[time-nuts] Changing ADEV, (was Phase, One edge or two?)
Bob Camp
kb8tq at n1k.org
Sat Oct 25 20:16:01 UTC 2014
Hi
> On Oct 25, 2014, at 3:34 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>
>> How many hours / days/ months / years had the OCXO been off power before the run was started?
>>
>> How soon after turn on did you start taking data?
>
> Hi Bob,
>
> On the ocxo.dat data set, the frequency drift rate was down to just 5e-11 a day so it's likely the OCXO had been powered for many days, even weeks. I don't know for sure w/o finding an old log book. The web page says the data came from "run3004/log50187.txt", which is was a free-running TBolt in November 2008 measured with a TSC 5120 against a locked HP 58503B. I can re-run the measurement if you wish.
>
> Why do you ask?
I ask because the most common place I see ADEV changing over time without systematic issues is when they have been off power for a long time. If you warm them up and run them for a while, the ADEV tends to become much more predictable.
Bob
> I have many data sets here, both with lower drift (e.g., rubidium or masers), or higher drift, or a variety of phase measurement instruments. Lots of samples is usually better than few samples, but it doesn't take a lot to pin the stability of an oscillator down to a couple of dB.
>
> In some cases, computed ADEV is not a number that gets more precise or more accurate the more data you collect. You can hit a floor and it starts to diverge if you collect for too many weeks or months. This is expected. HDEV would be better.
>
> An analogy: no one is interested in the mean of 1,000,000 days of earth temperature data. Yes, it will be a "very precise" number, if you apply the mindless sqrt(N) rule-of-thumb. But once you get enough data, looking at periodicity, jumps, outliers, and trends over time is usually far more important than blindly calculating a simple mean or standard deviation from an entire data set.
>
> You can argue all day if the ADEV(tau 1000s) should be 3.7e-12 or 3.75e-12 or 4e-12. Regardless, it's clearly about halfway between 1e-12 and 1e-11. Below 1 dB, the rest is what day it is, what hour you started the run, how long you collected data, or how your lab feels that day. Here's an example of ADEV(tau 1000) from adev6:
>
> C:\Tmp>adev6 /a < ocxo.dat 1000
> 1000 0/400000 a 3.706451e-012 398000
>
> C:\Tmp>adev6 /a < ocxo.dat 1000 40000
> 1000 0/400000 a 4.100519e-012 38000
> 1000 40000/400000 a 3.912714e-012 38000
> 1000 80000/400000 a 3.736134e-012 38000
> 1000 120000/400000 a 4.413685e-012 38000
> 1000 160000/400000 a 3.050424e-012 38000
> 1000 200000/400000 a 3.692079e-012 38000
> 1000 240000/400000 a 3.367214e-012 38000
> 1000 280000/400000 a 3.223972e-012 38000
> 1000 320000/400000 a 3.742055e-012 38000
> 1000 360000/400000 a 3.897041e-012 38000
>
> C:\Tmp>adev6 /a < ocxo.dat 1000 4000
> 1000 0/400000 a 7.804138e-012 2000
> 1000 4000/400000 a 4.085721e-012 2000
> 1000 8000/400000 a 3.368610e-012 2000
> 1000 12000/400000 a 2.890283e-012 2000
> 1000 16000/400000 a 2.408464e-012 2000
> 1000 20000/400000 a 5.823737e-012 2000
> 1000 24000/400000 a 4.127749e-012 2000
> 1000 28000/400000 a 4.310555e-012 2000
> 1000 32000/400000 a 3.375545e-012 2000
> 1000 36000/400000 a 4.166632e-012 2000
> 1000 40000/400000 a 3.052641e-012 2000
> 1000 44000/400000 a 4.718652e-012 2000
> 1000 48000/400000 a 4.238576e-012 2000
> 1000 52000/400000 a 5.275587e-012 2000
> 1000 56000/400000 a 5.695453e-012 2000
> 1000 60000/400000 a 3.669497e-012 2000
> 1000 64000/400000 a 3.107038e-012 2000
