QUESTION:
What is the difference between cesium and rubidium timing products?
ANSWER:
Both Cesium and Rubidium devices are atomic clocks. That is, the frequency characteristics are determined by the behavior of atoms. The manner in which these devices are constructed is somewhat different and they have somewhat different characteristics and therefore different applications.
In a Cesium Beam Tube, a reservoir containing Cesium is heated and vaporized. This results in a beam of cesium atoms that is "interrogated" by a microwave field generated using a (controlled) crystal oscillator. The interrogation process identifies whether the microwave frequency is less than, equal to, or greater than the frequency associated with the movement of cesium atoms between two atomic states. A servo loop then maintains a crystal oscillator at the right frequency. When the cesium atom moves between the two states it either absorbs or emits a photon whose frequency is determined by the energy difference between the two states ( a quantum effect) and therefore is very precise and accurate.
As long as good care is taken in the design and construction of the tube and associated electronics, the resulting accuracy of the crystal oscillator frequency is very precise, accurate, and stable. Consequently a Cesium beam tube device is often called a "Primary Atomic Standard". In a telecommunications environment, a clock built using a cesium beam tube will provide an output whose frequency is very accurate, more accurate that 1 part in 10E11, the stratum-1 accuracy specification. Therefore, a Cesium clock can be used as a Primary Reference Source directly "out of the box" and does not need to be routinely calibrated or adjusted. However, when the reservoir of cesium empties, the tube must be replaced.
A Rubidium atomic clock uses a vapor cell method. Rubidium (the two isotopes, Rb-57 and Rb-55) is stored in a cell and heated so that the cell contains rubidium vapour and a "buffer gas" (an inert gas). The "interrogation" process involves detecting how much light, generated by a lamp using Rb-87, is absorbed by the cell as the cell is excited by a microwave frequency. As in the case of the cesium beam device, a servo loop maintains the crystal oscillator at the right frequency. However, the movement of the atoms (Doppler) and impact of buffer gas may introduce a small frequency offset. Most rubidium devices will have a frequency offset of less than 1 part in 10E9 directly "out of the box". Therefore, rubidium devices are often called "Secondary Atomic Standards".
However, they are very, very, stable and so if they can be "calibrated" or "disciplined" using a known good reference (traceable to a primary reference source), they create wonderful telecommunications clocks. Rubidium fits the mold of "Type II" (ITU-T Rec. G.812) or "stratum-2" or "stratum-2E". They are excellent holdover oscillators because of their stability. Even if the PRS-traceable reference goes away, they will maintain stratum-1 accuracy for weeks! The excellent stability also allows the clock to act as a very narrow-band filtering clock. Filter time constants of the order of days can be implemented if the oscillator is a rubidium because of the stability. Therefore it can filter out essentially all accumulated wander on the reference. In contrast, quartz oscillators, even expensive ones, will provide stratum-1 quality holdover only for a few hours. Rubidium devices also act as excellent firewalls. A rubidium oscillator cannot be made to change its frequency by very much, typically 1 part in 10E8 offset from absolute truth, and therefore cannot be steered very wrong in the event of a reference going "rogue". The equivalent limit for quartz is about 1 part in 10E6.
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