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Small Atomic Clocks Chart New Horizons

Chip-scale time keepers offer accurate frequency location, lower power requirements for messaging, detection and navigation equipment.

A device the size of a sugar cube may revolutionize military communications and sensor systems. The technology is a micro-scale atomic clock designed to help spectrum-hopping radios synchronize their frequencies and access signals from navigation satellites. This prototype time keeper is undergoing testing to determine its readiness for military applications. 

The goal of the Defense Advanced Research Projects Agency’s (DARPA’s) Chip-Scale Atomic Clock (CSAC) program is to create miniaturized, low-power atomic time and frequency reference devices for high-security ultrahigh frequency communications, jam-resistant global positioning system (GPS) receivers, sensors and guided munitions. To achieve these objectives and to enable the technology to fit in handheld military radios and other small form factors, the Arlington, Virginia-based agency’s researchers have designed a miniature atomic clock that measures 1 cubic centimeter (CC). Their work tapped a range of micromechanical technologies and nanotechnologies to develop the ultra-miniaturized device.

An atomic clock keeps highly accurate time, which is essential for modern military communications, explains CSAC program manager Amit Lal. He notes that most modern radios operate by transmitting data in small packets. If many users in a group, such as an infantry platoon, are communicating, differing times are allocated to the radios to allow transmission on the same frequency. The data packets have guard bands that protect individual packets from overlapping. This timing feature ensures communications, even if all of the radios are not synchronized. A CSAC can provide such accurate timing per radio that it reduces the guard bands, allowing twice the information to be transmitted, Lal shares. “Radios need to be synchronized. The better time you keep, the faster the network can be kept in synchronicity,” he says.

DARPA’s clocks also are designed for low-power operations in handheld communications and personal navigation equipment. Lal says that commercially available atomic clocks consume between 5 watts and 10 watts of power. In addition, size is a critical issue. The smallest available atomic clocks have a volume of 200 CCs, but he points out that high-quality atomic clocks are larger, with volumes of 300 CCs or more, and have an average power consumption of 10 watts. The CSAC program not only reduced the size of these clocks by 200 times down but also cut power requirements down to 30 milliwatts.

Lal believes that CSAC represents a major step forward for communications systems. He explains that communications systems need specific frequencies to operate. These frequencies are typically generated by phase-locked loops and oscillators. He predicts that in the future, radio receiver architectures could use CSACs to generate precision frequencies while putting much less burden on phase-locked loop gains. This combination would provide lower power consumption and higher reliability for radio reception.

The CSAC program is now in phase four—operational testing—of its development. This stage is rare for DARPA programs because most efforts end at phase three, but Lal shares that DARPA director Dr. Anthony Tether is so confident about the technology’s great potential that he cleared the effort for additional development. Lal says this additional phase will ensure that there are no reliability issues with the clocks that might emerge in a full production development program. “The program was very successful in achieving the original milestones. But rather than just leaving it at this point, it would be worthwhile to explore the reliability of these atomic clocks,” he says.

Phase four testing is taking place at the U.S. Army’s Communications Electronics Research Directorate in Fort Monmouth, New Jersey. The devices will be tested to military specifications, what Lal refers to as “shaking and baking,” to measure the technology’s ability to withstand the vibration and heat encountered in airborne and ground-based platforms. The devices also must withstand temperature variations from zero to 50 degrees centigrade and must provide the frequency stability necessary for applications such as radio synchronization. If the clocks are able to meet military standards, they will spiral into radios and other applications.

Phase four will last roughly a year, and its success will determine whether the clock moves on to a manufacturing program to develop mass production methods or if the program returns to DARPA for additional fine-tuning. If the tiny clocks prove ready for deployment, the agency will conduct manufacturing research to bring the price of an individual CSAC down to less than $100.

The full version of this article is published in the April 2008 issue of SIGNAL Magazine, in the mail to AFCEA members and subscribers April 1, 2008. For information about purchasing this issue, joining
AFCEA or subscribing to SIGNAL, contact AFCEA Member Services.