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Cool Chips Boost Satellite Terminal Performance

May 2007
By Henry S. Kenyon

 
The experimental all-digital receiver is a 1-centimeter superconducting niobium microchip capable of processing high frequency satellite signals directly into digital format. The technology allows designers to eliminate many of the analog components currently used in satellite communications equipment.
Cryogenic technology offers increased gain from smaller dishes.

Anew type of digital receiver driven by a superconducting microprocessor could greatly increase the sensitivity of U.S. military satellite communications terminals. By directly converting signals from the antenna into data, the device eliminates the need for analog conversion systems, saving equipment space and reducing airlift and maintenance costs.

The X-band all-digital receiver (ADR) was tested operationally in February at the U.S. Army Communications–Electronics Research, Development and Engineering Center (CERDEC), Fort Monmouth, New Jersey. The heart of the ADR is a superconducting niobium microchip developed by Hypres Incorporated, Elmsford, New York. The processor is cooled to 4 Kelvin (–452 degrees Fahrenheit) and consists of 11,000 Josephson’s junctions, which are two layers of niobium linked by thin nonconducting oxide barriers. The junctions can be arranged in different arrays to form superconducting rapid single flux quantum circuits that move picosecond-long magnetic pulses.

According to Hypres officials, when cooled to their operating temperature, superconducting circuits are about 1 million times more sensitive to radio frequency signals than are conventional semiconductor devices. The niobium analog-to-digital converter (ADC) can sample at clock speeds of 20 gigahertz to 40 gigahertz.

The technology can replace a variety of ADC components used in satellite communications systems, says Rick Dunnegan, CERDEC’s test and integration technical lead for the project. The cryocooled ADR eliminates the need to lower, or downconvert, the frequency for sampling. Military satellite communications stations use downconverters to convert the super high frequency/ X-band signals to a lower intermediate frequency. This process is necessary because current modems cannot collect data, or demodulate, directly at X-band frequencies.

Even fully digital communications systems still have analog components, says Richard Hitt, Hypres’ president and chief executive officer. Hitt notes that other firms have directly converted analog signals to digital, but their systems are generally restricted to ultrahigh frequencies of 400 megahertz or lower. These other converters also operate in a relatively narrow bandwidth—several kilohertz to perhaps a megahertz. But the Hypres ADC used in the demonstration operates at higher frequencies. Hitt explains that this increased capability is important because the bandwidth of some of the new satellite beams is as high as 125 megahertz.

Hypres’ direct conversion takes advantage of the niobium processor’s high speed. Hitt says that this feature provides systems engineers with added flexibility. Instead of having to engineer the system to the ADC’s requirements, they now can modify the converter to suit their needs.

In the current system design, the ADC is located in the modem. Hitt notes that large enterprise satellite communications systems use banks of modems, each dedicated to receiving a specific analog signal. The Hypres ADR eliminates the need to split the signal because the signal already is digital.

The niobium chip also eliminates the downconversion process. A fully equipped large satellite terminal can use up to 56 downconverters, which cost approximately $28,000 each and can occupy an entire trailer. Dunnegan adds that the Hypres ADR may allow CERDEC engineers to replace another component, the low-noise amplifier that sits behind a satellite antenna.

Engineers have used the Hypres ADR to increase by 70 decibels degraded signals leaving the low-noise amplifier. Dunnegan explains that an increase of three decibels doubles the transmission power, and reducing the decibels cuts the power in half accordingly. An increase or decrease of 10 decibels is a signal power multiplier of 10. He is excited because, even with a signal degraded by 70 decibels, the receiver still was able to process data. “We were able to achieve a certain amount of sensitivity by operating at low signal levels without using a frequency conversion that is common throughout the world today,” he says.

The development of a fully digital ADR is significant for several reasons, Dunnegan explains. Once a signal is digitized, it can be easily manipulated and copied for analysis and transmission purposes. Digitizing signals allows a satellite communications terminal to manage several simultaneous telephone calls on one communications path. This economy can be achieved by having several high-speed samplers operating off of circuits and in one mission-bit stream. He adds that CERDEC and Hypres are improving the technology with the goal of digitizing signals at higher frequencies. A concurrent target is reducing power requirements to spare satellite-based resources.

Citing his experience as an Army satellite control technician, Dunnegan observes that satellite resources are highly coveted by the services. Satellites have limited payloads and capabilities that must be managed for efficient use. Key requirements for transponded satellites in geosynchronous orbit are power and bandwidth, two elements that always are limited. “We try to use as little power and bandwidth in all scenarios. That way, what power we don’t use at one frequency and bandwidth, we can use to bring up other people on that same transponder or satellite and keep adding people until we use up these resources. The less power and bandwidth we use per transmission, the more people we can serve with that communications satellite,” he shares.

Hypres has focused its business case on upgrading Army and U.S. Navy large ground terminals to make them compatible with the new generation of military communications satellites such as the Wideband Global Satellites that will enter service this year. In late 2008 or early 2009, CERDEC’s project manager, Defense Communications and Army Transmission Systems, will begin upgrading its military enterprise terminals with an improved version that will provide additional throughput and the capability to handle the high bandwidth and frequencies transmitted from the new satellites.

