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Space-Qualified Rubidium Frequency Standards

Space Qualified Rubidium Frequency Standards

The Excelitas Rubidium Atomic Frequency Standard (RAFS) is an exceptionally high performance and high reliability space-qualified rubidium atomic clock developed for the GPS Navstar navigation satellites. Over 100 space qualified RAFS have been delivered to customers to support GPS-IIR, GPS-IIF, and QZSS navigation satellites.

It 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 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.

The Excelitas RAFS was developed specifically for space applications where a highly stable 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. The addition of a thin film spectral filter increased the signal-to-noise ratio even higher with the design efforts for the GPS-IIF RAFS. Calorimetric Rb lamp process control and screening assures long life. The "natural frequency" output (13.4 MHz) permits the use of a low complexity, single loop design for improved reliability. The 10.23MHz design adds a phase locked loop and synthesizer locked to the natural frequency, which slightly degrades the short term stability, but is negligible in the long term stability. 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. The low aging rate (< 5 x 10-14/day) is exceptionally smooth and modelable.

A crystal oscillator at the desired output frequency produces the output signal via an output amplifier. This RF output path is hardened against transient radiation. The 13.4 MHz 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.

The 10.23 MHz design is larger due to the addition of the phase locked loop and synthesizer. The internal workings, flicker floor, and long term drift are identical. The subtle differences in the two designs are:

 

Heritage 13.4 MHz

Current 13.4 MHz

Current 10.23 MHz

1 Second Stability

2.5x10-12

1x10-12

2.5x10-12

Flicker Floor

2x10-14

<5x10-15

5x10-15

Freq Drift Rate

<5x10-14/day

<5x10-14/day

<5x10-14/day

1Hz Phase Noise

<-85dBc

<-100dBc

<-85dBc

Volume

226.6in3 (3.7 liters)

226.6in3 (3.7 liters)

290.7in3 (6.15 kg)

Weight

11.6 lbs (5.26 kg)

11.6 lbs (5.26 kg)

13.55 lbs (6.15 kg)