Frequency Stability
How much a clock’s frequency changes over a stated time interval or operating condition.
Stability measures variation, not the initial offset from the intended frequency. A clock can be stable while running slightly away from its nominal rate.
Frequency Stability — External Clock
For an External Clock, stability indicates how consistently its timing reference behaves under a specified basis. That basis might be a temperature range, ageing over a day or year, or short-term variation measured over a defined interval.
Read each figure with its basis. A one-second Allan-deviation value, annual ageing figure and temperature tolerance describe different behaviours and cannot be ranked as though they were the same measurement. Smaller variation is better only within a comparable specification.
Keep units and conditions attached when comparing products. A parts-per-billion figure without its interval or temperature range is incomplete. Stability alone does not describe absolute frequency accuracy or all timing noise, and it cannot establish an audible benefit from using an external clock.