Comment by rfdave

(also a time-nuts lurker, who’d occasionally worked on related things) The actual atomic frequency you’re locking to in an atomic standard is an obnoxious frequency in the GHz Range. For example Cs is 9,192,631,770 Hz. To get a 10 MHz reference frequency out of that requires a very high quality quartz oscillator and a PLL loop, so there’s a PLL right off the bat.