[time-nuts] What is "accuracy"? (newbie timenut, hi folks!)

Azelio Boriani azelio.boriani at gmail.com
Fri May 6 08:19:30 UTC 2016


For a visual feeling of what are accuracy and stability see page 6 of this PDF:
<http://www.ieee-uffc.org/frequency-control/learning/pdf/Lutwak_AtomicClocks_Tutorial_pdf.pdf>


On Fri, May 6, 2016 at 6:22 AM, wb6bnq <wb6bnq at cox.net> wrote:
> Hello Belinda,
>
> First off there is no such thing as accuracy, in and of itself.  I know many
> people on this list will call me on that, but accuracy requires a point of
> reference.  With regard to frequency that reference point has been defined
> by some World committee as a certain number of oscillations in a Cesium atom
> controlled within a specific set of conditions.
>
> The 100 ppm statement is talking about a change in frequency due to
> temperature,  The typical 100 ppm statement is saying for every change of 1
> degree (usually "C") the oscillator (or other components such as resistors,
> capacitors, etc.) will shift in value by a worst case of 100 ppm (parts per
> million).  This has nothing to do with accuracy except that it would not be
> considered accurate relative to a reference point.  What it does address is
> specifically the stability, but is not the only condition affecting
> stability.
>
> With respect to accuracy and stability, they are not related.  That is to
> say you could have extreme stability (say parts in 10 to the minus 21) and
> it could be way off from the recognized standard reference.  In the other
> direction you could have something that is adjusted to be precisely in
> agreement with the reference standard but will only hold that value for a
> very brief period of time.  The first case is a very good (and quite
> expensive) oscillator and the second example is a poor (and not expensive)
> oscillator.
>
> With regard to precision, the best example would be shooting at a target and
> how tight the grouping is maintained.  The tighter the grouping the better
> the precision.  You could have a tightly well defined small group of holes
> from the bullets but they could be anywhere on the paper target.  The only
> time you have accuracy (with respect to the shooting) is if all the bullets
> were directly in the bulls eye (center of target).  And if you can
> consisterntly repeat hitting just the bulls eye then you have stability.
>
> Bill....WB6BNQ
>
>
> 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.
>>
>>
>>
>> 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.
>>
>>
>>
>> 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.
>>
>>
>>
>> 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?
>>
>>
>> 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.
>>
>>
>> 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?
>>
>>
>> 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.
>>
>> 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).
>>
>>
>>
>> In the meantime, if anyone could provide some clarity on this topic or set
>> my crooked thinking straight, my gratitude will be bountiful.
>>
>>
>> Thanks.
>>
>>
>>
>> Belinda
>>
>>
>>
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>
>
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