Hewlett-Packard Cesium II Technology Ever since the Egyptians brought the sundial into use nearly 3000 years ago, the ability to measure time accurately has played a pivotal role in the advancement of civilization. By knowing the time, the movements of people, armies and assembly lines can be coordinated, the seasons, plantings and harvests become manageable, and oceans and heavens can be navigated. In today's post-industrial society, however, the need to move large quantities of information more and more quickly is one of the primary forces driving the need for clocks of increasingly high precision.
The new HP 5071A Primary Frequency Standard, a cesium-beam atomic clock from Hewlett-Packard, sets new accuracy and stability standards for commercially available atomic clocks. It achieves this unprecedented capability through HP's new Cesium II technology, combining an advanced cesium-beam tube design with sophisticated, microprocessor-controlled performance monitoring.
Keeping Time With Atoms
Around 1913, the young Danish physicist Niels Bohr, working in England with Ernest T. Rutherford, developed the original concept of atoms comprising a central nucleus with orbiting electrons, like planets circling the sun. Bohr then proposed the revolutionary idea that the electrons circling the nuclei of atoms did not gain or lose energy in a gradual way, like a spring winding down, but did so in lumps by jumping between distinct, allowable orbits. He also proposed that the orbital change is accompanied by the release or absorption of a distinct ``quantum'' of energy, and that this energy corresponds to a particular frequency of electromagnetic radiation.Cesium-beam atomic clocks utilize this quantum mechanical aspect of the cesium atom -- which has a suitable ``hyperfine'' resonant frequency in the microwave range of 9,192,631,770 vibrations per second, or hertz (Hz) -- to measure time with a high degree of accuracy.
The single, outermost electron of the cesium atom spins on its axis like a top as it orbits the atom's nucleus. The combined motions of this valence electron, spinning and orbiting around the nucleus, produces a magnetic field at the center of the atom called the hyperfine field. The nucleus, which is itself like a spinning magnet, aligns itself in the hyperfine field, the direction of alignment depending on the energy state of the atom. In one energy state, the nucleus and the hyperfine field are aligned in the same direction, while in the other state the two are opposed. The quantum of energy between these two states is called the hyperfine splitting, and corresponds to electromagnetic radiation at the hyperfine frequency of the cesium atom, 9,192,631,770 Hz.
By passing a beam of cesium atoms through an enclosure or ``cavity'' containing microwave energy, the atoms can be made to change energy states, either absorbing or emitting energy. When the frequency of the microwave energy is precisely equal to the hyperfine frequency of the cesium atom, the greatest number of transitions will occur.
Atomic clocks capitalize on this phenomenon by using a stable oscillator to produce microwave energy, and then passing a highly controlled beam of cesium atoms through these microwaves. Then, by monitoring how many cesium atoms change energy states, the oscillator can be ``tuned'' like a radio to produce the greatest number of transitions. At this point, the oscillator is producing microwaves at precisely the hyperfine frequency of the cesium 133 atom.
From Theory to Reality
The cesium (a silvery metal), the microwave cavity, magnets for properly steering the cesium atoms through the microwaves, and devices for detecting the desired transitions are contained in a cesium-beam tube. The tube produces an electrical current proportional to the number of cesium atom transitions occurring. An atomic clock also contains a crystal oscillator and microwave frequency synthesizer for driving the microwave cavity inside the tube. The electronics in an atomic clock tune the crystal oscillator and frequency synthesizer to produce the greatest tube output, and then lock onto this frequency.But even atomic clocks are not perfect. Changes in the clock's physical environment, such as temperature and humidity or movement of the clock, as well as certain atomic phenomena can affect its accuracy and stability by causing the tube's maximum output to occur at some frequency other than the standard. As a result, the crystal oscillator locks onto the wrong frequency.
Two types of undesired, frequency-pulling transitions have particularly significant effects on cesium-beam atomic clocks: Rabi pulling and Ramsey pulling. When cesium atoms are passed through the microwave field in the cesium-beam tube, some of the transitions that occur are other than those corresponding to the desired 9,192,632,770-Hz frequency. These undesired transitions have the effect of ``pulling'' the atomic clock's frequency, shifting it slightly above or below the standard.
Temperature changes cause the microwave chamber, or cavity, the cesium atoms pass through to expand or contract slightly. This tends to ``fool'' the electronics that monitor the number of cesium atom transitions and control the crystal oscillator into ``thinking'' the maximum number is occurring at some frequency other than the correct one. This also pulls the frequency off standard.
HP's Cesium II technology allows the HP 5071A to achieve greater levels of stability and accuracy by reducing the clock's susceptibility to these effects. The Cesium II technology -- comprising an improved cesium-beam tube design and microprocessor-controlled digital electronics -- doubles the accuracy of the HP 5071A over previous designs, and makes it the first cesium-beam atomic clock to specify stability for averaging times longer than a day.
Reducing Ramsey Pulling
Ramsey pulling occurs because the entire cesium beam cannot pass through the precise center of the microwave field in the cavity. Because some atoms move more quickly than others, the magnets that steer the atoms through the microwaves bend the beam slightly more or less, causing the beam to spread. This results in a larger number of undesired energy state transitions, and this pulls the frequency.Many tradeoffs are made in the design of any cesium-beam tube. Before Ramsey pulling was discovered, tube designs for reducing the effects acceleration had on the accuracy of the clock could be simpler. Many cesium-beam atomic clocks took advantage of the intrinsic characteristic of the two-beam design that automatically compensates for the acceleration effect.
