Characteristics and performance of miniature NBS passive hydrogen masers

Recent data on the miniature (30 kg) passive hydrogen masers developed at the National Bureau of Standards (NBS) in cooperation with the Naval Research Laboratory (NRL) indicate a frequency stability of σ²y(τ) ≤ (1.5 × 10⁻¹²τ^−1/2)² + (5 × 10⁻¹⁵)² 1 s < τ < 5 × 10⁵ s. These masers also have extremely low sensitivity to changes in the external magnetic field or temperature. The sensitivity to barometric pressure and or humidity, although very small, does dominate the residuals in one of the miniature masers for times beyond about a week. Frequency drift is so small as to be hidden in the present measurement precision. The fractional reproducibility under all changes, excluding the storage bulb, appears to be better than 5 × 10⁻¹³. The concepts behind the physics and the electronics that made these advances possible as well as present limitations are explained in some detail.