1983e-010 oadev(n=399990)
> 10 tau, 3.1578e-010 adev(n=39998), 3.1549e-010 oadev(n=399980)
> 20 tau, 1.6531e-010 adev(n=19998), 1.6534e-010 oadev(n=399960)
> 50 tau, 7.2513e-011 adev(n=7998), 7.3531e-011 oadev(n=399900)
> 100 tau, 4.0029e-011 adev(n=3998), 4.0618e-011 oadev(n=399800)
> 200 tau, 2.1512e-011 adev(n=1998), 2.1633e-011 oadev(n=399600)
> 500 tau, 9.2193e-012 adev(n=798), 9.1630e-012 oadev(n=399000)
> 1000 tau, 4.9719e-012 adev(n=398), 4.7750e-012 oadev(n=398000)
> 2000 tau, 2.6742e-012 adev(n=198), 2.5214e-012 oadev(n=396000)
> 5000 tau, 1.0010e-012 adev(n=78), 1.1032e-012 oadev(n=390000)
> 10000 tau, 6.1333e-013 adev(n=38), 6.1039e-013 oadev(n=380000)
> 20000 tau, 3.8162e-013 adev(n=18), 3.2913e-013 oadev(n=360000)
> 50000 tau, 1.0228e-013 adev(n=6), 1.5074e-013 oadev(n=300000)
> 100000 tau, 5.8577e-014 adev(n=2), 6.7597e-014 oadev(n=200000)
>
> 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:
>
> 1 tau, 3.0257e-009 adev(n=199998), 3.0257e-009 oadev(n=199998)
> 2 tau, 1.5373e-009 adev(n=99998), 1.5345e-009 oadev(n=199996)
> 5 tau, 6.3147e-010 adev(n=39998), 6.3057e-010 oadev(n=199990)
> 10 tau, 3.3140e-010 adev(n=19998), 3.3067e-010 oadev(n=199980)
> 20 tau, 1.7872e-010 adev(n=9998), 1.7810e-010 oadev(n=199960)
> 50 tau, 7.9428e-011 adev(n=3998), 8.1216e-011 oadev(n=199900)
> 100 tau, 4.2352e-011 adev(n=1998), 4.3265e-011 oadev(n=199800)
> 200 tau, 2.2001e-011 adev(n=998), 2.2593e-011 oadev(n=199600)
> 500 tau, 9.6853e-012 adev(n=398), 9.5441e-012 oadev(n=199000)
> 1000 tau, 5.0139e-012 adev(n=198), 5.0387e-012 oadev(n=198000)
> 2000 tau, 2.7994e-012 adev(n=98), 2.7090e-012 oadev(n=196000)
> 5000 tau, 1.4280e-012 adev(n=38), 1.2214e-012 oadev(n=190000)
> 10000 tau, 7.4881e-013 adev(n=18), 6.5814e-013 oadev(n=180000)
> 20000 tau, 7.6518e-013 adev(n=8), 3.7253e-013 oadev(n=160000)
> 50000 tau, 2.4698e-014 adev(n=2), 1.3539e-013 oadev(n=100000)
>
> 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:
>
> 1 tau, 3.0411e-009 adev(n=199998), 3.0411e-009 oadev(n=199998)
> 2 tau, 1.4985e-009 adev(n=99998), 1.4999e-009 oadev(n=199996)
> 5 tau, 6.1964e-010 adev(n=39998), 6.2010e-010 oadev(n=199990)
> 10 tau, 3.1315e-010 adev(n=19998), 3.1339e-010 oadev(n=199980)
> 20 tau, 1.6499e-010 adev(n=9998), 1.6495e-010 oadev(n=199960)
> 50 tau, 7.1425e-011 adev(n=3998), 7.3416e-011 oadev(n=199900)
> 100 tau, 3.9940e-011 adev(n=1998), 4.0730e-011 oadev(n=199800)
> 200 tau, 2.1488e-011 adev(n=998), 2.1558e-011 oadev(n=199600)
> 500 tau, 8.4809e-012 adev(n=398), 9.0886e-012 oadev(n=199000)
> 1000 tau, 4.9223e-012 adev(n=198), 4.7104e-012 oadev(n=198000)
> 2000 tau, 2.4335e-012 adev(n=98), 2.4515e-012 oadev(n=196000)
> 5000 tau, 1.0308e-012 adev(n=38), 1.0861e-012 oadev(n=190000)
> 10000 tau, 5.9504e-013 adev(n=18), 6.1031e-013 oadev(n=180000)
> 20000 tau, 3.6277e-013 adev(n=8), 3.1994e-013 oadev(n=160000)
> 50000 tau, 1.0630e-013 adev(n=2), 1.6715e-013 oadev(n=100000)
>
> 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,
>>
>> You've asked a couple of questions. Let me start with this.
>>
>> 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
>> (aka phase) measurements made about the same time once
>> 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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