Bob,
On 10/25/2014 08:15 PM, Bob Camp wrote:
Hi
On Oct 25, 2014, at 12:19 PM, Magnus Danielson <mag...@rubidium.dyndns.org>
wrote:
Bob,
On 10/25/2014 02:02 PM, Bob Camp wrote:
On Oct 24, 2014, at 6:25 PM, Magnus Danielson <mag...@rubidium.dyndns.org>
wrote:
Tom,
On 10/24/2014 11:31 PM, Tom Van Baak wrote:
ADEV most certainly does change with time, even for short tau's.
Can you elaborate?
Such as when, why, what kind of change, how much change,
at how short of tau's, over how long of time,
and using what type Oscillators?
Do you know what in the freq or Phase plot is causing the ADEV to change?
I'm happy to let Bob answer his own claim here. I'm curious as well. Unless
he's talking about thermal noise, in which case I now believe him 100%.
OTOH, for time intervals of minutes to hours or days, the plotted ADEV can often vary.
When in doubt, enable error bars in your ADEV calculations or use DAVAR in Stable32, or
use "Trace History" of TmeLab to expose how little or much the computed ADEV
depends on tau and N.
In general, never do an ADEV calculation without visually checking the phase or
frequency time series first.
You should make sure that you remove all forms of systematic effects before
turning the residue random noise over to ADEV.
If you have random noise being modulated in amplitude, you need to measure long
enough for the averaging end not to have a great impact on the result.
Is days long enough for a 1 second tau? If you define 1,000 x tau as “long
enough” you are being way more
conservative than just about anybody out there. My claim is that rather than
telling everybody to run for 10,000 or
100,000 x tau, simply accept that ADEV does / may change.
I did not say that you need to do 1000xtau, that was what someone else said. If
you paid attention I said that the number of samples N and the tau0-multiple m
for a particular dominant noise (of that tau) creates a certain degree of
freedom for a particular ADEV estimator algorithm. Discussing the length of the
measure without discussing which estimator algorithm you're using and what
confidence interval you aim to reach is just taking a single value and run with
it without thinking about it.
For ITU-T telecom standards, the measurement length is 12 times the maximum
tau, using the overlapping estimator (see O.172, §10.5.1 for limit and G.810
§II.3 for TDEV algorithm). That was chosen to ensure comparability between
different implementations for the same type of measure. See O.172 for other
relevant details on limits for implementation, tau0 has an upper limit, so does
bandwidth. Naturally, these limits is for this specific purpose, algorithms
etc. which may not fit the needs of other needs or choices.
If you are using under 100 samples for the test (overlapping or not), your
confidence is not as high as it might be.
You can see ADEV “drift in” over a period of days, even with a lot more than 10
samples.
Yes. One needs to look at what happens to judge when you can trust the
values. In the standard case, there would be a lot of samples, with a
minumum of 360 for the extreme-case.
Yes indeed you can find FCS papers with all sorts of interesting “adjustments”
or no processing at all. The consensus
seems to be that if you go past drift correction, you really should have a
footnote.
When you do not make a drift compensation, and that line shows up, you better
explain that too.
In the end, ADEV is overused to represent things for which it is not a good
tool. You will need other tools in the tool-box to build a good estimation of
how that oscillator will behave at some tau.
Except that ADEV is used by many as an acceptance test on systems and
oscillators. Saying it’s OK to pull data out
of a test run makes for a very interesting test design. We certainly use ADEV
(without subtractions) here on the list
to compare things like GPSDO’s at the system level.
I use ADEV, TDEV, phase-plot and frequency plot to best illustrate and
understand what is happening. Would be using FFT for long-term if only
TimeLab would support it for normal counter measures. Would be using
phase-noise more if I had a TimePod at work.
Cheers,
Magnus
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