[time-nuts] frequency stabilty question
Bob Camp
lists at rtty.us
Mon Aug 15 16:01:11 UTC 2011
Hi
One more nit to pick...
ADEV looks at frequency differences. You would take the standard deviation
of the delta frequencies (F0-F1 etc) rather than the standard deviation of
the frequency (F0,F1...) readings.
Bob
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Jim Lux
Sent: Monday, August 15, 2011 9:31 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] frequency stabilty question
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. t = 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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