More than a few microwave engineers probably remembering being told that something "couldn't be done." The mere physics of building structures to support the small wavelengths needed at RF/microwave frequencies might have made production of repeatable discrete microwave circuits unlikely at one time, let alone in monolithic integrated-circuit (IC) form. At the very least, it seemed improbable they'd ever be affordable enough for consumer-electronics products.
And yet, cellular telephones have microwave circuits and ICs and are produced for consumer in the millions at prices that allow service providers to practically give these portable electronic devices away. The cellular telephone is just one example of an end-product that many thought couldn't be done. But the RF/microwave industry also has a history of being highly adaptive and resilient, switching between commercial and military applications as market needs dictate. Oftentimes, it is the ability to adapt and create that provides a solution for technical challenges that might have seemed impossible at one time.
Numerous current microwave technologies might have been considered as unlikely solutions some 50 years ago, until improvements in materials, fabrication processes, or simple understanding occurred. As the look back at microwave design software pointed out, early computers and microprocessors operated in the MHz clock range. Few then would have projected clock rates in the GHz range for consumer-level portable laptop computers within such a relatively short time. But advances in CMOS and other process technologies have enabled the design and fabrication of digital electronics that routinely operate at microwave clock speeds.
The RF/microwave industry has dabbled with numerous technologies sometimes referred to as "black magic," including superconductors. A superconducting material will tend toward zero resistance as it approaches a temperature of absolute zero (0 K). Research on superconductors has taken place since the early part of the 20th Century, typically using liquid helium, which has a temperature of 4 K (-269C). A breakthrough occurred in 1986 when researchers at IBM discovered that some materials exhibited low resistance at temperatures somewhat higher than absolute zero; most of these materials were some form of ceramic blended with copper, called cuprates. These high-temperature-superconductor (HTS) materials could then be used in more practical cooling systems.
The US government has been extremely interested in the practical application of superconductors in components such as filters and oscillators as well as in making complete receivers that are extremely sensitive. Founded in 1989, Illinois Superconductor is an example of a firm started to develop superconducting materials and solutions. Another superconductor company, HYPRES, Inc. (Elmsford, NY), has made numerous advances in low-temperature superconducting components and systems, earning several contracts from the US Department of Defense (DoD). For example, HYPRES' model HYDR-01 digital RF receiver (Fig. 1) is designed to operate at 4.2 K cooled by a commercial cryocooler. The rack-mounted receiver can directly digitize microwave input signals in bands from 800 MHz to 21 GHz.
Most recently, HYPRES demonstrated the world's fastest arithmetic logic unit (ALU), a digital superconducting IC fabricated within the firm's foundry capable of 8-b resolution at 20 GHz. The device was designed and fabricated in collaboration with researchers at Stony Brook University, and is a building block for a potential high-speed superconductor-based computer. The cell-level ALU design is also scalable to 32- and 64-b architectures.
Back at room temperature, one of the materials to benefit from a good deal of vision during the 1980s was gallium arsenide (GaAs). Integrated-circuit developers were attracted by the high electron mobility of this III-V compound semiconductor material. They could foresee the fabrication of microwave-frequency analog functionssuch as amplifiers and frequency mixers, microwave-clock-speed digital circuits , and even high-speed optical components based on the material. Some companies, such as instrument manufacturers Tektronix and Hewlett-Packard Co., had the resources to investigate GaAs ICs internally, for possible use in next-generation test instruments. During a period of financial prosperity in the mid-1980s, Tektronix spun out its GaAs operation as TriQuint Semiconductor, with the intent of independently producing analog and digital GaAs ICs and devices.
Numerous companies were started to explore the possibilities of GaAs-based ICs, including GigaBit Logic (Newbury Park, CA) in 1981. The digital-GaAs-IC manufacturer caught the attention of supercomputer supplier Cray Computer, which needed the speed advantages of GaAs over silicon to make a huge jump in performance from its Cray-2 computers to its Cray-3 model. The Cray-3 computer, although not a commercial success, was the earliest example of a digital computer using GaAs ICs.
GaAs technology was also the basis for companies such as ORTEL (now EMCORE) to explore the development of high-speed laser diodes and receivers for use in wideband, high-speed communications links over optical fibers. The firm, for example, has applied the technology to the development of specialized system designs for the military, including a spectrometer capable of sweeping a full 2-THz bandwidth with frequency resolution of 0.25 GHz. The system is ideal for threat detection, but also has applications in materials and medical research.
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The GaAs "explosion" of the 1980s was greatly fueled by the US Defense Advanced Research Projects Agency (DARPA) and its GaAs Microwave and Millimeter-Wave Monolithic Integrated Circuits (MIMIC) Program. Not only did this program help create GaAs devices for military applications, but it most likely enabled the commercialization of the technology to also support the growth of commercial cellular communications. At the time, it seemed that every major military electronics contractor was involved in some way in the development of GaAs semiconductor technology, including Hughes Aircraft Co., Raytheon Co., and TRW Semiconductors. The list of smaller companies involved in GaAs development was even longer, and included Alpha Industries, Avantek, Harris Microwave Semiconductor, M/A-COM, Motorola, Varian Solid State, Vitesse Semiconductor, and Watkins-Johnson Co. (Fig. 2).
