Daphna G. Enzer, David W. Murphy, Eric A. Burt, Jet Propulsion Laboratory, California Institute of Technology

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We have improved an atomic-clock model discussed in [1] to calculate a new noise metric, useful for characterizing clock-environment interactions. Atomic clocks operate by comparing a local oscillator (LO) to an atomic frequency standard at regular intervals and applying corrections each iteration. We found that the Allan deviation (ADEV) [2] of these corrections reveals different information than the more typically reported performance metric: the ADEV of clock-output frequencies. Either of these ADEVs can be raised above its noise floor due to imperfect LOs or due to the numerous disturbances encountered in space clocks, for example. In this talk, we specify the relationship between the two ADEVs, in an effort to help better identify the source of such excursions. First, we derive the expected shot-noise limited noise-floor for this new ADEV. We also derive the noise floor of the more widely used output-ADEV, and validate both with our computer model. Next, we examine model results that diverge from either noise floor by quantifying the size of expected ADEV peaks (on both) due to sinusoidal disturbances. Then we do the same for disturbances due to drift rather than sinusoids. Importantly, these lead to a simple diagnostic: we can now compare the size of any peak or drift on the two ADEV plots to glean more robust information about the source of the disturbance. LO disturbances appear similarly on the two ADEV plots, but ion reference frequency disturbances appear much more prominently on the output-ADEV. This new way to analyze ADEVs will help with identifying the source of environmental disturbances in clocks, and perhaps aid in designing future clocks to be less sensitive to them. [1] D. G. Enzer, W. A. Diener, D. W. Murphy, S. R. Rao, R. L. Tjoelker “Drifts and environmental disturbances in atomic clock subsystems: quantifying local oscillator, control loop, and ion resonance interactions,” IEEE Trans. Ultrason., Ferroelectr., Freq. Control (TUFFC), vol. 64, no. 3, pp. 623-633, Mar. 2017. [2] D. W. Allan, “Statistics of Atomic Frequency Standards,” Proceedings of the IEEE, vol. 54, no. 2, pp. 221–230, Feb. 1996.