[time-nuts] frequency stabilty question

Bob Camp lists at rtty.us
Mon Aug 15 16:08:03 UTC 2011


The "how long can I go?" part of the question goes immediately to "how good
do I want?". For a good confidence level, you might want 100 samples at your
longest Tau rather than 5...
For long Tau, that can indeed get pretty nasty. 100 samples at 10,000
seconds is a very long time. It is worth keeping in mind when you look at
most plots. The confidence drops off significantly at the longer Tau. 


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Tom Van Baak
Sent: Monday, August 15, 2011 11:42 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] frequency stabilty question

> Is it correct to assume that if you collect data every 10 ms for
> 10 seconds (1000 data points in total), you could actually split
> that data set into 100 sets of data at 1 seconds for 10 seconds
> (10 data points in each set), which would have the same statistical
> quality (I am sure caveats would apply) as a single data set of
> 1000 data points collected at one second interval over a total
> period of 1,000 seconds?
> Didier KO4BB

If you look every 10 ms and collect data for 10 seconds you will
be able to make a log-log ADEV plot for points from 0.01 s to 2
seconds, no more, no less.

In order to plot ADEV points less than tau 0.01 s you need to
re-measure using an instrument that gives data faster than one
every 10 ms.

In order to plot ADEV points more than tau 2 seconds you simply
need to let the measurement run much longer than 10 seconds.
For example, if you measure for 4 or 5 hours then you can plot
ADEV out to 1 hour.

In other words, the leftmost point on any log-log ADEV plot is
determined by how *fast* you can collect data and the rightmost
point is determined by how *long* you collect data.


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