The Physics of the Environmental Sensitivity of Rubidium Gas Cell Atomic

Environmentalsensitivity is of&n the most significant limitation to the practicalstability oJrubidium frequency standards(RFS). For example, temperuture sensitivity can cause a rapid frequency change of several parts in 10'' for a tactical RFS that has an aging of only l ~ l O ~ ~ / m o n t h . Ot er importunt envirorzmerztul factors w e buromtric pressure, vibrution, magneticfield, and nuclear rudidion. This paper considers the physical mechanisms that lie behind these environmental sensitivities, und rehtes them to the performance of actual rubidium frequency standards. It i.v part of an efSort currently underway under NIST and IEEE sponsorship toward a stundard charucterizing such environmentul sensitivities. For the systems designer, a better understanding of the reasons for RFS environmental sensitivity will he& in making program tradeofi. For the user of these devices, a better knowkdg of the causes for Rb clock instability will aid in their testing and proper application. For the time and frequency specialist, a review of these factors may prove useful toward improving RFS design. Some of the M S environmental semitivities are due to simple physical mechanisms like the effect of ak magnetic$eld on the Rh hyperjne resonance frequency. For these, an unulysis cur2 be bared on physicul principles and struighforward design factors. Other environmental factors, like temperuture sensitivity, ure more complex combinations of marly eflects, both physicul und prudicul, and the urzalysis often takes the form of an error budget with lurge iirzit-to-unit variutions. Today's rubidium frequency stundurds span u wide performunce rungeJrom smull, inexpensits units with pp10" error budgets to larger, higher performance versions 0 f f e r i n ~ ~ ~ l 0 ~ ~ stubilities. For both extremes, however, environmentul sensitivib can be the most sign$cuwt pe~jiormunce limitation. This paper helps explain why, und offers some insight into how to make improvements.