High-temperature superconductivity for EW.
Figure 1 shows the areas in an EW system where HTS technology potentially will have an effect. After a brief discussion of the benefits of HTS materials and microwave device technology, this article will address the major HTS development and insertion programs for EW.
Superconductor technology has several performance benefits over conventional passive device technology approaches. Superconductor materials and passive devices possess the properties of no direct current (DC) loss, minimal microwave loss and virtually no frequency dispersion. Each of these properties can be exploited to produce high-payoff microwave EW system insertions.
No DC Loss - The physical property of no DC loss allows the fabrication of very high power superconducting magnets for focusing electron beams in microwave traveling wave tubes. These magnets, once charged by a charging coil, can provide high-quality beam shaping similar to solenoid magnets but require no prime power to operate (solenoids can require as much as a kilowatt of prime power to operate). The only cryogenic heat load is the heat leakage into the cryogenic package through electrical input/output connectors.
Very Low RF Loss - Superconductor materials' very low microwave loss can be exploited to produce low insertion loss/high quality factor (Q) devices for microwave systems. Because of the low microwave losses, smaller microwave devices can be fabricated using coplanar, stripline and microstrip device geometries while still maintaining the high Q and low loss characteristics of much larger and more expensive technologies such as waveguide. These lower losses will also increase system sensitivity or effective radiated power (ERP) and permit signal processing functions to be performed directly at microwave frequencies. This minimizes signal distortion and eliminates complex down conversion hardware.
No Dispersion - Superconductor materials are also virtually non-frequency dispersive well up into the terahertz frequency range. This means devices can be fabricated with extremely wide operating bandwidths. Fewer components are therefore required to cover a given frequency band. The wide bandwidth combined with the ability to process at direct microwave frequencies will reduce overall system size and complexity. Although superconductor devices must usually be fabricated on dielectric substrates that possess non-superconductor properties, devices have been demonstrated using both high- and low-temperature superconductor technology with much wider bandwidth than conventional technology approaches.
EW HTS DEVELOPMENT
A number of EW HTS development efforts have been awarded to various vendors in the EW and superconductor industry. Figure 2 shows the chronological evolution of HTS development efforts for EW. Each block in the figure will be discussed. A number of supporting efforts will be mentioned where appropriate.
EW HTS Studies
Wright Laboratory (WL) initiated a Program Research and Development Announcement (PRDA) in May 1988 titled, "EW Superconductivity Study." Two contracts were awarded under this PRDA. The first award was funded by the EW Division of Wright Laboratory, and went to TRW in September 1988 to evaluate airborne EW applications of superconductivity. A second award was made possible by funding from the Joint Technical Coordinating Group on Aircraft Survivability (JTCG/AS). This study was awarded to SRI International in June 1990 and evaluated EW and aircraft survivability applications of superconductivity. Both studies have been completed and are available through the Defense Technical Information Center (DTIC).
Channeled Cuing Receiver
Westinghouse Electric Corp. has been awarded a program through the Office of Naval Research ONR) to develop an HTS channelized receiver. The program is being funded by DARPA and technically directed by WL and ONR personnel. The scope of the effort is depicted in Figure 3.
Under the program, two identical banks of four 50-MHz thin-film filters centered at 4 GHz will be fabricated. Figure 4 shows the response of two contiguous six-pole filters previously fabricated by Westinghouse. What is not shown is that each filter is contained in a separate cryogenic package.
All filters will be on the same substrate and in the same cryogenic package. Two identical microwave delay lines will also be fabricated for each filter bank to operate over the 2- to 6-GHz frequency range. The receiver will then be married to an existing state-of-the-art signal encoder/processor and evaluated for amplitude and phase tracking between the two sets of filter banks and delay lines. The development will prove the applicability of HTS technology to direction finding systems as well as the ability to design and produce HTS devices to exact specifications in the same cryogenic package.
Multi-Octave Beam Forming
TRW Corp. and Sandia National Laboratories have been awarded contracts also through ONR to develop a microwave phased-array beam forming network (BFN). These programs are being jointly funded by DARPA and WL. Again, WL and ONR are providing technical management for the program.
Figure 5 shows the scope of the effort, which involves the fabrication of three critical components and their integration into an actual system implementation. The required components are 90 [degrees] hybrids, broadband power dividers and vernier controlled phase shifters. The components will provide higher power handling capability, lower insertion loss and broader operating bandwidths than conventional beam forming networks.
Sandia has demonstrated coplanar switched line and hybrid phase shifters operating from 4 to 20 GHz with maximum insertion loss and phase deviations of 1.8 dB and 8 [degrees], respectively. Since HTS components are low-loss, ultra-broadband and ideally implemented in planer technology, antenna systems can be made more compactly and less expensively than conventional systems. By the beginning of FY 1994, TRW will have four array elements to be tested for insertion loss, bandwidth, beam quality and beam-switching speed.
Under a Wright Laboratory, EW Division, Small Business Innovation Research (SBIR) Phase I program, Superconductor Technology Inc. (STI) has demonstrated both high power and low loss in fixed HTS microwave phase shifters. The phase shifters maintain complete linearity and low loss up to STI's maximum TWT amplifier output capability of 36 W at 6 GHz.
