On 8/14/11 8:10 PM, Paul Cianciolo wrote:
Folks,
I amtrying to understand some of the terms used here quite often.
I quoted this from Wikipedia
An Allan deviation of 1.3×10−9 at observation time 1 s (i.e. τ = 1 s) should be
interpreted as there being an instability in frequency between two observations
a second apart with a relative root mean square(RMS) value of 1.3×10−9.
Does this mean the observations made were at the very begining and the very end
of the 1 second time.
If so what value about all the values in between? What happens if the
oscillator deviated far worse than this during the interrim.
A measurement at tau=1 second doesn't say anything about what happened
at shorter intervals.
Or does the measurement consist of making measurements every cycle during that
1 second and then entering all those values into a formula that accounts for
them all??
Nope..
It's if you measured the frequency (instantaneously) at one second
intervals, and calculated the standard deviation, that would be the ADEV
for tau=1 second.
measure at 10 second intervals and you get ADEV(Tau=10sec), etc.
That's why you typically see an ADEV as a series of performances at
different taus.
You can also fill in the gaps in the curve, to a certain extent, because
physical oscillators have constraints on what the ADEV can do (i.e.
you're not likely to see 1E-9 at tau=1 second, 1E-5 at tau=2 seconds,
1E-10 at tau=3 seconds)
In fact, if you do the ADEV measurement and it's NOT a nice curve and
has spikes and weirdnesses, that starts to tell you have either a
measurement system problem or a problem with your frequency standard.
(sort of like spurs in a phase noise plot from 120Hz line interference,
or reference clock leakage)
As an example of measurement system problems, it's pretty common to see
a "hump" in ADEV around 500-1500 seconds (around 15-20 minutes) because
of temperature effects on the test equipment or unit under test as the
airconditioning/heating cycles on and off.
Maybe a very basic tutorial on this topic would help but I cant find one
Signed very confused,
Thank You
PauLC
W1VLF
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