Re:For precision, the sapphire clock outshines even the best...
by adrian_b on Saturday August 21, 2021 @02:35AM
(#28254786)
What they meant by accuracy and precision was actually about which is the average value and which is the variance of the duration of a clock period.
What you want is that both the average duration of a clock period is exactly a certain fraction of a second and also its variance is very small.
However, you cannot have both in the same clock device.
For atomic clocks, you know very well which is the average value, but the duration of the clock periods has a large variance.
For sapphire clocks, the variance of the duration of the clock periods is very small but the average value is known only approximately.
According to the terminology used in the article, someone who shoots accurately spreads the shots over a large area, but the center of the area is the same as the center of the shooting target.
On the other hand, someone who shoots precisely sends all the shots through the same hole in the shooting target, but that hole is far from the center of the target.
The result of the difficulty of having both high accuracy and high precision in the same clock device is that whenever you measure short times, you must use either quartz clocks for a cheaper solution or sapphire clocks for higher precision (or also hydrogen masers in the past, before the development of sapphire clocks).
Because the average frequency of the precise clock is not known, you must either measure it periodically with an atomic clock and correct digitally the measurement results or you must use a PLL loop to adjust continuously the frequency to be in a certain ratio with the average frequency of the atomic clock.
What you want is that both the average duration of a clock period is exactly a certain fraction of a second and also its variance is very small.
However, you cannot have both in the same clock device.
For atomic clocks, you know very well which is the average value, but the duration of the clock periods has a large variance.
For sapphire clocks, the variance of the duration of the clock periods is very small but the average value is known only approximately.
According to the terminology used in the article, someone who shoots accurately spreads the shots over a large area, but the center of the area is the same as the center of the shooting target.
On the other hand, someone who shoots precisely sends all the shots through the same hole in the shooting target, but that hole is far from the center of the target.
The result of the difficulty of having both high accuracy and high precision in the same clock device is that whenever you measure short times, you must use either quartz clocks for a cheaper solution or sapphire clocks for higher precision (or also hydrogen masers in the past, before the development of sapphire clocks).
Because the average frequency of the precise clock is not known, you must either measure it periodically with an atomic clock and correct digitally the measurement results or you must use a PLL loop to adjust continuously the frequency to be in a certain ratio with the average frequency of the atomic clock.