HTS Notch Filter Bank/Microwave
Under a Phase ISBIR program through ASD/XR (Wright-Patterson), STI also fabricated and tested a 28-nsec microwave delay line (Figure 6). The delay line demonstrated operation up to 3.6 GHz (limited by substrate mode) with a maximum insertion loss of less than 8 dB (Figure 7). Under the Phase II effort, STI is fabricating a 100-nsec delay line that will operate up to 10 GHz with less than 10-dB insertion loss. The device will be ready for laboratory and flight testing by early FY 1995.
As a second part of the Phase II program, STI is teamed with Electro-Radiation Inc. to develop a direct microwave frequency, switched HTS notch filter bank. The purpose is to reject unwanted microwave emitter signals from wideband EW receiver systems. Many weapon systems have been identified where interfering signals area major source of system problems. Tunable YIG filters are used in many systems to filter out these signals but they are large and temperature-sensitive and can require up to several milliseconds to tune and settle on a desired frequency. Superconductivity allows very small, high Q (narrowband) filters to be fabricated directly at microwave frequencies.
Many narrowband HTS filters can be placed in a small cryogenic package and switched in and out of the frequency spectrum in less than a microsecond to reject interfering signals from wideband microwave receiver/processor systems. ERI is evaluating various weapon system requirements and STI is fabricating the filter bank to ERI's design specifications. Figure 8 shows the frequency responses of a 1 cm x 1 cm four-pole HTS notch filter and an identical filter fabricated using gold conductor.
By the first quarter of FY 1993, STI will have 10 switchable filters fabricated for testing at Wright Laboratory's Integrated Defensive Avionics Laboratory (IDAL). The IDAL facility contains many types of wideband microwave receiver assets as well as a hardware simulator able to generate an extremely dense and complex hostile/friendly signal environment in excess of 500,000 pulses per second.
In the fourth quarter of FY 1993, it is planned that ERI will integrate a 30-filter HTS notch filter bank with a "host" receiver system and perform IDAL and flight testing in conjunction with a system program office (SPO). If further technology validation is required, a 100-filter HTS notch filter bank flight test unit will also be developed.
HTS Multi-Chip Module and
The Aeronautical Systems Division is managing the HTS Multi-Chip Module (MCM) and Microwave Insertion Program. The program is a dual-task program entirely funded by DARPA and is intended to demonstrate and insert HTS technology into operational EW systems.
The MCM task is being managed by E-Systems/Melpar and is to develop HTS interconnects between conventional semiconductor digital chips. To increase digital processing speeds, it is necessary to pack semiconductor chips closer together to reduce signal transfer time between them. The faster clock speeds, shorter interconnect lengths and reduced pulsewidths associated with these faster processors all work to increase surface resistance in the interconnects between the chips. To reduce this resistance to permissible levels, up to 50 signal planes of copper may be required, each with line widths of up to 50 [mu]m. This resulting complex structure is a non-trivial manufacturing, reliability and repair challenge.
Using superconducting interconnects between the chips, however, the number of signal planes can be reduced to only two layers with line widths of only 2 jam. It is much simpler and cheaper to fabricate and repair this two-signal-plane structure compared to the conventional interconnect version.
As an added benefit, CMOS and GaAs digital chips operate two to three times faster at liquid nitrogen temperatures than they do at conventional system operating temperatures. Candidate EW system applications include IR focal plane array processors and digital microwave receivers.
E-Systems/Greenville Division is heading up the HTS Microwave Insertion portion of the program, which involves multiple tasks. The first task is to acquire state-of-the-art HTS microwave devices from various superconductor vendors and test them for reliability and performance. Optional follow-on tasks are: 1) identifying potential high-payoff microwave system insertions, 2) choosing a high-payoff application and 3) developing the application and inserting it into an operational system. The two most promising candidates for the development are a microwave receiver or a true time-delay phased-array antenna system.
Superconductivity can also be used to produce digital circuits that can operate at speeds 10 times faster than semiconductor technology. A number of Japanese companies have fabricated microprocessors that clock at speeds in excess of 1 GHz. The problem is that these processors have needed to be cooled to liquid helium temperatures (4.2 K) to operate. This would clearly be impractical for airborne EW applications.
Recently, a new digital superconductor technology has emerged, known as Rapid Single Flux Quantum, that can be implemented in both the high- and low-temperature superconductor materials. Figure 9 shows a concept developed by Wright Laboratory for a digital EW system that could potentially be implemented using HTS. The key benefit derived from superconductivity would be direct microwave frequency digitization and signal processing. Demultiplexing is required, however, to match the ultra-high-speed superconductor circuitry with conventional semiconductor memory and processors.
HTS technology will affect both analog and digital avionic EW systems. Analog system applications are more near-term than digital applications. Successful technology insertion will depend largely on sufficient maturation of cryogenic cooler and packaging technology to make the fast-developing HTS technology practical for system insertion.
Paul A. Ryan received his bachelor's degree and is currently working toward his master's in electrical engineering at the University of Dayton. Since 1987, he has been working on digital radio frequency memories, antiradiation missile countermeasures as well as developing superconductivity applications for electronic warfare at the Electronic Warfare Division of Wright Laboratory, Wright-Patterson AFB, OH. Ryan is active in the Kittyhawk Chapter of the AOC and is his division's AOC national convention chairman.
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|Title Annotation:||electronic warfare|
|Publication:||Journal of Electronic Defense|
|Date:||Aug 1, 1992|
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