[time-nuts] Phase, One edge or two? (was Digital mixing with a D Flip Flop)
Tom Van Baak
tvb at LeapSecond.com
Wed Oct 22 22:40:06 UTC 2014
> You need more than 1 sample per day for ADEV plots left of 100,000 K seconds.
Correct. What I sometimes do is collect data for just a few minutes at 1000 samples per second. That's enough to make an ADEV plot for tau 0.001 to 1 or 10 seconds. Then I'll collect data for a couple of days at 1 sample per second. That gives you an ADEV plot for tau 1 to 100 k seconds. For really long-term tracking you don't need samples every millisecond or every second. This approach means you don't tie up expensive high-resolution instruments with long-term measurements. The key point here is you don't need to use the same instrument or the same sampling rate or the same resolution for short- and long-term measurements.
> Suppose you have lots and lots of data at, say, 1 second samples. You can
> turn that into an ADEV plot. Does anybody scan the data in clumps, say a
> day, to see if the short-tau picture changes over time?
Yes, Stable32 implements DAVAR (dynamic AVAR) which gives an impressive 3D ADEV plot. This is useful for timing sources where the stability, for whatever reason, changes over time.
John's TimeLab software implements another form of this: you can specify a "trace history" value. For example, instead of computing ADEV of a million points, you compute 10 ADEV's on successive groups of 100,000 points each. This is a great way to show the confidence in your statistics. It also works well on oscillators that are warming up, over hours and days.
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