[time-nuts] frequency stabilty question

[Paul A. Cianciolo]

Hi Tom,

Ok .. what you say makes good sense.  If one is interested in the
performance over a specific period of time,  than do a test with that
specific period of time in mind.
I notice in a later email you posted a chart showing the ADEV beginning .01
seconds.  As expected it is worse at that short interval and gets better as
the averaging time increases.

This seems to me to be a little like and aliasing problem in the video
world.  If you are recording a rotating wheel(say at 1 rev per second) with
an index mark to indicate when the wheel is at zero degrees,  the sampling
rate had better not be 2 X the rotating rate or the wheel will appear to not
rotate. And who knows what happened between the 2 samples.  If you need to
find out what happened in the period between rotations one will need to
increase the sampling rate.

So use the proper Tau for the information you wish to acquire. Or use a
group of readings as in your Chart.

Thank you Tom,


Paul A. Cianciolo
W1VLF
http://www.rescueelectronics.com/
Our business computer network is  powered exclusively by solar and wind
power.
Converting Photons to Electrons for over 20 years



-----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 2:20 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] frequency stabilty question

> Does this mean the observations made were at the very begining and the 
> very end of the 1 second time.

Correct.

> If so what value about all the values in between? What happens if the 
> oscillator deviated far worse than this during the interrim.

Oscillator behavior in between 1 second intervals is unknown. As you say it
could be much worse, or it could be much better. If you need to know for
sure, then you must re-run the measurement and collect data at shorter
intervals this time.

Specifically, if you make only one measurement per second you can compute
ADEV(1 second), or actually ADEV(N seconds).

By contrast, if you collect 100 measurements per second you can compute
ADEV(0.01 x N seconds). This allows you to see how well the oscillator
performs for intervals less than a second.

> 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??

If you had measurements every cycle it would be great. But not all
measurement systems can measure every cycle. In the real world there are
limitations on the rate of measurement. There are also limitations on the
resolution of each measurement.
With some instruments you can increase the rate but you then loose
resolution, or visa-versa.

These two limits dictate how short a tau you can plot and how low an ADEV
you can observe. I can provide examples if you wish.

/tvb

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