Effects of Helium Exposure on the Frequency Output of a Space System OCXO

The output frequency of a space system oven controlled crystal oscillator (OCXO) was measured as a function of helium gas exposure pressure and baseplate temperature. The exposure of an OCXO to ambient gasses during storage, transport or mating to the launch vehicle has the potential to effect on orbit performance. The current study examines the physical basis for the observed frequency output changes. Helium diffusion into a sensitive component of the OCXO was determined to be the most probable source of the frequency shifts. It was observed that after temporary gas exposure the OCXO exhibits a change in frequency output on the order of 10−10, but returns to nominal operation after only several hours. The magnitude and transient nature of the disruption suggests that a similar event occurring during a mission could be corrected with minor adjustments to the OCXO control voltage.