Here is an clip from one of the space qualified ones Bob mentioned. "The RAFS employs classical rubidium gas cell atomic frequency standard principles. It utilizes a physics package with a discrete isotopic filter cell for best stability. The relatively large, cool absorption cell provides exceptionally high signal-to-noise ratio and excellent short term stability. The addition of a thin film spectral filter increased the signal-to-noise ratio even higher." Highlights are: -"classic" architecture, meaning a separate discrete filter cell. -"large cool absorption cell", the 5065A cell is large and runs at around 65 degrees C. -"thin film spectral filter", this is what the "super" mod adds to the 5065A Telecomm comparisons are: (not universal) -small size combined filter and absorption cell, This requires a "hot" cell, well above 65 degrees. -only a few have spectral filters. Conclusions: - A bigger cell is more stable than a smaller - a cooler cell is more stable than a hotter - a separate discrete filter sell allows higher stability than a combined arraignment. - These requirements run counter to the telecomm requirements of low power, small size, fast warmup, and LOW cost. Cheers, Corby