Recent research, however, declared Ramsey pulling a significant factor affecting the accuracy of cesium-beam tubes, and described its cause. The challenge then was to design a tube less susceptible to the effects of Ramsey pulling that also maintained an adequate degree of acceleration compensation while balancing technical sophistication with manufacturability.
With a two-beam design, the two streams of atoms cannot pass through the precise center of the microwave energy, resulting in Ramsey pulling. A single cesium beam reduces Ramsey pulling because it can be more precisely located at the center of the microwave energy field, but this design lacks the automatic acceleration compensation found in dual-beam tubes.
HP used sophisticated computer modeling to optimize the geometry of the cesium-beam tube and the deflection system used to steer the cesium atoms through the microwaves. Using this technique, HP scientists and engineers were able to try different design approaches and refine their designs without having to go through the costly and time-consuming process of building prototypes and testing them. The resultant tube design provides the required acceleration compensation while reducing Ramsey pulling.
The improved tube design enables the HP 5071A to achieve a stability of less than 2 parts in 1014 for averaging times of greater than five days under laboratory conditions with the high-performance Option 001 tube. As a result, the HP 5071A can maintain time consistently to within 1 second in 1.6 million years.
The improved tube also makes more efficient use of its cesium supply and has a higher signal-to-noise ratio. The cesium oven holds 20 percent more cesium and is operated with reduced power consumption. With the tube's optimized beam optics and magnetic state selectors, HP anticipates the life expectancy of the tube will increase by at least 25 percent.
Rabi Pulling and Temperature Compensation
Rabi pulling is another form of frequency shift cause by an undesirable energy-state transition of the cesium atom. Laboratory atomic clocks employ a variety of modulation techniques to minimize the effects of Rabi pulling.Frequency shifts are also caused by mistuning of the microwave cavity as it expands and contracts due to temperature changes. It is well known, however, that by adjusting the power of the microwave energy the cesium beam passes through, it is possible to minimize these effects. In fact, there exists one correct microwave power level for any temperature at which these temperature-induced frequency-pulling effects disappear.
Until now, however, no one has been able to devise a way to combine modulation techniques to reduce Rabi pulling with the microwave power control necessary to minimize temperature effects. Only one of the two could be used at any given time.
HP's breakthrough Cesium II technology employs a sophisticated modulation technique to suppress Rabi pulling while simultaneously implementing the microwave power control necessary to eliminate the effects of cavity mistuning due to temperature changes. As a result, the HP 5071A is extremely insensitive to environmental changes, and can power up automatically to full specifications in only 30 minutes under full environmental conditions without operator adjustments.
Remotely Programmable
To accommodate the increasing use of atomic clocks as integral parts of telecommunication, satellite communication and navigation systems as master clocks, the HP 5071A provides complete remote control capabilities, enabling it to be easily operated and maintained in the remotest locations. All instrument functions and parameters can be interrogated via the standard commands for programmable instruments (SCPI) language and a standard RS-232C asynchronous serial communications port.The HP 5071A's remote programmability combined with its frequency steering capability -- the ability to adjust the clock rate up or down without compromising accuracy -- enable the creation of practical, real-time clock ensembles. An ensemble of clocks, under the control of a computer, creates a ``super clock'' capable of greater accuracy and stability of any of the individual clocks.
Traditional HP Reliability
HP integrates reliability into every phase of the atomic clock life cycle, from research and design to production, testing and field service. Since HP began producing atomic clocks and related products in 1964, HP's atomic clocks have proven their reliability in over 100 million field operating hours in critical commercial and military applications.The HP 5061B, predecessor to the clock announced today, currently demonstrates a mean-time-between-failures of 90,000 hours, or one failure in more than 10 years. Cesium II technology, because of advancements in tube design, automated maintenance and extensive use of digital circuitry, offers even greater reliability than that achieved by the HP 5061B. HP backs up this reliability with a five-year warranty on the standard cesium-beam tube and a three-year warranty on the optional high-performance tube.
When tubes are changed, HP's Cesium II technology provides a new level of ease of use. Each tube is completely tested and characterized by HP before it is shipped, and these characterization parameters are stored in a read-only memory (ROM). The HP 5071A's microprocessor-controlled circuitry then reads the information in the ROM and automatically compensates for the new tube, eliminating all the manual adjustments and circuit modifications previously required when changing tubes.
Atomic clocks play a vital role in modern communications, synchronizing the rapid movement of information through telephone systems and computer networks. They provide the accuracy required to properly generate the signals transmitted by virtually every radio and television station in the world. And state-of-the-art, satellite-based navigation systems used by everything from oil tankers to space shuttles rely on the precise timing provided by atomic clocks. With space navigation and communication, as well as the work of astronomers already pushing the limits of today's atomic clocks, HP's 5071A primary frequency standard and HP's Cesium II technology provide the accuracy, stability and reliability necessary.
# # #