Model
E10-GPS
Operation Manual
E10-GPS Manual A5 4 December 2013
Page 6
2 Rubidium Frequency Reference
2.1 Rubidium Frequency Reference
A Rubidium frequency reference owes its outstanding accuracy and superb
stability to a unique frequency control mechanism. The resonant transition
frequency of the Rb 87 atom (6,834,682,614 Hz) is used as a reference
against which an OCXO output is compared. The OCXO output is
multiplied to the resonance frequency and is used to drive the microwave
cavity where the atomic transition is detected by Electro-optical means. The
detector is used to lock the OCXO output ensuring its medium and long-
term stability.
The first realised Rubidium frequency reference arose out of the work of
Carpenter (Carpenter et al 1960) and Arditi (Arditi 1960). It was a few years
until the first commercial devices came onto the market and this was
primarily due to the work of Packard and Schwartz who had been strongly
influenced by the work of Arditi a few years before on Alkali atoms (of
which Rb 87 is one). Unlike much of the research done into frequency
references at that time, practical realization of a Rubidium maser was high on
the researchers’ agenda. This was mainly due to an understanding that such a
device would have extremely good short-term stability relative to size and
price. In 1964, Davidovits brought such research to fruition, with the first
operational Rubidium frequency reference.
The Rubidium frequency reference, like its more expensive cousin, the
Hydrogen maser, may be operated either as a passive or as an active device.
The passive Rubidium frequency standard has proved the most useful, as it
may be reduced to the smallest size whilst retaining excellent frequency
stability. The applications for such a device abound in the communication,
space and navigation fields.
The Rubidium frequency reference may be thought of as consisting of a cell
containing the Rubidium in its vapour state, placed into a microwave cavity
resonant at the hyperfine frequency of the ground state. Optical pumping
ensures state selection. The cell contains a buffer gas primarily to inhibit wall
relaxation and Doppler broadening. The Rubidium frequency reference
essentially consists of a voltage controlled crystal oscillator, which is locked
to a highly stable atomic transition in the ground state of the Rb 87 atom.
There are several reasons why Rubidium has an important role to play as a
frequency reference. Perhaps more important is its accuracy and stability.
