Chip-scale atomic clock approaches performance of modules

If you think atomic clocks require a rack-sized unit or even just a somewhat smaller module, think again. The PC-board mountable Quantum SA 45s chip scale atomic clock (CSAC) (actually, it's an oscillator, of course) from Symmetricom, Inc. operates from a single 3.3 V supply (115 mW) and provides a 10-MHz high-impedance output, in a package which is 1.39 × 1.45 × 0.45 inches high (35.3 × 36.8 × 11.4 mm high), for a volume of about 15 cm³.

Developed using technology provided by partners Sandia National Laboratories and Draper Laboratory, stability of this oscillator is 2 x 10^-10 at 1 sec. Compared to a conventional temperature- and oven-controlled crystal oscillator (TXCO and OXCO), the vendor claims the unit's power consumption is 1/10^th to 1/20^th as much, and yields performance which is 10 to 100 times better. Compared to conventional atomic clocks (including Symmetricom's own units), they say the CSAC is 1/3^rd to 1/30^th the size, at just 120^th to 1/100^th the power. {Note that the vendor is already a leading supplier of cesium-based atomic clocks, and acquired that well-known clock product line from Agilent Technologies--previously known as Hewlett-Packard--in 2005.]

So, how did they do it? Figure 1 details the construction of the core cell.

Functional elements include a tensioned polyimide suspension, and a microfabricated silicon-vapor cell, with a low-power vertical-cavity surface-emitting laser (VCSEL) whose output passes through the vapor. The entire package, Figure 2, includes electronics, RS-232 interface, a low-power DSP, a double-layer magnetic shield (atomic resonance frequency is affected by such fields), all in a hermetically sealed, through-hole PCB-mountable package

In use, on a representative PCB, it looks like Figure 3.

The basic architecture is deceptively simple, Figure 4. The "natural" atomic microwave resonance frequency is synthesized from an RF local oscillator (LO), and a control loop locks the LO frequency to the atomic resonance.

Which applications need a CSAC, with GPS-based clocks so available and prevalent? First, there are applications where the GPS-based timing may not be accurate enough. But there are also many applications where GPS is unavailable, such as underwater exploration, underground drilling, geophysical research, and EMI shielded rooms. There are also in-the-field military situations where GPS and all EM waves are deliberately jammed by patrols, to prevent remote triggering of improvised explosive devices (IEDs), yet there is a need for precise communication-equipment synchronization among combat teams.

[Despite the complex construction, the CSAC is rugged, and will survive shock of up to 500 g in any axis. It is tested to the vibration profile described in MIL-STD-810, Method 514.5, Procedure 1, Category 24, minimum integrity, 7.7 g rms @0.04 g/Hz, 20 Hz to 1 kHz, -6 dB/octave 1 kHz to 2 kHz, 15 minutes/axis.]

Availability and pricing: The CA 45s CSAC may use vapor but it is not a "vapor" or vaporware product. It is shipping in two versions:

• Option 001: -10°C to +70°C, primarily for commercial applications
• Option 002: -40°C to +85°C, primarily for military applications

Prices begin at $1500 each in small quantity.

For more information: contact Symmetricom, Inc.