Most of the GaAs device work focused on metal-epitaxial-semiconductor field-effect transistor (MESFET) structures, which became widely applied in high-frequency circuits in commercial and military applications. But Rockwell Science Center (Thousand Oaks, CA) took a different look at GaAs, applying a more traditional bipolar device configuration to this high-speed material. The firm was the first to develop a GaAs heterojunction bipolar transistor (HBT) structure in 1984, with an impressive transition frequency (fT) of 50 GHz.
Rockwell (which would become Conexant) and others would also explore the high-frequency possibilities of another III-V semiconductor material, indium phosphide (InP), which was viewed as an attractive candidate for millimeter-wave devices. Rockwell developed an HBT process on InP and would eventually collaborate with Kopin Corp. to commercialize the process, in addition to encouraging the development of high-frequency devices and ICs on InP. Military customers such as the DoD and DARPA have particular interest in InP for what are known as terahertz monolithic integrated circuits (TMICs), intended for use at frequencies through 300 GHz, and in such applications as secure communications and missile guidance.
Millimeter-wave technology (30 to 300 GHz) has often been categorized as one of those technologies "just over the horizon" for at least 30 years. Ironically, with more available RF bandwidth being consumed by wireless communications, the need to place communications channels somewhere has made affordable millimeter-wave technology even more attractive. Of course, the key word here has always been "affordable," and that has been a stumbling block to the widespread use of millimeter-wave frequencies in short-range communications, sensors, and other applications. The US Federal Communications Commission (FCC) has cleared the use of various millimeter-wave bands for commercial use, but it may require the incentive of a MIMIC-like government-sponsored program to encourage RF/microwave companies to make the required investments to develop lower-cost millimeter-wave circuit and device technologies.
A technology that has intrigued all branches of the Armed Forces as well as many commercial researchers is microwave power transmission (MPT), which involves collecting power at one location and beaming it across space to another location where the power would be used. Especially with the current interest in "harvesting" energy from the environment, some means of transferring the energy from one location to another would be useful.
MPT was demonstrated to the American public in 1964 by Raytheon's Bill Brown, a major proponents of the technology. Brown appeared on national television with newscaster Walter Cronkite to show how a microwave transmission could carry power to a model helicopter which flew via MPT during the television broadcast. Unlike traditional microwave communications (where the received energy beam can be relatively low), the purpose of MPT technology is to transfer as much of the transmitted energy as possible to the receiver (with the highest possible efficiency).
The technology was proven at higher power levels in 1975 by means of tests performed by NASA's Jet Propulsion Laboratory (Pasadena, CA) at their Goldstone Deep Space Communications Complex (Goldstone, CA). During those tests an 85-ft. parabolic (dish) transmit antenna (Fig. 3) normally used for deep-space communications sent 320 kW power to a rectenna antenna array more than 1 mile away. The MPT beam was sent at 2.388 GHz. The measured conversion efficiency was over 80%. The rectenna array (Fig. 4) consisted of 17 subarrays, with each subarray containing 270 dipole antenna elements and Schottky diode rectifiers to convert the received RF back to DC power. A 450-kW klystron amplifier was the source of the power. NASA has long been interested in MPT technology for its potential of transferring power through space, for example, to a base on the moon or to an in-orbit space station (Fig. 5).
Contributors to Microwaves & RF such as Dr. A. Kumar of AK Electromagnetique have helped further MPT technology by designing high-gain, high-efficiency rectifying antennas known as rectennas. Kumar notes that the US Department of Energy, NASA's JPL, the Canadian Communications Research Center, and various European agencies are developing programs for MPT at 2.45 GHz, due to low atmospheric attenuation at that frequency. Other Industrial-Scientific-Medical (ISM) frequency bands are available, with some organizations preferring the smaller wavelengths and components required for use at 5.8 GHz, which also suffers low atmospheric attenuation.
As a variation of this MPT technology, military customers have pursued the use of microwave beams in directed-energy weapons. The US Air Force, for example, has supported research into the use of high-power microwave energy as a weapon through its Counter-Electronics High-Powered Microwave Advanced Missile Project (CHAMP) program. The initial intent is to create a weapon that could fire bursts of high-power microwave energy to disable the electronics of an opponent's systems, without causing harm to nearby people or infrastructure. CHAMP's microwaves could be delivered from pods on airplanes, unmanned aerial vehicles (UAVs), or even retrievable cruise missiles programmed to safely land near their points of origin. The US Office of Naval Research (ONR) High-Power Microwave Directed Energy Weapons Program has also been a strong supporter of developing new technologies for the advancement of directed-energy weapons systems.