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Chilled Electronics Race Ahead

November 2005
By Henry S. Kenyon

A cryogenically cooled technology can directly sample and process multiple complex radio signals. The very low temperatures allow the system’s microelectronics to superconduct, operating at clock speeds of up to 40 gigahertz. The high processing speed enables high frequency waveforms to be translated directly to digital data, avoiding analog translation processes that can distort the signal.
Superconducting system opens way to all-digital signal processing.

An advanced microelectronics technology may allow future communications equipment to receive and process multiple high frequency waveforms easily. Relying on superconducting processors in a sealed refrigerated container, the system translates analog radio signals directly to digital information, preventing the data and efficiency losses found in semiconductor-based applications. Unconstrained by performance-limiting issues such as thermal interference, the frigid superconducting chips permit prototype devices to receive, sample and transmit gigahertz-range signals across much of the military’s spectrum.

The need to provide greater volumes of data to battlefield platforms and warfighters is beginning to push many existing communications systems to their limits. Next-generation radios such as the Joint Tactical Radio System (JTRS) also are beginning to encounter engineering difficulties as designers try to increase performance in smaller packages. A new approach to system architecture may provide a solution to current development hurdles and open the door to faster and more efficient radios.

Developed by Hypres Incorporated, Elmsford, New York, the technology uses superconducting microelectronics to provide an entirely digital receiving and transmission system for radio frequency (RF) communications. Richard Hitt, the firm’s president and chief executive officer, explains that even the most advanced digital radios still must use analog processes to convert RF signals from the antenna to an intermediate frequency before they can translate the data. Although great strides have been made in the digital processing of waveforms and memory systems, he contends that the basic architecture for converting analog signals to digital signals and digital signals to analog signals, known as heterodyning, has remained unchanged for nearly 30 years.

The company’s RF system features superconducting chips cooled to 4 Kelvin (-452 degrees Fahrenheit). Unlike semiconductors, which use transistors to process data, digital superconducting systems use structures, or Josephson junctions, that can operate at higher speeds using less energy. The junctions form circuits called superconducting quantum interference devices (SQUIDs). When cooled to their operational temperature, SQUIDS are approximately 1 million times more sensitive to RF signals than conventional semiconductor devices.

Josephson junctions produce predictable quantum effects, allowing extremely accurate measurements of electromagnetic energy. One of the company’s early technologies is the Voltage Standard System, a calibration device that allows scientists to measure voltage in numbers up to nine digits. Hitt notes that the device is used to accurately define standard voltage in the United States and in many other nations.

When applied to radio systems, this highly sensitive signal detection and measurement can convert high frequency analog signals directly to digital formats. This is important because the traditional radio architecture loses signal fidelity as it transfers the signal down to intermediate frequencies that can be sampled and read by semiconductors. “We want the signal as it directly comes off the antenna. We don’t want to boost it. We don’t want to convert it. We don’t want anything done with it because we like it just the way it is,” Hitt maintains.

Direct signal conversion has been attempted before using semiconductor devices. Hitt notes that these systems do not operate fast enough to create a viable digital representation for high frequency military RF signals. Using superconducting chips and systems, he claims, Hypres’ technology represents a major increase in reception sensitivity and significant performance advantages for transmission.

Besides the distortion analog signal conversion processes cause, thermal noise presents other performance issues for conventional radios. Because most radios operate at room temperature, thermal distortion creates a minimum level of noise in a given system. Direct digital conversion avoids thermal noise because the heat-generating analog systems are not present. Once the signal is converted to digital data, it cannot be distorted and can be replicated and manipulated as many times as the user wants, he says.

The potential advantages that superconducting microelectronics offer have attracted the interest of the U.S. Defense Department. According to Deborah Van Vechten, program officer for superconductive electronics at the Office of Naval Research, Arlington, Virginia, the technology’s high clock speeds allow equipment to receive RF signals directly up through the Ka band. This structure also increases data acquisition and simplifies receiver architecture, decreasing the number of parts and related costs.

Logic processing in the 100-gigahertz range facilitates wideband transmitter linearization, which is needed for power-efficient simultaneous transmission of multiple signals, Van Vechten says. She adds that the quantum properties of superconducting systems allow digital filtering that provides enhanced signal resolution. Such capability permits the development of software-defined, wideband modular and universal receivers. She speculates that these receivers could provide needed interoperability of military systems while enabling the introduction of new waveforms—a necessity for network-centric warfare. The technology also would simplify dynamic bandwidth management and increase the number of channels that can be used simultaneously, increasing communications capacity and reducing battlespace coordination and data sharing.

But cooling is the technology’s Achilles’ heel. Traditionally, superconducting electronics are cooled by immersing the processors in containers of liquid helium or nitrogen. In the mid-1990s, a cryocooling method that operates like a standard air conditioning system was developed using compressors to circulate cold helium gas over the electronics.

Although cryocoolers offer significant weight and power advantages over liquid helium systems, they are still relatively bulky. A commercial cryocooler consists of a compressor about twice the size of a toaster and an insulated bottle that houses the electronics. The bottle is roughly three times the size of a standard thermos. When placed in a standard 19-inch x 6-foot electronics rack, the most compact cryocoolers are roughly 50 inches in height, Hitt says.