> 1000 68000/400000 a 4.863025e-012 2000
> 1000 72000/400000 a 1.882393e-012 2000
> 1000 76000/400000 a 2.395768e-012 2000
> 1000 80000/400000 a 1.606562e-012 2000
> 1000 84000/400000 a 6.180515e-012 2000
> 1000 88000/400000 a 3.201972e-012 2000
> 1000 92000/400000 a 2.023414e-012 2000
> 1000 96000/400000 a 1.515005e-012 2000
> 1000 100000/400000 a 2.343072e-012 2000
> 1000 104000/400000 a 4.249873e-012 2000
> 1000 108000/400000 a 2.676816e-012 2000
> 1000 112000/400000 a 1.656133e-012 2000
> 1000 116000/400000 a 2.411179e-012 2000
> 1000 120000/400000 a 4.081474e-012 2000
> 1000 124000/400000 a 2.997803e-012 2000
> 1000 128000/400000 a 2.095393e-012 2000
> 1000 132000/400000 a 5.760947e-012 2000
> 1000 136000/400000 a 7.075811e-012 2000
> 1000 140000/400000 a 1.769521e-012 2000
> 1000 144000/400000 a 3.358276e-012 2000
> 1000 148000/400000 a 4.893182e-012 2000
> 1000 152000/400000 a 1.936321e-012 2000
> 1000 156000/400000 a 1.578596e-012 2000
> 1000 160000/400000 a 3.601683e-012 2000
> 1000 164000/400000 a 2.287769e-012 2000
> 1000 168000/400000 a 3.073412e-012 2000
> 1000 172000/400000 a 2.291148e-012 2000
> 1000 176000/400000 a 5.813071e-012 2000
> 1000 180000/400000 a 3.669111e-012 2000
> 1000 184000/400000 a 1.766833e-012 2000
> 1000 188000/400000 a 2.527836e-012 2000
> 1000 192000/400000 a 1.982012e-012 2000
> 1000 196000/400000 a 2.387086e-012 2000
> 1000 200000/400000 a 4.483388e-012 2000
> 1000 204000/400000 a 1.825970e-012 2000
> 1000 208000/400000 a 1.405565e-012 2000
> 1000 212000/400000 a 5.431766e-012 2000
> 1000 216000/400000 a 1.325479e-012 2000
> 1000 220000/400000 a 1.874571e-012 2000
> 1000 224000/400000 a 8.372485e-012 2000
> 1000 228000/400000 a 4.277784e-012 2000
> 1000 232000/400000 a 2.616340e-012 2000
> 1000 236000/400000 a 3.765100e-012 2000
> 1000 240000/400000 a 1.840977e-012 2000
> 1000 244000/400000 a 2.921888e-012 2000
> 1000 248000/400000 a 1.532576e-012 2000
> 1000 252000/400000 a 2.774957e-012 2000
> 1000 256000/400000 a 5.713711e-012 2000
> 1000 260000/400000 a 4.725035e-012 2000
> 1000 264000/400000 a 3.428511e-012 2000
> 1000 268000/400000 a 2.549448e-012 2000
> 1000 272000/400000 a 8.913688e-013 2000
> 1000 276000/400000 a 4.408449e-012 2000
> 1000 280000/400000 a 2.503479e-012 2000
> 1000 284000/400000 a 1.883790e-012 2000
> 1000 288000/400000 a 3.782682e-012 2000
> 1000 292000/400000 a 3.132628e-012 2000
> 1000 296000/400000 a 2.913452e-012 2000
> 1000 300000/400000 a 2.021695e-012 2000
> 1000 304000/400000 a 3.370930e-012 2000
> 1000 308000/400000 a 2.043129e-012 2000
> 1000 312000/400000 a 5.285278e-012 2000
> 1000 316000/400000 a 3.020556e-012 2000
> 1000 320000/400000 a 3.454389e-012 2000
> 1000 324000/400000 a 5.324399e-012 2000
> 1000 328000/400000 a 4.481485e-012 2000
> 1000 332000/400000 a 1.773463e-012 2000
> 1000 336000/400000 a 4.372101e-012 2000
> 1000 340000/400000 a 5.504813e-012 2000
> 1000 344000/400000 a 3.117502e-012 2000
> 1000 348000/400000 a 5.678955e-012 2000
> 1000 352000/400000 a 2.934761e-012 2000
> 1000 356000/400000 a 3.119271e-012 2000
> 1000 360000/400000 a 2.867365e-012 2000
> 1000 364000/400000 a 5.534214e-012 2000
> 1000 368000/400000 a 1.696574e-012 2000
> 1000 372000/400000 a 1.954016e-012 2000
> 1000 376000/400000 a 3.051420e-012 2000
> 1000 380000/400000 a 5.290654e-012 2000
> 1000 384000/400000 a 3.908393e-012 2000
> 1000 388000/400000 a 2.984008e-012 2000
> 1000 392000/400000 a 2.348755e-012 2000
> 1000 396000/400000 a 4.209189e-012 2000
>
> /tvb
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