Power Supply

Power supply variations can induce frequency changes in at least two ways. Residual voltage changes after imperfect voltage regulation cause shifts by means of voltage coefficients. LOs with voltage-variable tuning capacitors are particularly susceptible. Input voltage variations will cause variations in the power dissipated in the voltage regulators, and the resulting temperature changes can induce frequency changes.

Good quartz LOs have typical voltage coefficients of ±5 × 10−11 per volt. Rubidium gas cell devices are perhaps five times better. The best cesium beam standards will change less than ±1 × 10−13 over the full voltage range.

Radiation

In space-based clocks, radiation effects are a significant determinant of clock performance and lifetime. Background radiation particularly affects the performance of the quartz crystals, whether they are used as clocks or as the LO, by inducing defects in the crystals over time.

FIGURE 2.3 Short-term stability of some state-of-the-art standards.



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