Hypres is working with Lockheed Martin to develop a smaller cooling system. Known as the Compact Cryocooler program, it is adapting a space-based cryocooling system developed for NASA for use in terrestrial tactical communications applications. The program is replacing the piston-driven compressors with solid-state components called pulse tubes. Hitt explains that the goal is to fit the entire cryocooler into a

box 19
inches wide, 10 inches high and 24 inches deep—roughly 4,000 cubic inches. He believes that this system will have the operational functionality of the front-end equipment of between four and 10 conventional analog radios.

The company is working with the U.S. Army to provide its strategic satellite communications terminals with direct digital signal conversion. Hitt notes that Army programs such as the Dedicated Communications Advanced Transmission System (DCATS) currently use large trailers filled with analog frequency converters. “We can eliminate all of that with one of these boxes,” he says.

Replacing the analog front end of many tactical and strategic systems would provide the military with many advantages, Hitt says. He adds that roughly 80 percent of the parts in communications systems such as Link-16 or single channel ground and airborne radio systems (SINCGARS) are devoted to analog processes. Because this analog equipment represents most of the size, weight and power costs of these and other related systems, the Defense Department could achieve greater savings by reducing the existing analog architecture in these military radio systems than through further cuts in existing digital components, he says.

Hypres Incorporated is working with Lockheed Martin to reduce the size of the current cryogenic cooling system, which relies on pistons and stands about 50 inches tall. The Compact Cryocooler program will develop a processing and cooling package that will fit in a box that is 19 inches wide, 10 inches high and 24 inches deep. By reducing the size and weight of the system, the superconducting microelectronics will be able to operate in a variety of communications applications such as processing high frequency satellite signals.
Although digital radios such as JTRS have made great strides in converting hardware components to software and algorithms, Hitt maintains that these developments affect only the 20 percent of the device that is already digitized. Systems like JTRS still use analog front-ends to process up to a dozen signals for their modems to sample. These design limitations have led to delays and complications in advanced radio programs such as JTRS.

Hitt believes that digital superconducting systems can eliminate the complications semiconductor-based technologies face when sampling high frequency military waveforms. “We want to build one RF front-end that can handle all the signals for a system like JTRS. We can convert any of their signals directly to digital and eliminate almost all of the analog processing at the front end,” he shares.

Hypres has developed devices operating at clock speeds of 20 gigahertz to 40 gigahertz. Hitt notes that the fastest semiconductor systems used for digital conversion have sampling speeds of 1 gigahertz to 2 gigahertz. The semiconductor devices also tend to operate across a narrow frequency band of 10 megahertz to 100 megahertz. By comparison, the superconducting digital samplers the company is developing can sample signals from across gigahertz-wide frequency bands.

Hitt explains that the faster a device’s sampling speed, the higher frequency signal it can digitize. This is important for military communications systems operating at extremely high frequencies up to 44 gigahertz. He notes that to directly digitize these signals, a device must operate at about four times the frequency. Although Hypres is several years away from being able to digitize very high frequencies, its current devices can process complex signals such as the JTRS waveform. “We’re trying to develop our first generation of products with the goal of eventually being able to digitize the entire military spectrum,” he says.

Besides providing enhanced reception for radio communications, superconducting microelectronics also offer advantages for signals transmission. The technology can mitigate co-site interference, which is caused when antennas are placed too closely together. Engineers currently solve co-site interference by placing analog filters on receivers to cancel out the interfering transmitter. But Hitt maintains that a better approach is to clean up the transmitting signal. “If you’ve got a transmitter that’s 10 percent efficient, it means 90 percent of the energy you’re putting on the antenna is wasted energy that is trashing up the spectrum. If you have a 50 percent or 60 percent efficient signal, you are eliminating most of the problem before it starts,” he says.

By replacing almost all of a radio’s analog components, Hitt notes, considerable efficiencies could be gained in terms of weight, power consumption and efficiency. Until the analog architecture of communications systems changes, he believes that they will continue to encounter difficulties when processing high frequency signals.

As the Defense Department moves toward higher bandwidth communications as envisioned by the transformational communications architecture, engineers will begin to encounter difficulties when they try to develop systems that can receive and transmit multiple high frequency signals, Hitt predicts. The transformational architecture is envisioned in three layers: space systems, high-flying aircraft and a lower tier of low-flying platforms and ground-based systems. Although the initiative calls for the space and high-altitude segments to use lasers for communications, he notes that the lowest part of the architecture must rely on RF systems.

Hypres is targeting the architecture’s terrestrial communications segment, Hitt says. He notes that until steps are taken to move toward fully digital communications, the frequency limitations of semiconductor-based devices will continue to limit the Defense Department’s ability to implement the architecture. Direct digitization is optimal in platforms that must transmit or receive high speed, multichannel and complex waveform signals. “We’re not out to replace individual handheld SINCGARS radios. We are looking at command and control nodes where you’ve got 20 channels worth of communications and you are trying to interconnect people using SINCGARS, the emergency position-reporting location system, Link-16, wideband networking waveform and airborne networking waveform,” he says.

The company recently sent a prototype all-digital receiver with a 20-gigahertz clock speed to the U.S. Navy’s Space and Naval Warfare Systems Command in San Diego for evaluation. Hitt relates that Hypres also is building an additional digital receiver for the Navy and one each for the Army and the U.S. Air Force. The firm is working on developing an operational system for the Army’s DCATS program by 2008.


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