[time-nuts] Changing ADEV, (was Phase, One edge or two?)
Tom Van Baak
tvb at LeapSecond.com
Sat Oct 25 12:35:02 EDT 2014
Let's take a real example.
Use your own phase data, or grab any of my large data sets (http://leapsecond.com/pages/gpsdo-sim) like ocxo.dat.gz which is a good example of real-life OCXO performance (400,000 seconds of data).
Attached are Stable32 plots of frequency, ADEV/MDEV, and dynamic ADEV. As a 3D plot, the latter shows how ADEV(tau) varies during the run. In this case the full data set is broken into about 90 pieces and ADEV is computed for each segment of data ("window"). If you study the frequency plot you may be able to convince yourself why the DADEV plot looks like it does; ADEV(small tau) is quite constant, while ADEV(larger tau) varies quite a bit. To me, this is as it should be, given how the raw data looks.
To explore dynamic ADEV without Stable32 or to go deep with the effects of sample size, see adev6.c / adev6.exe in my tools directory (www.leapsecond.com/tools).
Most programs compute ADEV based on the entire data set. But adev6 will compute ADEV(tau) in user defined subsets of data. So, for example, instead of computing ADEV(tau 1) from 400,000 points, you can compute ADEV(tau 1) 400 times in blocks of 1000 points each, or 4000 times in blocks of 100 points each, etc. The default is back-to-back segments but you can specify overlapping segments. Using various combination of parameters, it's pretty instructive to see the "noise" in computed value of ADEV.
The 4th attachment is a TimeLab plot of the data set with trace = 1 (default), 10, and 100. In some cases I prefer this sort of display. One can get complacent with simple error bars and forget that 1 - 68% = 32% of the points must always lie outside the error bars, by definition.
/tvb
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