At Raytheon’s Radio Frequency (RF) Components facility, engineers are producing the next generation of Gallium Nitride (GaN) Monolithic Microwave Integrated Circuit (MMIC) amplifiers that will provide advanced capabilities to our defense systems in the areas of RF power, efficiency, and reliability at millimeter-wave (mm-wave) frequencies. Raytheon is at the forefront of GaN technology, having developed microwave GaN amplifiers that provide a five-times boost in power over legacy Gallium Arsenide (GaAs) amplifiers, meeting the challenging requirements of key emerging defense systems such as Raytheon’s Air Missile Defense Radar (AMDR) and the Next Generation Jammer (NGJ).
Building from this strong foundation, Raytheon has developed higher frequency, mm-wave versions of the GaN process that operate through W-band (higher than 30GHz). This W-band mm-wave GaN process has enabled the development of highly reproducible watt-level solid-state MMIC amplifiers at W-band. By combining these MMIC amplifiers, Raytheon has demonstrated solid-state amplifier power levels previously attainable with only vacuum tube based systems. In particular, the free-space combining of 8,192 of these amplifiers has produced a solid-state transmitter capable of 7 kilowatts (kW) of power at W-band frequencies. This is more than two orders of magnitude higher than the highest published W-band solid-state power to date (less than 40 watts). This high power amplifier capability is an enabler for advanced future systems, including high power solid-state mm-wave radars and solid-state active denial systems.
Vacuum Tubes’ Last Frontier
Invented in 1904, vacuum tubes were the key building block for early computers, radios, televisions and radar transmitters. Soon after its invention in 1947, the transistor began replacing vacuum tubes in one application after another, which resulted in compact, lighter weight and more reliable systems than the tube-based predecessors. However, one area where vacuum tubes are still predominantly used is for very high power (kilowatt-level) mm-wave sources. For a transistor to produce gain at mm-wave frequencies, it must have small dimensions, which in turn limits its power capability. Historically, the highest power transistor amplifiers at mm-wave frequencies have been in the milliwatt level, limiting their ability to compete with vacuum tubes for kilowatt level mm-wave applications. GaN is now changing this.
RF power performance improvements afforded by new semiconductor technologies have been evolutionary, offering incrementally more power as RF semiconductors have evolved from Silicon to Gallium Arsenide (GaAs) and Indium Phosphide (InP). Gallium Nitride (GaN) technology, however, is truly revolutionary, resulting in dramatic (more than five times) improvement in amplifier power. Like Silicon and GaAs, GaN is a semiconductor, but with a tighter crystal structure and a higher breakdown electric field, allowing for operation at higher voltages and producing more RF power. Raytheon has pioneered the development of W-band (95 GHz) GaN MMIC power amplifiers, demonstrating the world’s first watt-level W-band MMICs.1 Figure 1 shows a comparison of published output power of MMIC amplifiers of the various semiconductor technologies at 95 GHz.
Raytheon’s mm-wave Solid-State Power Amplifiers (SSPAs)
While the higher power capability of GaN technology makes kilowatt level power amplifiers possible, producing kilowatts of output power at W-band requires the low-loss combining of the output power of thousands of MMICs. To accomplish this, Raytheon has developed a patented modular, free-space combining approach (figure 2). This modular design approach provides economy-of-scale, enabling significant production cost reductions, as well as scalability to configure a variety of different-sized systems from the same basic building block. As shown in figure 2, eight 1 watt (W) MMICs are free-space combined to produce a tile-able 7W sub-module. Next, a four-by-four array of these sub-modules is constructed to produce a 100W module. Multiple 100W modules can then be arrayed to produce kilowatt levels of output power. In the figure, an eight-by-eight array of modules was used to produce 7 kilowatts of RF output power at W-band. To put this remarkable result in perspective, the previous high-water mark for W-band power was less than 40W.
System Application and Reliability
The 7 kilowatt system’s array aperture is compact (approximately 25 inch by 25 inch) and is presently planned for use by the U.S. Army as a Solid State Active Denial system. Active Denial is a nonlethal, counter-personnel weapon utilizing high-power mm-wave (approximately 95GHz) technology. Application of millimeter waves causes rapid skin heating, quickly inducing activity-disrupting pain. The pain stops when the beam is turned off, or the person leaves its path. Traditional active denial weapons, such as “System 1” (Figure 3, left), utilize vacuum electron device (VED) transmitters. These systems require dedicated vehicles to house the relatively large VED and ancillary support equipment. Solid State Active Denial Technology (SSADT) enables smaller adjunct stand-alone tactical systems that can be mounted on top of a variety of different type vehicles (Figure 3, right). In addition to the active denial application, the mm-wave SSPAs are also finding application for vacuum tube replacement in radar and electronic warfare applications.
Historically, a concern of tube-based millimeter power amplifiers has been shorter-than-desired operational lifetime of both the tube and the required high voltage power supply. By contrast, the GaN MMICs have very good reliability. Raytheon’s W-band GaN technology has been shown, through testing, to have an extrapolated lifetime of approximately 1 million hours at typical transistor junction temperatures. In addition, unlike the single point of failure seen with the vacuum tube in tube-based transmitter systems, the modular power combining approach of the SSPA allows for graceful degradation of system output power in the event of a MMIC failure. Based on a system lifetime criteria of 1dB of power degradation, it is calculated that the kilowatt level GaN W-band SSPA will have a system lifetime of over 30 years.
Raytheon is a leader in W-band solid-state power combining, producing state-of-the-art W-band GaN MMIC power amplifiers and pioneering the efficient power combining of solid state MMICs to produce two orders-of-magnitude more solid-state power than previously shown. The kilowatt level power amplifiers produced in this manner have application in tube replacement for both radar systems and active denial systems. Raytheon’s future work in this area will focus on extending the output power of these systems to tens of kilowatts by increasing the power of the individual GaN MMICs, as well as on demonstrating the feasibility of GaN SSPAs at even higher frequencies, such as 140 GHz.
— Nick Kolias,
— Andrew Brown
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