A Miniature, Low Cost Rubidium Frequency Standard

Quartzlock UK Ltd.
Totnes, Devon, UK

Many of today's wireless communications systems require a compact, high precision frequency source. Although the use of a crystal frequency standard for these applications can provide excellent short-term stability, this type of source falls short in terms of long-term drift due to the inevitable aging of the crystal. Rubidium frequency sources solve the long-term stability problem but, in the past, have been both too bulky and too expensive to be a practical solution by themselves.

To address these issues, a new low cost rubidium instrument has been developed that is only one-third the size of previous sources of this type without sacrificing performance. The model A10-B rubidium frequency standard features high accuracy, small size, light weight and low power consumption, making it suitable for mobile radio applications. The unit features frequency stabilities on the order of 10-13/day and 10-11/year; 1, 5 and 10 MHz sine wave and 1 Hz and 1, 5 and 10 MHz square wave outputs; a 1 pps synchronized output; and a built-in six-output distribution amplifier. In addition, a Global Positioning System (GPS) synchronizing input enables a cesium beam performance upgrade using a GPS time and frequency standard.

The Frequency Control Mechanism

The new rubidium frequency standard derives its impressive frequency accuracy and stability from a unique frequency control mechanism that employs a vacuum-enclosed two-envelope lamp for much lower temperature operation, higher stability and longer life. The resonant transition frequency of the rubidium 87 atom (6,834,682,614 Hz) is used as a reference against which an oven-controlled crystal oscillator (OCXO) output is compared. The OCXO output is multiplied to the resonance frequency and used to drive the microwave cavity where the atomic transition is detected by electro-optical means. The detector is utilized to frequency lock the OCXO output, ensuring its medium- and long-term stability.

Performance

The model A10-B unit is part of the A10 series of rubidium sources, which includes the rubidium frequency sources shown in Figure 1 . All models feature high stability, small size and low cost. In addition to mechanical setability, the units feature an electrical adjustment range (-5 to +5 V control range) of 4 ¥ 10-9. Phase noise for the model A10-B instrument is shown in Figure 2 . Some general specifications that apply to the A10-B rubidium frequency standard are listed in Table 1 .

Table I
A10-B Specifications

Output frequency (MHz)

10

Accuracy at shipment

±2x10-11

Retrace after 24 hours

±2x10-11

Aging/month

±4x10-11

Stability at 2-sigma interval
1 s
10 s
100 s
1 day


3 x 10-11
1 x 10-11
3 x 10-12
1 x 10-11

Temperature Sensitivity
-65º to +65ºC


±2x10-10

Voltage Sensitivity ±10%

±8x10-12

Frequency setability

±2x10-9

Output level into 50 ohms (dBm)

+8 ±2

Harmonic distortion (dBc)

-30

SSB phase noise at 10 kHz (dBc)

-150

The unit operates from a 22 to 30 V DC supply and consumes 10 W of steady-state power at 25°C. In addition, the source features an external 24 V redundant battery backup input that is seamlessly switched in the event of a supply power failure. Standard operating temperature range is -10° to +55°C and the warm-up time is five minutes to achieve 5 x 10-10 stability.

Applications

Applications for the rubidium frequency standard include frequency and time calibration and telecommunications network synchronization as well as use in cellular telephone base stations and satellite navigation and GPS receivers. The unit also can be used in television broadcasting, radio transmitter and secure communications applications, and ground and satellite communications equipment.

The model A10-B rubidium frequency standard and 10A-R rack-mount version sell for $6900. The A10-B unit requires a few weeks for delivery; the rack-mount version is available from stock.

Quartzlock UK Ltd.,
Totnes, Devon, UK and
Lake Mary, FL
+44 (0)1803 862062.