Plugged my rubidium frequency standard in again to check some stuff on the board, and this time I had the oscilloscope hooked up to the sinewave output during its startup phase. And it's neat, you can see it do the thing!

This module works by looking for the frequency of the hyperfine transition of rubidium atoms. It uses a lamp to shine microwave energy through a little glass cell that contains a vapour of rubidium atoms. A photodetector on the other side measures how much microwave energy makes it through the cell.

When the frequency of the emitted microwaves exactly matches the frequency of rubidium's hyperfine transition (a little over 6.8GHz) the rubidium atoms absorb a tiny amount of the light. This shows up as a minuscule dip in how much the photodetector sees. So, effectively, the "physics package" is a black box where you feed in a frequency of microwaves to generate, and you get back a signal that dips a tiny bit when the input frequency is bang on target.

The module uses a more ordinary (but still pretty fancy) 60MHz crystal oscillator to generate the input frequency to this "physics package". On startup, the module sweeps the oscillator through a range of frequencies around where the hyperfine transition should be, looking for that telltale dip in the output. When it finds the dip, it locks onto it and keeps the oscillator at exactly the frequency required to stay there. It now has an extremely precise 60.0000...MHz clock, which it can further divide by six to produce its lab-standard 10MHz reference sine wave.

And you can see this happen! During module startup, before it reports that it's locked on, the oscilloscope's frequency counter shows the sinewave output sweep back and forth multiple times from ~9.9990MHz to ~10.0005MHz, looking for the correct frequency. After a few minutes, the sweeps get narrower and slower, and finally stop on 10.0000MHz a few seconds before the "Locked" LED lights up.

I expect it's even more impressive when you have a better frequency counter with more digits, it must be extremely satisfying to see the increasingly tiny fractions slowly get nudged towards a perfect zero.