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Theory of
rubidium oscillators
Precision
Test Systems manufacturers various types of rubidium
frequency standards. The heart of our frequency
standards is a rubidium frequency reference.
This
document gives a brief outline of the theory of rubidium
oscillators.
Rubidium
is an alkali metal (like lithium, sodium, potassium and
cesium). There are two naturally occurring isotopes of
rubidium, Rb85 and Rb87, which have relative abundances
of 72% and 28% respectively. The metal has a melting
point of 39°C.
The
alkali metals behave similarly: they have one electron
outside an inert core. Most of the chemical, electronic
and spectroscopic properties of these elements are
determined by this outer electron. The deep red glow of
a low power rubidium discharge lamp is due to the
resonance line transitions of the outer electron as it
emits a red photon and drops back to the ground state.
The
ground state of Rb87 is split by a very small energy due
to the relative orientation of the magnetic spins of the
electron and the nucleus. The split corresponds to the
energy of a photon with a (microwave) frequency of
6.834,682,612,8 GHz. It is this hyperfine transition
frequency which is used to stabilize the 10 MHz output
of our rubidium frequency standards.
The
physics package consists of a discharge lamp, an
isotopic filter, and a resonance cell. The amount of
light which passes through the resonance cell to the
photodetector can be reduced when the resonance cell is
exposed to microwaves at the hyperfine transition
frequency.
If we
assume (to make things easier) that the light from the
Rb87 discharge lamp consists of just two lines
corresponding to transitions from a single excited state
to the split ground state. The filter cell contains Rb85
vapor which also has a split ground state and an
isotopic shift (relative to Rb87) as well. An important
coincidence exists: one of the lines from the Rb87
discharge corresponds to one of the transitions in Rb85.
This allows the intensity of this line to be reduced, by
passing the Rb87 discharge light through the Rb85 vapor.
Normally, atoms in the ground state will be equally
distributed between the split states, as the splitting
is much less than the thermal energy of the atoms in the
vapor. This distribution is modified by the filtered
light from the discharge, by a process called “optical
pumping”.
Let
suppose the filter can completely remove one of the two
discharge lines. The remaining light can be absorbed by
Rb87 atoms in the resonance cell which are in the lower
ground state, moving them to the upper state. When they
decay from the upper state, they fall with equal
probability into either ground state. As this continues,
population will be moved from the lower ground state to
the upper ground state, circulating through the upper
state. As the population in the lower ground state is
decreased, the amount of light which reaches the
photodetector will increase, as the number of atoms
which can absorb the radiation is
reduced.
Now, if a
microwave field is applied at the frequency
corresponding to the hyperfine transition frequency
(6.834,682,612,8 GHz), the populations in the ground
state will mix, and the amount of light reaching the
photodetector will decrease.
Precision
Test Systems uses an “integrated filter” topology:
rather than a separate filter cell, the resonance cell
contains a mixture of the two rubidium isotopes, along
with a buffer gas. The lamp also contains a mixture of
isotopes. The isotopic mixtures, buffer gases, and
operating conditions are chosen so as to minimize
temperature coefficients and intensity shifts of the
apparent hyperfine transition frequency.
The
apparent transition frequency will be shifted by about 3
kHz from the natural transition frequency by the buffer
gas and discharge lamp spectral profile. The transition
frequency differs slightly for each unit, depending on
the fill pressure, etc. The transition frequency is also
tuned over a few Hertz by a magnetic field which may be
varied.
In our
rubidium, the physics package acts as a very stable
frequency detector for a frequency around 6.834 GHz. By
using a microwave frequency synthesizer which is
referenced to a 10 MHz OCXO, the 10 MHz may be
stabilized to the rubidium hyperfine transition
frequency.
Our
rubidium exhibits exceptional low phase noise, up to 30
dB lower than many competitors. Coupled with our
ultra low noise distribution amplifiers, our rubidium
frequency standards can be used in the most demanding
situations.
Also all
our rubidium oscillators have been designed for a 20
year life, typically double that of most other rubidiums.
And all
this is achieved and a cost effective price. Email
Precision Test Systems for more information.
Click here to be taken to our
frequency standards web page.
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