Hitt explains that the Army is interested in Hypres because the company’s technology represents major savings in recurring and operations costs. The digital system will eliminate much of the analog processing currently used in satellite communications systems. “The demonstration showed that we can take the signal right off of the antenna before it goes through extensive analog processing and feed the data directly to a modem,” he says.

The niobium microchips allow increased sensitivity for satellite antennas. Large antennas can be made more sensitive, and smaller antennas can become as acute as larger ones. Hitt notes that the Army is seeking to reduce antenna size for logistics, airlift and installation purposes, adding that some enterprise dishes can be 60 feet across and housed in geodesic domes set on concrete pads. “These aren’t things that you can set up in a couple of hours,” he states.

The Hypres technology also requires less equipment for the ground-based component of a communications system. Conversely, as fewer satellite resources are used, more military customers will be able to move away from commercially leased bandwidth, directly saving money.

Dunnegan notes that the Army has been trying to develop technologies to reduce its satellite communications burdens for several years. But while cutting a mobile system’s footprint is very useful, it is not as necessary for large, fixed facilities. Though he cautions that it may be many years before the cryocooled niobium technology is ready for mobile applications, he believes that it will be most useful in reducing the size and increasing the sensitivity of smaller mobile and deployable antennas. The ADR was integrated and demonstrated in a large, fixed enterprise terminal, but he says that CERDEC’s next goal is to install it in tactical satellite communications platforms.

 
The niobium microchip is housed in a compact cryocooler designed to fit in a standard equipment rack. The cooler chills the processor down to 4 Kelvin (–452 degrees Fahrenheit), allowing it to superconduct.
CERDEC and Hypres currently are focusing on improving the ADR, which Dunnegan believes is the most important part of the research. He explains that the figure of merit for any satellite communications terminal is referred to as gain over thermal noise, or G/T. The better the gain over temperature, the more sensitive the terminal. He notes that a terminal’s transmission power is not as important as its ability to receive a signal from orbit. “The better it can receive, the less power from the satellite has to be transmitted to it,” he maintains.

Besides increased sensitivity, another motivation for using Hypres’ cryocooled technology (SIGNAL Magazine, November 2005) is that it will save taxpayers’ money. Dunnegan explains that the ADR system allows operators to preserve spacecraft resources by maximizing usage and throughput. He notes that some estimates indicate that leasing commercial satellites is costing the U.S. Defense Department more than $1 billion dollars per year. “I think we need to be doing more of this [research] to reduce costs. The only way we’re going to do that is by getting more ground assets off of commercial satellites and onto military satellite communications platforms,” he says.

February’s operational test represents a milestone for Hypres’ cryocooled chip technology. Dunnegan observes that successful laboratory operation is one thing, but an application must function under operational conditions. “The program has enjoyed an extreme amount of success,” he says.

The next step in the CERDEC program is to test a Hypres ADR with a 30-gigahertz microchip. Dunnegan expects the new device to increase sampling fidelity and sensitivity while simultaneously demodulating several frequencies.

Hypres also is developing microchips that can process multiple frequency bands. Hitt says that his firm has chips that can manage C, L, X, high frequency and very high frequency bands. Engineers have put up to three analog-to-digital conversion modulators on a chip, which allows one device to cover a variety of frequencies. This feature also permits processors to be custom-designed to meet user requirements. “That kind of engineering freedom has never been available before,” he states.

Hitt believes that as the technology matures, unusual frequency combinations will be built into systems to meet specific mission requirements. “It’s not just eliminating analog components; it’s the ability to go to digital at whatever frequency you want to operate. Once you get into the digital domain, it’s just so much cheaper and faster to operate,” he says.

Digital superconducting technology has been in use for many years, but it was conducted using chips immersed in liquid helium. These cooling needs were viewed as impractical for military applications. Cryocoolers operate similar to refrigeration units, without the size or power requirements of traditional cryogenic cooling systems. Hitt credits recent advances in cryocooler technology, which have greatly reduced the size of the equipment, as a major factor in Hypres’ success.

Hypres currently is using a Japanese-designed commercial cryocooler for its prototypes. This system takes up half the space of a standard 19-inch equipment rack. The company is developing a compact military cryocooler with Lockheed Martin Corporation. This unit will measure 19 inches x 20 inches x 22 inches—roughly half the height and depth of the commercial unit. The ultimate goal of the military cryocooler program is to develop a device that is 7 inches x 9 inches x 11 inches. Hitt adds that his company soon will receive its first Lockheed Martin cryocooler.

Hypres’ work with CERDEC was part of a Phase 2 Small Business Innovative Research contract launched in August 2004. The firm was given a $2 million contract to design a direct conversion X-band receiver.  Army officials were impressed enough with the demonstration to declare the system at technology readiness level 6, which indicates that it is mature enough for further development. Hitt explains that because the cryocooled ADR was unproven technology, to achieve level 6 on the first test is remarkable. “We made history. No one anywhere, ever, has directly converted 7.5 gigahertz before. We’ve had this working in our labs since July. To prove that it works in a meaningful way on a real system and that it can interface with regular antennas and modems is huge. That’s coming a long way in a short period of time,” he says.

 

Web Resources
CERDEC: www.monmouth.army.mil/CRDC
Hypres Incorporated: www.hypres.com