RAFS

Description

The PerkinElmer RAFS Rubidium Atomic Frequency Standard is an exceptionally high performance and high reliability space-qualified rubidium clock developed for the GPS Navstar navigation satellites. RAFS offers the highest stability ever reported for a device of this type, along with the practical size, weight, power and life advantages of a rubidium gas cell atomic frequency standard. While the RAFS may be operated in air, it achieves its best performance in vacuum which eliminates barometric sensitivity. All other environmental sensitivities (temperature, voltage, etc.) are negligible. The design of RAFS  has been fully documented and qualified for all space requirements, including radiation. The RAFS offers exceptional performance as a precise time and frequency reference for demanding space applications. This PerkinElmer Rubidium Atomic Frequency Standard was developed specifically for space applications where a high stability and extremely reliable atomic clock is needed. It is the highest performance device of this type currently available, combining exceptional stability and low drift with negligible environmental sensitivity, while offering the low size, weight and power advantages of a rubidium frequency standard. The RAFS employs classical rubidium gas cell atomic frequency standard principles. It utilizes a physics package with a discrete isotopic filter cell for best stability. The relatively large, cool absorption cell provides exceptionally high signal-to-noise ratio and excellent short term stability (typically 2 x 10-12 at 1 second). Calorimetric Rb lamp process control and screening assures long life. The "natural frequency" output permits the use of a low complexity, single loop design for improved reliability. Operation at low fixed magnetic bias field improves stability and reduces magnetic and radiation sensitivity. An integral baseplate temperature controller reduces the overall temperature sensitivity to below the noise level. When operated in a vacuum environment that eliminates barometric sensitivity, the extremely low temperature, magnetic, radiation and voltage sensitivities mean that the unit is essentially free of any measurable environmental sensitivity. This provides a very low flicker floor (typically 2 x 10-14). The low aging rate ( A crystal oscillator at 13.40134393 MHz, 1/510 of the Rb hyperfine resonant frequency, produces the output signal via an output amplifier. This rf output path is hardened against transient radiation. The crystal oscillator also excites the Rb physics package via a phase modulator and frequency multiplier chain. This produces a discriminator signal that is processed by a servo amplifier to lock the crystal oscillator to the Rb atomic resonance. Temperature controllers, a lamp exciter and a precision C-field source support the operation of the physics package, while a dc/dc converter and linear regulators provide power for the RAFS circuits. An integral controller stabilizes the RAFS baseplate temperature and several analog monitors are available to assess the operation of the unit.