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High-temperature superconductivity (HTS) was hailed as a great scientific breakthrough and received enormous media attention when it was discovered in 1986. For more than 15 years, worldwide materials and applications development has sought to put high-temperature superconductors to work in a variety of industries. By the late 1990s, RF applications emerged as the first place where these materials could play a significant role. With its explosive growth and increasing technology demands, the commercial wireless industry represented the first sizable opportunity for this new technology.

Successive generations of wireless systems based on HTS technology continue to offer increasing performance, high reliability and continued tailoring to the specific needs of today's wireless networks. Thousands of HTS systems are deployed today in wireless networks and the rate of adoption of the technology continues to grow.

As HTS technology gradually becomes established in the commercial wireless infrastructure, interest is growing in the technology for a variety of government applications. There are challenging RF requirements in multiple military, intelligence and law enforcement applications where HTS technology may have an important role to play. With recent developments in the areas of tunable HTS filters, as well as continued progress in HTS filters in general, the technology can offer capabilities that cannot be achieved by alternative approaches.

In the government world, there exists a multiplicity of RF applications that stress receiver performance from many perspectives. Sensitivity, ability to reject out-of-band interference, and wide dynamic range are all important characteristics of receivers used in reconnaissance, surveillance, secure communications and other intelligence activities.

Superconductivity is not a new phenomenon, having been discovered in 1911. The hallmark of superconductivity is the abrupt vanishing of electrical resistivity when a superconductor is reduced in temperature below its so-called critical temperature.

For most of the thousands of materials that exhibit this behavior, this critical temperature occurs very close to absolute zero (-273° C) Such low temperatures meant that superconductivity could only be achieved by using costly and complex refrigeration techniques. As a result, practical applications of the phenomenon were few and far between and mostly limited to exotic laboratory devices. The only significant exception was the use of powerful superconducting magnets almost universally found in MRI medical imagers where the costs and complexities associated with refrigeration were acceptable in the face of the unique capability provided by these magnets. This was the state of affairs until the late 1980s when the discovery of a family of new materials meant that there were now superconductors with critical temperatures above 90 degrees Kelvin (-183° C).

While 90 K is still a cryogenic temperature, it can be reached and maintained far more cost-effectively and efficiently than the operating temperatures of the earlier superconductors. For this reason, such superconductors are known as high-temperature, although clearly the term is relative. Liquid nitrogen can be used to cool these materials because it is widely available commercially and provides a stable thermal bath at 77 K. More significantly, single-stage, closed-cycle cryogenic refrigerators are available that can provide refrigeration at similar temperatures on a continuing basis with high reliability and at a reasonable cost. It is this capability that makes HTS technology practical for use in RF applications.

HTS technology is attractive for use in RF applications because, although the surface resistance of a superconductor is not zero at radio frequencies (as opposed to the DC resistance of a superconductor, which is zero), it is typically several orders of magnitude lower than that of even cryo-cooled copper. In Figure 1, the frequency dependence of the surface resistance of the HTS material YBa₂Cu₃O_7-δ(YBCO) is compared with that of copper at 77 K.

At typical communications frequencies, the YBCO provides at least a 1,000-fold reduction in surface resistance. With such a material, resonators can be designed with quality factors far in excess of those fabricated from conventional materials. Unloaded resonator Qs in excess of 100,000 are routinely achieved in commercial filters used for cellular communications, for example.

Practical realizations of microwave components using HTS technology have, for the most part, centered on the use of microstrip designs using thin-film technology. Both YBCO and Tl₂Ba₂CaCu₂O_10-δ(TBCCO) have been used in thin-film form in the production of HTS microstrip filters, which are essentially solid-state devices fabricated in a similar fashion as microprocessors and other semiconductor components.

At cellular frequencies, for example, even complex microwave filters take up no more than a few centimeters of space on a wafer substrate using HTS thin-film technology. The low-loss properties of the HTS films, therefore, not only provide excellent RF performance, but also substantial size reduction compared with conventional filters. Of course, this miniaturization of the filters is offset by the necessary presence of a cryogenic refrigerator. However, once there are multiple filters in a system, HTS technology provides size reduction even taking into account the refrigerator.

