In your FE-5650A rubidium frequency standard, being “locked” means the electronics have successfully tuned the oscillator so that its frequency is being controlled by the rubidium atoms, rather than merely running as an ordinary quartz oscillator.
A simplified picture is:
Power ON → warm up → find rubidium resonance → LOCK → accurate 10 MHz output
Inside the unit, roughly this happens:
- A quartz crystal oscillator generates the basic frequency. Quartz is stable, but it slowly drifts.
- The unit heats its rubidium vapour cell and lamp to their operating conditions.
- The electronics interrogate the rubidium atoms and look for their extremely precise atomic resonance.
- A feedback system—essentially a PLL/servo control loop—adjusts the quartz oscillator until it corresponds with that atomic resonance.
- Once the feedback loop has acquired and is maintaining that condition, the unit is locked.
So there is an important distinction:
10 MHz present ≠ necessarily locked
Your FE-5650A can produce a very nice-looking and apparently stable 10 MHz signal on your Rigol immediately after switch-on, while the rubidium physics package is still warming up. At that stage you’re principally seeing the quartz oscillator.
Once locked, you have:
Quartz oscillator → continuously corrected by rubidium → extremely stable 10 MHz reference
That’s also why your observation that the frequency looked stable on the oscilloscope immediately after power-up doesn’t prove atomic lock. An oscilloscope simply doesn’t have sufficient frequency resolution over a short measurement to reveal the tiny difference.
In your experiments, the particularly interesting moment has been when the 15 V power consumption falls substantially after warm-up and the LOCK/BITE voltage changes state. Those observations give us much more information about whether the rubidium system has actually acquired lock than simply seeing 10 MHz on the scope.
A useful analogy is a quartz wristwatch being continually checked against an extraordinarily accurate atomic clock. Before lock, the watch is keeping time by itself. After lock, someone is continuously comparing it with the atomic reference and making microscopic corrections.
And this connects directly with the PLL you were asking about earlier: the feedback loop is what keeps the oscillator tied—or “locked”—to the atomic resonance.