Bill,
On 05/06/2016 06:22 AM, wb6bnq 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.
We do have an agreed point of reference, the SI second as realized in
the forms of TAI and UTC we can get traceability of measurements all the
way down to parts in 1E-16 which is where the currently best clocks are,
and as a constellation it is even better. It's by far the best
realization of any SI units, and because of that, more and more of the
units becomes re-defined in terms of the SI second and the precision and
accuracy of those will improve. At the same time redefinition of the SI
second is being discussed, as the best optical clocks is now pushing
into the low parts of 1E-18 and we soon expect 1E-19 numbers to be reported.
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.
Yes and no. 100 ppm oscillators have pretty bad temperature coefficient
(tempco) because they don't need to be better. There is also variation
in their manufacturing that is not adjusted for, so that's why they are
cheap.
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.
This is one of the pictures often shown. The more entertaining is a play
with Willhem Tell. :)
Cheers,
Magnus
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