Stable and precise frequency control has been a requirement for many broadcasters; analog TV stations on channels 2 and 3 use precise frequency control to minimize interference from distant co-channel stations. For ATSC digital TV, precise frequency offsets are used to reduce interference to lower adjacent channel analog TV stations. Precise frequency control is essential for distributed digital transmission systems.
Today GPS is most often used for precise frequency control, but some TV stations may still use rubidium frequency standards. Thanks to the work of scientists at the Commerce Department's National Institute of Standards and Technology (NIST), it may soon be possible to include an atomic clock with a stability of one part in 10 billion in handheld devices! For comparison, it is very difficult to achieve long-term stabilities better than five hundred parts per 10 billion (0.05 PPM) using crystal oscillators. Not only is this NIST micro-clock accurate, it also consumes very little power--less than 75 mW.
Physicist John Kitching, principal investigator for the project, commented, "The real power of our technique is that we're able to run the clock on so little electrical power that it could be battery-operated and that it's small enough to be easily incorporated into a cell phone or some other kind of handheld device, and nothing else like it even comes close as far as being mass producible."
The chip-sized atomic clock consists of a sealed cell, about the size of a grain of rice, containing cesium vapor that is probed with light from a miniature infrared laser. As in much larger clocks such as the NIST-F1 fountain clock, the micro-clock is based on the natural vibration frequency of the cesium vapor. The NIST page on the clock's Overall Design and Basic Physics Research describes how the clock works:
"The injection current of a diode laser is modulated at a frequency equal to one-half of the ground-state hyperfine splitting of the atoms. This modulation (a combination of FM and AM) produces sidebands on the optical carrier separated from the carrier by the modulation frequency. The two first-order sidebands are therefore separated by a frequency equal roughly to the atomic resonance frequency. These two first-order sidebands form the L optical configuration needed to excite the CPT resonance. The modulated light is passed through the atomic vapor, and the transmitted power is detected with a photodiode. As the modulation frequency is swept over the first subharmonic of the atomic resonance, a change in the transmission through the cell is observed. This change in transmission can then be used to determine how far the modulation frequency is from the atomic oscillation frequency, and to correct the modulation frequency as needed."
NIST said that even though the chip-sized atomic clock is less accurate than larger atomic clocks, its small size, low power dissipation, and potentially low cost could make it useful for improving network synchronization in wireless communications systems, improving the precision of satellite-based navigation systems and even replacing crystal oscillators in products such as computers.
A complete description of how the clock works, including photos and block diagrams, is available on the NIST Web page Chip-Scale Vapor-Cell Atomic Clocks at NIST.
