Hi Belinda,
On 05/06/2016 03:34 AM, BJ wrote:
Hi Time Nuts,
I'm fairly new to the fascinating world of time and frequency, so I
apologise profusely in advance for my blatant ignorance.
We have all been there, so don't worry about it!
Your questions is actually really relevant and you've done your homework.
When I ask "what is accuracy" (in relation to oscillators), I am not asking
for the textbook definition - I have already done extensive reading on
accuracy, stability and precision and I think I understand the basics fairly
well - although, after you read the rest of this, you may well (rightly)
think I am deluding myself. It doesn't help matters when some textbooks,
papers and web articles use the words precision, accuracy and uncertainty
interchangeably. (Incidentally, examples of my light reading include the
'Vig tutorial' on oscillators, HP's Science of Timekeeping Application note,
various NIST documents including the tutorial introduction on frequency
standards and clocks, Michael Lombardi's chapter on Time and Frequency in
the Mechatronics Handbook and many other documents including PTTI and other
conference proceedings). Anyway, you can safely assume I understand the
difference between accuracy and precision in the confused musings that
follow below.
That's not a bad set of readings, so you're doing good there.
BTW. I ended up eating dinner with John Vig this sunday as fellow
time-nut Francis Grosz was meeting up with us both.
For the terms themselves, the real reference you will find at BIPM where
the "International Vocabulary of metrology" VIM (JCGM 200) and
"Evaluation of measurement data - Guide to the expression of uncertainy
in measurement" GUM (JCGM 100) is the real definitions, but at times
people is sloppy about it.
On the other hand, VIM and GUM is sloppy in that it assumes white noise,
where as we know we have non-white noise and have had to invent new
statistical tools such as Allan Deviation and Modified Allan Deviation,
kind of ironic but that's where it is.
David Allan and I had a discussion about it the other day and agree that
something needs to be done about it. It's an interesting topic in its
own right, so let's come back to it later.
Being at the International Frequency Control Seminars (IFCS) several
presentations start with illustrations of accuracy and precision, some
even with nice cartoons, just to make the point again and again... so
that nobody confuses the two.
What I am trying to understand is, what does it REALLY mean when the
manufacturer's specs for a frequency standard or 'clock' claim a certain
accuracy. For ease and argument's sake let us assume that the accuracy is
given as 100 ppm or 1e-4 ....
As per the textbook approach, I know I can therefore expect my 'clock' to
have an error of up to 86400x1e-4= 8.64 s per day.
That would be accuracy, as it is about how good the average aim is.
But does that mean that, say, after one day I can be certain that my clock
will be fast/slow by no more than 8.64 seconds or could it potentially be
greater than that? In other words, is the accuracy a hard limit or is it a
statistical quantity (so that there is a high probability that my clock will
function this way, but that there is still a very small chance (say in the
3sigma range) that the error may be greater so that the clock may be
fast/slow by, say, 10 seconds)? Is it something inherent, due to the nature
of the type of oscillator (e.g. a characteristic of the crystal or atom,
etc.) or does it vary so that it needs to be measured, and if so, how is
that measurement made to produce the accuracy figure? Are environmental
conditions taken into account when making these measurements (I am assuming
so)? In other words, how is the accuracy of a clock determined?
By tradition and also put into standard, as far as I recall it, the
given accuracy is a 3-sigma value, that is 99,7% of all devices is
within this accuracy limit.
To the degree environmentals is taken into it can... vary. Some
sales-men take a more opportunistic approach to it, but there is
standards for what goes where. I'm not the master of the standards, but
I'm sure fellow time-nuts Bernd Neubig can enlighten us, as he was
awarded for his 40 years of work, and in particular his many
contributions to the standards.
Note that I am conscious of the fact that I am being somewhat ambiguous with
the definitions myself. It is my understanding that the accuracy (as given
in an oscillator's specs) relates to frequency - i.e. how close the
(measured?) frequency of the oscillator is to its nominal frequency - rather
than time i.e. how well the clock keeps time in comparison to an official
UTC source.... but I am assuming it is fair to say they are two sides of the
same coin.
Indeed. The precision given is relating to it's frequency value. Then,
as you measure it for some time you integrate it up to phase and time,
but that will also integrate environmentals which is a separate
systematic effect.
Does accuracy also take stability into account (since, clearly, if an
oscillator experiences drift, that will affect the accuracy - or does it?)
or do these two 'performance indicators' need to be considered
independently?
Frequency-offset and drift is relatively easy properties to establish
and compensate, it's systematics. The stability is a random variations,
the noise processes that goes on top of that. If you look into the VIM
and GUM you discover the Type A and Type B types of precision, and it is
exactly about this where you have random noise vs. systematic effects.
I am guessing that the accuracy value is provided as general indicator of
oscillator performance (i.e. the accuracy does REALLY just mean one can
expect an error of up to, or close to?, a certain amount) and that stability
(as indicated by the ADEV) is probably more significant/relevant.
They are quite distinct properties, and very relevant as separate
properties.
If you take a SEC/Stratum 3 oscillator, it should be within +/- 4.6 ppm,
including the linear drift for say 15 years. It then have stability as
indicated by ADEV, or actually by the TDEV limit curve. The actual specs
is in ITU-T Recommendation G.813.
It is also entirely possible I am asking all the wrong questions. As you can
see, confusion reigns. I am hoping things will become clearer to me as I
start playing around with hardware (fingers and toes crossed on that one).
You ask very relevant questions. You are on a good path to learn more,
and you have lots of fun in front of you! :)
In the meantime, if anyone could provide some clarity on this topic or set
my crooked thinking straight, my gratitude will be bountiful.
That is what we are happy to help with here, so don't worry about it.
Keep the questions going and remember that there is at least 10 others
silently reading this list that will learn from the discussion.
Cheers,
Magnus
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