[time-nuts] frequency locking and phase locking in atomic clocks

Attila Kinali attila at kinali.ch
Fri Dec 28 22:10:36 UTC 2012


Common wisdom says that you want to use a PLL to lock two oscillators
together instead of an FLL, to avoid small frequency offsets or
randomly shifting phase.

Almost all atomic clocks use a technique to frequency lock to an
transition frequency of the atom in question. The frequency gets
probed and when it's spot on, the output signal is at a maximum.
This is true for the Cs beam and fountain clocks, the classical Rb,
CPT designs and the passive H maser.

Now, the active H maser is the only one, where the atoms are let
to "oscillate" at their natural frequency and the resulting radiation
is coupled out to a PLL that steers a crystal oscillator.

But i've read that the passive H masers are inherently more stable
than the active H masers.

Could someone enlighten me why this is the case?

			Attila Kinali

There is no secret ingredient
         -- Po, Kung Fu Panda

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