Rubidium Frequency Standard: RFS-10

The RFS-10 provides either a 5 or 10 MHz output locked to the rubidium atomic resonance. A 20 MHz crystal oscillator drives the Rb physics package via a synthesizer and multiplier chain and produces the output through a divider and output amplifier. The discriminator signal from the physics package is processed by the servo amplifier that produces a control voltage to lock the crystal oscillator. Lamp exciter and temperature controller circuits support the operation of the physics package, while an adjustable C-field current source provides frequency adjustment. A dc/dc converter and linear regulators provide power to all RFS circuits.

The units feature a patented miniaturized physics package with separate filter and absorption cells, both of which are located inside the microwave cavity. Zero net temperature coefficient is achieved by cancellation of the individual filter and absorption cell coefficients. The discrete filter cell allows optimization of the optical pumping process to improve physics package performance. The design also incorporates a custom Excelitas developed, low noise photodetector and an efficient microwave multiplier. These design features all ensure that this atomic rubidium frequency standard provides high-stability performance.

The physics package is double-shielded for low magnetic sensitivity and is supported by silicone rubber mounts that provide shock and vibration isolation. The four main electronic boards surround the physics package and plug into a motherboard at the front of the unit. The RFS-10's modular design uses MIL spec avionics grade parts, conservatively derated to assure high-reliability operation.

Very low power consumption (12 Watts operating at 25°C and 34 Watts during warm-up) is a major advantage of the RFS-10 line compared with other currently available rubidium frequency standards. This feature is achieved by highly efficient switching circuitry for oven controllers and power supplies, simplified and efficient electronic circuitry and lamp excitation, and a combined cell and crystal oscillator oven. The lamp and cell ovens have low thermal mass to aid fast warm-up (5 minutes at 25°C).

A low g-sensitivity, fast warm-up quartz crystal is employed. A wide servo bandwidth is used to tightly lock the crystal oscillator to the rubidium resonance which further enhances performance under vibration. The RFS-10 has a high stability 5 or 10 MHz output. The output and power/monitor connectors and frequency adjustment are all located on the front of the unit. Built-in test (BIT) capability is provided via the monitor connector. Mounting is via tapped holes in the back surface or by an optional heat sink assembly. Efficient power circuits, discrete filter and absorption cells, a low-noise photodetector and a resiliently mounted, miniaturized physics package set the RFS-10 line apart from all other available rubidium frequency standards as a high-performance, low-power, small, light weight and reliable unit. The standard interface and rugged design make the RFS-10 ideal for use in tactical communications and navigation systems or other military applications needing the high precision of an atomic frequency standard.

A sister unit, the RFS-10-7, is identical to the RFS-10 except for the external package which has a larger footprint to more easily facilitate the direct replacement of a discontinued heritage atomic frequency standard.

Excelitas' precise rubidium atomic frequency/time standards are ideal, whether on earth or in space for:

• Tactical communications systems
• Navigation systems
• Global positioning