What is "accuracy"? (newbie timenut, hi folks!)
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
Belinda,
It is often good to ask such questions. So, please do not apologize.
Accuracy is defined as how close a claimed parameter of a device or
system (frequency, time, etc.) is to a standard. This claim has to be
supported by measurements made to a certain precision, (there's that
other word) which is the quantifying of the possible uncertainty in the
measurement.
As with any parameter, the CONDITIONS under which the claim is made can
vary from creator/manufacturer to creator/manufacturer.
For example, I can claim my clock is accurate to 1 part in a trillion
(1e-12) UNDER THE CONDITION that the operating temperature is kept
between 10C and 30C. Outside of those limits, the accuracy is outside
this range.
To make this claim, however, I would have to also make measurements to a
PRECISION that is much smaller than the ACCURACY I claim.
An example of this is measuring the accuracy of my clock's 1 second
pulse. That is, how close it is to "on time" as put out by my country's
bureau of standards. If I claim it is ACCURATE to +/- one millisecond,
but my measurement device can only measure in tenths of a second, i.e.
its PRECISION is in 0.1 seconds, my claim is on shaky ground. On the
other hand, if the precision of my measurements is to one microsecond,
then my claim has merit.
Also I can claim that my clock has this accuracy only 99% of the time
under these conditions. Now we have a more fuzzy boundary.
So, you see that while accuracy itself is well defined, the conditions
under which the claim is made can vary quite a bit.
You are correct that "precision" and "accuracy" are often confused and
misused.
It is good to clarify these two important principles. So, do lots of
reading and research. In a business focused on "precision" and
"accuracy", it is important to be precise and accurate in what you are
thinking/writing/speaking about.
Hi Belinda,
When a manufacturer builds the first N oscillators, they are tested,
usually over a range of temperatures, and some of them are characterized
over a long period of time. They will note the distribution in
oscillator frequency at (say 25 C) and decide where to make a cut, to
accept or reject each. The published frequency accuracy specification
is probably larger than the manufacturing specification limit. For
example, if the published spec is plus or minus 5 ppm, they might reject
any oscillator whose frequency is more than 3 ppm from nominal. From
the distribution one can tell how many will be scrapped. They are trying
to prevent a customer from getting a unit that "doesn't meet spec." As
a customer, if you buy a large quantity and characterize them yourself,
you'll probably see a truncated gaussian distribution.
Usually there's also a drift or aging spec that says that the oscillator
will remain within X ppm of nominal for a given period. Maybe for more
than one time interval, such as one year and ten years.
All these specs apply to a given temperature, but there may be other
specs that address how much the oscillator's frequency will change as a
function of ambient temperature, power supply voltage, changes of
loading on its output, etc.
An oscillator may also have various spectral purity specs, such as phase
noise at various offset frequencies. These address the oscillator's
behavior at much shorter time frames.
Dave
On 2016-05-05 18:34, 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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Well, I'll take a crack at this, although I'm no expert.
I hope it provides a base for others to build on.
First the basics. Accuracy is a property of the thing being measured.
Precision is a property of the measuring instrument.
A digital voltmeter may have a precision of one millivolt and an
accuracy of a tenth of a volt.
You know what the meter reads to one millivolt, but you only know the
voltage to an accuracy of 0.1 volt.
Time and frequency are mathematically related. If you know one, you know
the other.
They can be measured to an accuracy that is very near the precision of
the instrument because there is no analog to digital conversion, as
required by most physical values.
The accuracy is somewhat degraded by the zero crossing detector.
Otherwise, measuring frequency and time is simply a matter of counting
cycles of an oscillator.
A clock is a cycle counter with a fixed period of repetition.
When you want to know the accuracy of a clock with respect to a
standard, you are really interested in how well they match over a period
of time. You can watch your wall clock slow down with respect to WWV or
some other national standard. Then you can say the clock is accurate to
some value of minutes per day or other counts per period of time.
I've never really looked at Allan Deviation, but it seems to be a
statistical method for displaying variations in accuracy with time.
Perhaps ADEV is what you need.
Regards,
Bill Hawkins
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of BJ
Sent: Thursday, May 05, 2016 8:34 PM
To: time-nuts@febo.com
Subject: [time-nuts] What is "accuracy"? (newbie timenut, hi folks!)
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
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