Microwave resonators with extremely high quality factors result in the ability to make filters with very little insertion loss, even with multiple poles in the filter, or even when the filter bandwidth is extraordinarily narrow. When such filters are used in receiver front ends, it is possible to have maximum frequency selectivity and maximum receiver sensitivity at the same time. Conventional filter technologies sacrifice sensitivity when selectivity is increased. In contrast, filters made using superconductors provide the closest approximation to a perfect filter; namely, one that allows 100 percent of the desired signals to pass through and rejects 100 percent of the unwanted signals. Hence, such filters are ideally suited for rejecting out-of-band signals, particularly those that are very close in frequency to the desired band.

Because of the unique properties of superconducting filters, the most appropriate applications for the technology are either to produce filters with extraordinarily steep skirts (extremely rapid fall-off in transmission outside the band of interest) or to produce filters that are extremely narrow in bandwidth. In either case, such filters can still have very low insertion losses. Figure 2 shows the measured response of an HTS filter designed for the wideband CDMA spectrum near 1.9 GHz.

This filter was fabricated with 22 poles, but the addition of multiple transmission zeroes in the design results in greatly increased sharpness of the band edge. The attenuation in this filter is such that the rejection reaches 100 dB only 400 MHz from the band edge. Such a filter would be virtually impossible to make using conventional approaches, and in any case would have enormous losses if it were built at all.

The high quality factor of HTS films also allows efficient filters to be made that are extremely narrowband in nature. Figure 3 shows an HTS filter at roughly 700 MHz center frequency with a bandwidth of only 100 kHz.

Filters of this fractional bandwidth (less than 0.02 percent) have, up until now, not existed at this high a frequency, and the technology is quite capable of extending this kind of narrowband performance to considerably higher frequencies. Figure 4 is a nomograph depicting the bandwidth capabilities of a variety of filter technologies as a function of center frequency. The estimated currently available performance of HTS filters clearly shows the newly practical spectrum for ultra-narrow filters.

HTS technology, therefore, offers unique opportunities for interference mitigation under difficult circumstances, particularly when interference sources are located very close to signals of interest.

The ability to produce extremely sharp-skirted filters is important for protecting relatively wideband spectra, such as in commercial wireless bands. The ability to produce extremely narrowband filters is important when capturing specific weak signals in cluttered RF environments is important. A key element of this capability comes from the ability to combine HTS filter technology with cryogenically cooled semiconductor amplifiers (typically GaAs FETs) to provide an overall low-noise front end for a receiver.

The dramatic reduction in amplifier noise performance, coupled with the inherent low noise of the HTS filters themselves, results in unparalleled front-end sensitivity. Typical HTS-enabled front ends have noise figures below 1 dB. Systems at cellular frequencies have noise figures closer to 0.5 dB. With the standard system architecture having the HTS filter as the first component after the antenna, and only then followed by the amplification stage, the HTS-enabled receiver front-end can simultaneously offer extreme sensitivity, unmatched selectivity and superior dynamic range.

HTS technology has been proven to enhance both receiver sensitivity and interference immunity at the same time. In particular, the value of HTS technology for military applications lies in its ability to extend the range of surveillance receivers. The cryogenic front end allows the system to reduce overall noise figure without sacrificing system dynamic range. Therefore, the surveillance mission can be accomplished at more than double the range of previous systems. This benefit has been tested in airborne, shipboard, and ground-based applications, with a dramatic improvement to reconnaissance range in all cases.

In addition to the surveillance and reconnaissance applications, HTS technology can improve performance in collocation interference problems. For example, on board many Navy ships, certain satellite communications signals are disrupted when on-board radar systems are engaged. Narrowband planar filter technology is perfect for these applications since the compact, high performance filters can reject the unwanted interference. In particular, one C-band satcom system has been outfitted with a cryogenic filter that improved signal-to-interference ratio by more than 60 dB in the presence of an interfering radar signal.

