Somewhat off-topic:<p>I’ve often wondered if a hobby-class atomic clock can be built with “a less accurate gas” that is easy to excite and measure in a feedback loop simply because it’s available in a handy package that lends itself for experimentation without having to mess with melting glass and bottles of pressurised gas. E.g. neon, nitrogen or mercury vapour.<p>The reason I’m asking is because in RF we often need a stable reference, and these come in a clear $ for phase noise relationship: RC, LC, xtal, TCXO, GPSDO, YIG, Rubidium, …<p>Price-wise, all atomic clocks come after Rubidium. But would it be possible to build an atomic clock that sits between TCXO and Rb both for price and phase noise, by employing a non-exotic gas in a readily available lamp?
There are different types of atomic clocks, but in most common types, the output comes from a crystal oscillator, or more generally frequency synthesizer, which is then slaved to some spectral feature in the "physics package". That is to say, the phase noise is as good as that of the crystal in the unit, but the longer term frequency stability is much improved by the slaving.<p>An exception is an active hydrogen maser, which directly outputs the frequency of atomic transition. It has very good phase noise, but is a rare beast, which is only used where it is absolutely necessary.