[time-nuts] Characterising frequency standards
Tom Van Baak
tvb at LeapSecond.com
Thu Apr 9 14:39:58 UTC 2009
> I think the penny has dropped now, thanks. It's interesting that the
> ADEV calculation still works even without continuous data as all the
> reading I have done has led me to belive this was sacrosanct.
We need to be careful about what you mean by "continuous".
Let me probe a bit further to make sure you or others understand.
The data that you first mentioned, some GPS and OCXO data at:
was recorded once per second, for 400,000 samples without any
interruption; that's over 4 days of continuous data.
As you see it is very possible to extract every other, or every 10th,
every 60th, or every Nth point from this large data set to create a
smaller data set.
Is it as if you had several counters all connected to the same DUT.
Perhaps one makes a new phase measurement each second,
another makes a measurement every 10 seconds; maybe a third
counter just measures once a minute.
The key here is not how often they make measurements, but that
they all keep running at their particular rate.
The data sets you get from these counters all represent 4 days
of measurement; what changes is the measurement interval, the
tau0, or whatever your ADEV tool calls it.
Now the ADEV plots you get from these counters will all match
perfectly with the only exception being that the every-60 second
counter cannot give you any ADEV points for tau less than 60;
the every-10 second counter cannot give you points for tau less
than 10 seconds; and for that matter; the every 1-second counter
cannot give you points for tau less than 1 second.
So what makes all these "continuous" is that the runs were not
interrupted and that the data points were taken at regular intervals.
The x-axis of an ADEV plot spans a logarithmic range of tau. The
farthest point on the *right* is limited by how long your run was. If
you collect data for 4 or 5 days you can compute and plot points
out to around 1 day or 10^5 seconds.
On the other hand, the farthest point on the *left* is limited by how
fast you collect data. If you collect one point every 10 seconds,
then tau=10 is your left-most point. Yes, it's common to collect data
every second; in this case you can plot down to tau=1s. Some of
my instruments can collect phase data at 1000 points per second
(huge files!) and this means my leftmost ADEV point is 1 millisecond.
Here's an example of collecting data at 10 Hz:
You can see this allows me to plot from ADEV tau = 0.1 s.
Does all this make sense now?
> What I now believe is that it's possible to measure oscillator
> performance with less than optimal test gear. This will enable me to
> see the effects of any experiments I make in the future. If you can't
> measure it, how can you know that what your doing is good or bad.
Very true. So what one or several performance measurements
are you after?
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