PhaseStation 53100A User’s Manual V1.04 25-Apr-2022
43
“What’s all this ADEV stuff, anyhow?”
With apologies to the late Bob Pease and his editors at
Electronic Design
, here are a few tips to help you
understand what Allan deviation (ADEV) graphs really tell you, and how to get the most out of the
statistical deviation measurements made by TimeLab.
Allan deviation is defined as the
two-sample deviation of fractional frequency differences at a given time
interval
. While technically correct, such definitions may be unenlightening to new users. Often, the first
question asked is something like, “Why did my plot stop at 1000 seconds? It took an hour to run!”
The answer is that these plots are not simply linear depictions of frequency drift over time. (In TimeLab,
that’s the purpose of the
Measurement
→
Frequency Difference (
f
)
view.) Nor are they depictions of a
single Allan deviation measurement that describes the clock’s stability from one interval of a given
duration to the next. Instead, ADEV and similar statistical graphs usually portray an entire collection of
deviation measurements, each based on a list of phase or frequency data points spaced at an interval
unique to that measurement.
Imagine that you own a watch that exhibits a certain accuracy in parts per million from one minute to
the next. To arrive at this figure, you’ve checked your watch against a better clock several times, waiting
one minute between each trial, and then calculated the Allan deviation of the resulting list of phase
errors. (You might have used conventional standard deviation, but in this case you’ve heard, correctly,
that STDEV has major drawbacks for timekeeping work.)
Figure 8: Examples of ADEV measurements
Summary of Contents for PHASE STATION 53100A
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