Instability of an atomic clock

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Discussion Overview

The discussion revolves around the characterization of atomic clock stability, specifically the differences between popular representations of clock accuracy and more technical measures such as fractional instability and Allan deviation. Participants explore how these different representations relate to each other and the implications for understanding clock performance over time.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants note that popular papers often state atomic clock accuracy in terms of seconds lost over millions or billions of years, while professional papers use fractional instability measures like 10-14t-1/2.
  • One participant asserts that it is not proper to convert between these two representations, emphasizing the importance of measuring stability using Allan deviation, which requires a plot rather than a single number.
  • Another participant attempts to apply the suggested method by calculating the fractional instability for a 1-second measurement, questioning whether this results in a loss of 10-16% of a second during that time.
  • A later reply corrects the interpretation of the fractional instability, stating that 10-14 corresponds to 10-12% and discusses the limits of accuracy related to measurement time.
  • Participants highlight that the deviation is not fixed and that technical or physical limits influence the minimal deviation achievable.

Areas of Agreement / Disagreement

Participants express differing views on the appropriateness of converting between popular and technical representations of atomic clock stability. There is no consensus on whether the conversion can be made properly, and the discussion remains unresolved regarding the implications of these different measures.

Contextual Notes

Limitations include the dependence on specific definitions of stability and the unresolved nature of how to properly relate different representations of atomic clock performance.

Niles
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Hi

When I read "popular" papers on atomic clocks, many journalists write that the clock loses 1 second in XX million/billion years. But when I look at some professional papers, people talk about a fractional instability of e.g. 10-14t-1/2, the authors never use the former way of characterizing the clocks. How does one go from one "representation" to the other?
 
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You can't. At least not properly.
The correct way to measure the stability of an oscillator is to measure the Allan deviation; this will be in the form of a plot with the fractional stabilty on the y-axis and the integration time on the x-axis. Hence, there is no single number that can be used to characherize an oscillator/clock.

What you CAN do is of course to just take the stability for the integration time used to steer the actual clocks, and then calculate what this means in term of say change per million years.
 
f95toli said:
You can't. At least not properly.
The correct way to measure the stability of an oscillator is to measure the Allan deviation; this will be in the form of a plot with the fractional stabilty on the y-axis and the integration time on the x-axis. Hence, there is no single number that can be used to characherize an oscillator/clock.

What you CAN do is of course to just take the stability for the integration time used to steer the actual clocks, and then calculate what this means in term of say change per million years.

Thanks. The last thing you suggest I tried to do, and I just set t=1 as an example. So I get the fractional instability 10-14 during 1s measurement time. So this means that the clock loses 10-16% of a second during 1 second, right?Niles.
 
10-14 = 10-12% (as 10-2 = 1%). While accurary usually increases with measurement time, there are some technical or physical limits too - and those determine the minimal deviation.
Note that this deviation is not fixed - otherwise you could simply correct the error (and it would not be an uncertainty).
 

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