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New Wave Communications Emerge

December 2010
By George I. Seffers, SIGNAL Magazine
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This graphic shows the development of a soliton, which takes about 2.7 nanoseconds. Current begins passing through the center channel causing the magnetization to oscillate. The magnetization under the channel inverts to form the soliton, indicated by the red center.

Research may lead to a frequency agile oscillator for more secure communications.

Mathematical research conducted by the U.S. National Institute of Standards and Technology could lead to the development of military radios capable of hopping frequencies up to 1,000 times faster than conventional systems. The research also could result in more energy-efficient, interference-resistant cellular telephones than are available today, as well as improvements in many other modern communications devices.

Researchers have found evidence of a new way to generate the high-frequency microwaves that focuses on solitons, which are waves—ocean waves, electromagnetic waves, sound waves and so forth—that do not ebb and flow in a series. A soliton is a single wave formed by a bunched-up packet of energy, according to Thomas Silva, a National Institute of Standards and Technology (NIST) physicist involved in the research. Solitons travel at a constant speed over long distances and maintain their shape as they go. Tsunamis are a form of soliton in water. The window-rattling, house-jarring, ground-shaking sonic boom formed by a low-flying jet is a soliton of sound. The pulses of light racing along fiber optic cables and making modern-day communications possible are another form of soliton.

“Solitons are kind of weird. Instead of being many oscillations spread out in space, all the energy gets bunched up. It’s a big pile of energy in one wave,” Silva explains. “For fiber optics, you have a short duration pulse of light in a fiber. All the energy in the wave bundles up into a single packet, so that rather than seeing blue or red light waves traveling down the fiber, you get one little burst of energy that stays compact.”

Current soliton research builds on the research of John Scott Russell, a Scottish naval engineer, who first discovered the soliton phenomenon in 1834. Russell described seeing a wave formed when a boat being pulled through a canal by a pair of horses suddenly stopped. The engineer followed the resulting wave on horseback for a mile or two, noting that it traveled at a constant speed and maintained its shape for a long distance before being lost in the channel. Russell’s accomplishments include successes in ship hull design and conducting the first experimental study of the so-called Doppler shift of sound frequency of a passing train. His research into the solitary wave effect was not greatly appreciated until the mid-1960s when scientists began using modern digital computers to study nonlinear wave propagation.

The NIST scientists believe solitons could lead to a spin torque oscillator that uses the spin of electrons to generate microwaves—the electromagnetic waves in the frequencies used by mobile communications devices, such as cellular telephones. Oscillators are key components in a wide array of telecommunications systems.

“The unique property of this type of oscillator is that you can change its frequency of oscillation very quickly—very, very quickly. Most oscillators require a long time to change frequency, but these guys can change frequency in times on an order of magnitude of a nanosecond, or one one-billionth of a second. It’s about 100 to 1,000 times faster than other oscillators out there,” Silva says. “This is intriguing for military applications because you can have a radio system where you’re communicating at a particular frequency, and then the enemy starts to jam that signal, and it would rapidly change frequencies before the enemy could effectively block communications. It’s a subset of high-frequency agile technology.”

The NIST team’s research predicts that a soliton can be created within a multilayered “magnetic sandwich,” which includes one layer that is magnetized perpendicular to the plane of the sandwiched layers. An electric current is forced through a small conductive channel that feeds into a multilayer magnetic film, and once the soliton, which is known as a droplet soliton, is established in the magnetic film, the orientation of the spins within the droplet oscillates at more than one billion times per second, the same frequency as microwave communications devices.

NIST’s theoretical oscillator is projected to be more energy efficient than conventional systems, but Silva says he cannot yet quantify how much more efficient it might be. In addition, the frequency is predicted to remain constant even with variations in current, which would reduce unwanted noise.

Oscillator phase noise, which is caused by random fluctuations in the phase of a waveform, limits performance of electronics in a wide range of both military and civilian applications, and this translates into a reduction of overall performance. For example, phase noise limits geopositioning precision, radar detection of some targets, the density of frequencies that can be resolved from a radio spectrum, and the ability to detect chemical or biological agents. Phase noise especially is difficult because it cannot be filtered without reducing oscillation signal.

Oscillator phase noise is a significant enough problem on the battlefield that the Defense Advanced Research Projects Agency (DARPA) is searching for solutions. The agency published a broad agency announcement last year seeking proposals in the area of dynamics-enabled frequency sources. The goal is to develop an alternative architecture based on a nanoscale oscillator. Systems that could benefit from the technology include satellites, missile guidance systems, micro-aerial vehicles and handheld positioning systems, according to DARPA.

The next step for the NIST team is to build a prototype device and test it in the laboratory. Silva predicts that this could happen in the next year or so. First, the team must design a material that will maintain a sufficiently strong torque to keep the magnetization in the perpendicular direction. If the laboratory experiments prove successful, the results will be made publicly available, probably through a scientific publication. The team published its initial research in the August 30 edition of a physics-focused journal called Physical Review B. “We put the information out there for the public to use. We’re motivated by the potential practical applications, but we’re not working on product development.” NIST is known for doing the kind of research that establishes a firm understanding of the basics in a particular area. “Our customers are the taxpayers. Nobody in the private sector is going to do this kind of grunt work for the rest of the country,” Silva states.

NIST’s grunt work research could lead to an advance in a dynamic area of research known as spin transport electronics, or spintronics for short. Spintronics has led to a number of commercial technologies, such as magnetoresistance technology used in today’s disk drives and magnetic tunneling junctions used to develop magnetic random access memory.

Silva’s team also works with DARPA on spintronics research in the hopes of developing a universal memory superior to today’s technology, which requires enormous amounts of power and is too heavy, bulky, low-density and costly. DARPA’s Spintronics program has made progress in developing nonvolatile, radiation hard, random access, high-speed, low-power, high-density magnetic memory. The technology is expected to fulfill the Defense Department’s requirements and may compete with mainstream semiconductor memories, including flash, dynamic random access memory and static random access memory. DARPA’s memory technology will enable instant On and instant Off portable computing, as well as allow the development of robust system-on-a-chip wireless communication systems. The Spintronics program also has produced magnetic sensors that will be utilized for perimeter defense and unexploded ordnance detection as well as in electronic isolators and switches.

In June, Grandis Incorporated, a Milpitas, California, company focused on spin transfer random access memory, announced that it had received a DARPA contract valued at more than $8 million for the second phase of a research project developing spin-transfer torque random access memory chips. Grandis is collaborating with the University of Virginia, University of Alabama and College of William and Mary. The Naval Research Laboratory and Silva’s NIST team also support the effort. 

National Institute of Standards and Technology:
DARPA Spintronics Program: