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Comment by lend000 | Hacker News Reader

Nice description. I didn't know atomic clocks just sampled Cesium atoms for a second at a time every once in a while -- I always thought they were somehow continuously amplifying that oscillation. So in reality, they are just recalibrating a more traditional circuit at regular intervals, which serves as the clock for some digital circuit?

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Most atomic clocks work so, i.e. they serve only to calibrate periodically an oscillator, which is the only one which matters for measuring the time during short times, e.g. under an hour.

Many atomic clocks use just quartz oscillators (high quality Oven Controlled Crystal Oscillators).

The more expensive atomic clocks use either dielectric resonators (e.g. sapphire resonators), or superconducting cavities (e.g. from niobium or lead) or optical resonators (e.g. single-crystal silicon or germanium resonators for infrared light) which are cooled to very low temperatures (cryogenic resonators), to ensure much higher quality factors than possible with quartz resonators (the oscillators with cryogenic resonators can also have a better short-term stability than the active hydrogen masers).

The atomic clocks that produce continuous oscillations, so that they do not need another oscillator, are active masers or lasers. While there are many experimental types, the only commercial type are the active hydrogen masers.

The cheaper hydrogen masers are passive hydrogen masers, which are also used only to calibrate periodically another type of oscillator.

Usually something with good short-term stability, e.g. a hydrogen maser, generates a steady signal that can then be measured against a frequency reference such as a caesium fountain, and the output can be adjusted to correct for frequency drift.

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