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

[wb6bnq]

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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