In some cases, these filters have been switched on and off to allow receiver systems to look through collocated interferors. This is typically accomplished through the use of semiconductor switches, which often are cooled along with the superconducting filters to minimize noise figure contributions from the GaAs switches. These switched filter banks can be very effective at rejecting strong signals that would otherwise overload the receiver.

Extremely narrowband HTS filters can be used to create front ends that are ideally suited to capturing weak signals of interest in complex RF environments. If the frequencies at which such signals occur are small in number, then discrete HTS filters for each signal of interest may be a viable approach for system design. However, there are applications, particularly in the defense, intelligence and law enforcement areas for which there well may be a multiplicity of such signals. For such applications, the only practical way to provide the front-end performance offered by HTS technology is to be able to tune the HTS filters over a sufficiently broad frequency range.

Such frequency agile HTS filter technology has been under intense development by a number of engineering groups for several years and is now reaching the point where systems incorporating this capability are being readied for deployment. There are several techniques by which HTS filters can be tuned over a substantial frequency range. The critical requirement is to retain the unique characteristics of HTS filters with the addition of frequency agility. This requirement implies that the inherent high quality factor of the resonators cannot be excessively diminished by the tuning mechanism. The technique being employed in the first generation of tunable HTS filters makes use of the motion of superconducting tuning elements that inductively couple to the filter resonators to shift their frequencies. By using superconducting elements, the impact on resonator Q is minimized.

The fundamental physics behind this tuning technique allows for substantial frequency shifts. By positioning a tuning element closer and closer to the resonator, the resonator frequency increases more and more dramatically. Changing resonator frequency by a factor of two is within reach by this method. From a practical standpoint, the limitations of the technique come from the ability to precisely control tuning element position. At least sub-micron precision and high reproducibility are required for filter tuning over any substantial range, and the precision requirement increases as filter bandwidths become narrower. The current state of the art permits fairly narrowband HTS filters to be tuned over 20 percent or greater bandwidths, but there is continuing progress towards increasing tuning range for ever-narrower filters. Figure 5 shows a narrowband HTS filter being tuned across the standard cellular frequency band.

HTS front-end technology is well along the way towards becoming widely used in the commercial and government wireless worlds. It has moved from being an exotic laboratory demonstration to a technology being manufactured in volume in the form of reliable, cost-effective products.

The proliferation of RF systems of all types continues to create new interference problems for communications systems in use throughout the government world. HTS technology offers unique capabilities to solve challenging problems in high-performance receiver systems as well as to enable entirely new capabilities in RF systems. Over the next decade, this technology that saw much of its early development driven and funded by the government — and is now becoming a substantial presence in the commercial world — will find its place in a broad range of government applications.

1. Additional discussion of HTS materials and properties may be found in D. R. Chase, “High-temperature Superconductors: Online and Operational,” RF Design, June 2002, and in the references cited therein.

2. G. Tsuzuki, S. Ye and S. Berkowitz, “Ultra Selective 22-Pole, 10-Transmission Zero Superconducting Bandpass Filter Surpasses 50-Pole Chebyshev Rejection,” IEEE International Microwave Symposium Digest, Vol. 3, June 2002.

3. K. Dustakar and S. Berkowitz, “An Ultra-narrowband HTS Bandpass Filter,” IEEE International Microwave Symposium Digest, Vol.3, June 2003.

Randy Simon is the vice president of government business for Superconductor Technologies Inc. (www.suptech.com). He is responsible for strategic product direction and technology development for government applications. He has also managed the government business and served as chief technical officer while at at Conductus Inc., prior to its merger with Superconductor Technologies in 2002. He holds seven issued patents and has authored technical papers, articles and a book on aspects of superconductivity and superconductor technology. He obtained his Ph.D. in physics from the University of California at Los Angeles. He can be reached at rsimon@sv.suptech.com.