The Defense Advanced Research Projects Agency (DARPA) is making progress toward reaching a challenging portion of the radio frequency (RF) spectrum with a pair of programs involving vacuum electronic devices that could improve a host of radar, security and imaging programs.
One program, known as High Frequency Integrated Vacuum Electronics (HiFIVE), has a goal of developing an integrated, microfabricated vacuum electronic high-power amplifier circuit for use in high-bandwidth, high-power transmitters, according to DARPA. The program will demonstrate operation of such a circuit over a five gigahertz (GHz) frequency band centered at 200 GHz.
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| Blueprint of a solid state receiver. Photo: DARPA. |
Focuses of the HiFIVE program include high current density cathodes, high-power electron beam transport and microfabrication of an interaction structure to enable an integrated, microfabricated vacuum electronic high power amplifier circuit. Vacuum electronic devices are frequently used as high power amplifiers at frequencies approaching the millimeter wave (MMW) and sub-MMW part of the spectrum, which is of interest to the Defense Department because applications including high-bandwidth communications, collision-avoidance radar, security screening and high-resolution imaging can be improved upon due to challenges in accessing upper MMW and sub-MMW frequencies.
Traditional vacuum electronic devices are custom-built using numerous discrete components. For a vacuum electronics device to operate at MMW and sub-MMW frequencies and remain compact, they must be highly integrated and fabricated by utilizing state-of-the-art micromachining techniques.
“We were trying to take the state-of-the-art, which was relatively low power, relatively large form factor, and shrink it down into higher power, smaller form factor,” DARPA Program Manager for Microsystems Technology Devereux Palmer told Defense Daily yesterday after a presentation at the Association of Old Crows (AOC) conference on electronic warfare in Washington.
Palmer said the next goal for HiFIVE is to try to shrink the form factor even further while still maintaining a high power output while also moving toward continuous wave (CW) operation. HiFIVE, Palmer said, is currently pulse wave, which he said is useful in many applications, but that CW operation is more useful.
Palmer estimated DARPA’s investment in HiFIVE at $50 million over the full extent of the program. He said “multi-tens of millions of dollars” is a “roughly-standard” investment for DARPA programs.
“If you look at DARPA’s investments, especially in the microsystems technology office over time, there’s been a strong investment in improving device performance: transistor performance and, to a lesser extent, vacuum tube performance,” Palmer said. “Some people think that nobody uses vacuum tubes anymore, but really, they’re everywhere, especially in military applications.”
The other program is known as Terahertz (THz) Electronics, which has a similar goal to HiFIVE, but with higher frequency goals, Palmer said. Terahertz Electronics’ goal is for transistor-based electronics that will enable electronic capabilities at terahertz frequencies. The program focuses on two areas: terahertz transistor electronics and terahertz high power amplifier modules.
The success of the Terahertz Electronics program will lead to revolutionary applications by enabling coherent terahertz processing techniques, such as terahertz imaging systems; sub-MMW, ultra-wideband, ultra-high-capacity communication links and sub-MMW, single-chip widely-tunable synthesizers for explosive detection spectroscopy.
Palmer said while HiFIVE was a good demonstration that could work at a few hundred GHz, the goal of THz was a “considerable stretch.” Fortunately for DARPA, Palmer said, it was able to demonstrate that it is possible to develop the devices and technology necessary to realize small, high-performance circuits working at center frequencies exceeding 1.0 THz. DARPA hopes that will open interest from people in testing the components and figuring out new ways to use them.
“In the Terahertz Electronics case, it was explicitly called out that we were asking for development in the solid state arena and not just driver amplifiers, but also integrated ‘exciter’ and receiver circuits,” Palmer said during his presentation.
DARPA said researchers in July created the world’s first solid state receiver to demonstrate gain at 0.85 THz, representing progress toward the second major technical milestone on the way to 1.03 THz integrated circuits. Previous milestones included demonstrations at 0.67 THz.
DARPA Program Manager John Albrecht said on the agency’s website that the ability to coherently process signals at 0.85 THz provides a means to generate and radiate the high frequency signals needed for applications such as DARPA’s Video Synthetic Aperture Radar (ViSAR) program, which seeks to develop and demonstrate a targeting sensor that operates through clouds as effectively as today’s infrared (IR) sensors operate in clear weather. Albrecht said this would give United States warfighters an advantage in this difficult portion of the RF spectrum.