The effects of ambient temperature fluctuations on the long term frequency stability of a miniature rubidium atomic frequency standard

Rubidium gas cell frequency standards display a wide range of performance characteristics. At one extreme are relatively large and heavy standards developed for high performance applications such as GPS satellites. At the other extreme are miniature commercial units with sizes and weights less than 10% of the satellite clocks. Military and commercial satellites have growing needs for accurate frequency and time information. The new generation of miniature rubidium frequency standards (MRFS) appears attractive for future space applications where size, weight, and power considerations are of extreme importance. The focus of the current study has been to understand the origins of the frequency noise typically observed in MRFSs over the medium to long term. We have investigated the frequency stability of a MRFS under varying levels of ambient temperature control. We find that a major contributor to long term frequency noise are ambient temperature fluctuations. Under conditions of tight temperature control, the miniature units tested displayed excellent frequency stabilities, meeting anticipated needs for satellite communication systems. The "off-the-shelf" test unit did not perform well in the vacuum environment. We do not attribute this to its compact size but rather to specifics of its design which was not developed for the vacuum environment. The correlation between the standard's frequency stability and temperature fluctuations suggest that a compensation scheme based upon measurement of the ambient temperature might be useful. Tests performed are consistent with this expectation.