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 <[email protected]> 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 >> >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
