Smaller, more proficient devices provide sharper signal differentiation for both military and commercial uses.
Highly refined signal filters will open new vistas in applications ranging from complex intelligence gathering to cellular telephony. The advances emerge from high-temperature superconducting materials incorporated into semiconductor chips. Researchers at the Defense Advanced Research Projects Agency have moved some aspects of this technology to the private sector for production and commercialization.
The utility of the high-temperature superconductor filter lies in its ability to concentrate only on the signal of interest, instead of sorting a signal out of noise and interference. Many Defense Department systems require detecting small signals with predetermined characteristics that are within a dense background of interfering signals. This requirement applies to communications, navigation, radar, low probability of detection communications, and search and rescue, for example. It also applies to signals, measurement and signature, electronics and communications intelligence. Whereas other transmission and receiving needs benefit from performance improvements or greater portability, these intelligence disciplines gain novel advantages in detection and analysis that translate directly to new approaches on the battlefield.
By narrowing coverage to only the signal of interest, the superconductor-based band pass filters reduce the need for complex signal processing to weed out noise and interference. Placing these devices between a receiver—the antenna—and the detector filters out background signals before the detector receives them. This is accomplished without a filter adding noise to the signal. A low noise amplifier at 77 Kelvin placed after the filter amplifies only the desired signal, instead of its accompanying noise. Without any out-of-band signals entering the detector, distortion is eliminated. As a result, specific signals can be plucked out of a field heavy with background noise and adjacent band interference.
Conventional military and commercial users can operate wireless communications equipment more efficiently and for greater ranges. However, it is the intelligence community that actually sees its mission enhanced, and even altered, by the new capability enabled by these filters.
Dr. Francis W. Patten, program manager at the Defense Advanced Research Projects Agency (DARPA), explains that signals intelligence (SIGINT) and measurement and signature intelligence (MASINT) were the first applications for these filters. Users “were really amazed at the quality of the signal that they were getting,” he says. Receipt of a variety of signals was improved, including speech. He quantifies this enhancement as “much better than a factor of two.” The devices are being installed on aircraft, boats and land vehicles—“wherever anyone is doing signal collection in the Defense Department,” Patten states.
Incorporating superconducting materials onto microelectronics involves a procedure similar to conventional photolithography. A thin film of high-temperature superconducting material is deposited on a substrate. The film usually consists of yttrium-barium-copper oxide, and the substrate is lanthanum aluminate. The four-component superconducting film requires precise combination, Patten relates, especially if a fifth component is added. To fabricate a filter, standard lithography forms the necessary patterns to construct the resident structures that comprise the passband of the filter.
Once fabricated, the devices are cooled to operating temperature by a cryocooler. This is the most expensive component in the system, Patten allows, and can cost from $8,000 to $14,000, depending on the amount of cooling power necessary. These cryocoolers are required to deliver as many as 4 to 6 watts of cooling lift, as opposed to only the half watt required for similar coolers on forward looking infrared (FLIR) systems. FLIR coolers also require only about 2,000 hours mean time between failures, whereas the superconductor cryocoolers require 10,000 hours.
The same technology can be used to notch out unwanted signals, which Patten notes is much easier than providing a band pass. In one system, a series of 32 filters are switched in and out to efficiently reject signals. The filters that are switched in have low insertion loss, which has virtually no effect on the receiver.
Patten emphasizes that the program does not confine itself only to filters, but includes a variety of elements. These components may comprise resonators and delay lines, for example, that are incorporated into communications systems and subsystems.
Four vendors currently are exploiting the technology: Superconductor Technologies Incorporated (STI), Santa Barbara, California; DuPont, Wilmington, Delaware; Conductus Incorporated, Sunnyvale, California; and Superconducting Core Technologies, Golden, Colorado.
The filters themselves, being only thumbnail size, are small enough to be grouped in substantial numbers. Patten speculates that as many as 100 could be assembled to cover the same number of different signal frequency sites. A system employing these filters could switch among them to search for specific signals, or it could employ all of them concurrently in a massive noise-reduction operation.
Performance improvements aside, the devices are allowing intelligence users to expand their detection and analysis methods. Patten notes that these users were more familiar with advanced technologies and therefore were able to exploit the systems to great advantage. “They love the stuff,” he says, describing field reports of the systems’ operations. The ability to track a signal source improved, and the quality of information went “way up.” Key to this was a range enhancement of “at least a factor of three,” he adds. This especially is noticeable in an urban environment rife with signal noise.
The technology would prove useful in any situation that requires finding small signals buried in noise, Patten offers. Having the ability to detect very-low-level signals adds a new dimension to the effectiveness of SIGINT and MASINT operators. They could pick up signals otherwise not detectable at reasonable ranges. These signals could be emissions from virtually any device or platform that emits in the spectrum.
Especially vulnerable would be hardware or systems that users do not realize are emitters. A vehicle, for example, that produces a very small but identifiable signal could now, with the help of this equipment, possibly be detected far beyond audible and visual range. Little thought has been given to emission shielding for objects that would be seen or heard far sooner than detected by current signal monitoring techniques. This gives intelligence operatives a new weapon for alerting battlefield forces to the presence of hostile units. Once an object can be detected, it can be geolocated using interferometry or triangulation.
High-temperature superconductor filters are not limited to intelligence gathering, however. Patten notes that communication between two parties tends to require bilateral links of similar strength. On the other hand, if one receiver were capable of picking out a small signal from large amounts of noise, the transmitter need not be powerful. In effect, the receiver would do all the work. The transmitter could be the size of a button.
The U.S. Navy is installing the technology on some of its SIGINT aircraft, as is the U.S. Air Force. The Navy also is beginning to install it aboard some surface ships, which Patten says is an ideal setup for a platform bristling with antennas.
Commercial applicability is another key element of this program. Patten notes that many of the vendors have been involved with high-temperature superconductors since their discovery a little over a decade ago, and these firms are seeking to establish a commercial market for the technology. Defense Department work, while vital, will not support these firms over the long term.
One commercial market for these devices is cellular telephony. This involves the device that receives the handset signal from the cellular user. In the United States, cellular markets are divided into two carriers, A and B. Each carrier has separate receive and transmit sides in the 800- to 900-megahertz bandwidth area. In both areas, the B carrier band is split by a small portion of the A band. While the A band receive generally requires only two band pass filters, the B band must employ high-quality filters, usually a band pass filter and a notch filter.
Instead of using cavity resonance filters to fit the band shape, B-band carriers can employ superconducting filters. The newer devices take up one-tenth the space of their conventional counterparts. In addition, they provide better response performance through lower insertion loss.
Onboard sensors alert system operators about the potential demise of a unit’s cryocooler, which permits repair or rapid replacement before the unit actually fails. This is easily detectable when the operating current begins to rise.
The devices are incorporated into cellular base stations. Users report a 50 percent improvement in operating range and a significant decline in the number of dropped signals.
STI is manufacturing B-band cellular filter systems that incorporate high-temperature superconductors. Patten maintains that these devices duplicate spectrum better than other technologies. Other vendors also are exploring this market.
Future DARPA research aims to produce tunable high-temperature superconducting filters. Being able to adjust a single unit would obviate the need for massed numbers of different filters. This tunability would range about 20 to 30 percent, Patten suggests. Having this capability will open up new applications in both the military and commercial arenas, he states.