097-10603-01 Rev. A – November 2004 X72 Desiger’s Reference and User’s Guide 47
Installation and Operation
Theory of Operation
Theory of Operation
The Model X72 makes use of the atomic resonance property of rubidium (
87
Rb) to
control the frequency of a quartz crystal oscillator via a frequency-locked loop (FLL).
The FLL function block is shown in Figure 3-3. A microwave signal is derived from a
voltage-controlled crystal oscillator (VCXO) and applied to the
87
Rb vapor within a
glass cell. The light of a rubidium lamp also passes through this cell and illuminates
a photo detector. When the frequency of the applied rf signal corresponds to the
frequency of the ground-state hyperfine transition of the
87
Rb atom (an ultra-stable
high-Q rubidium atomic resonance), light is absorbed causing a change (decrease)
in photo detector current (I
PH
).
As the change in current is small, modulation techniques are required to be able to
extract the desired signal out of the noise background.
The photo detector current is used to generate a control signal with phase and
amplitude information, which permits continuous regulation of the VCXO frequency.
The servo section converts the photo detector current into a voltage, then amplifies,
demodulates, and integrates it for high dc servo loop gain.
Figure 3-3 X72 Rb Control Loop Block Diagram
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