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The future of high temperature superconducting microwave circuits.

What is the future of high-temperature superconductive (HTS) microwave circuits? Predicting the future is always risky, especially for cutting-edge technology. Because there are so many competing factors, any major breakthrough in one area can change the entire outcome. The microwave community had at least one negative experience regarding the little copper pipe called the T[E.sub.01] mode cylindrical waveguide in the 1960s. Because of the low loss and tremendous band-width that the T[E.sub.01] mode waveguide can provide in millimeter wave frequency, the T[E.sub.01] mode cylindrical wave-guide for high-volume communications was considered hot. It was predicted that all future long-distance surface communication links would be through these little copper pipelines. However, only a decade or so later, the prediction was proved wrong, and instead optical fiber became the backbone of long-distance telecommunications networks. Optical fiber provided several thousand times broader bandwidth with much lower loss than copper pipelines. In addition, optical fiber was less expensive and easier to handle. All of this is now history. The other problem with forecasting is that we tend to forget correct predictions and only remember the incorrect ones. If this is so, why predict at all? Clearly, predictions are necessary because without them we do not know which direction to go.

Let us look at some prior predictions about the future of high-temperature superconductivity in general. After the discovery of HTS some extremely optimistic predictions surfaced in a very euphoric atmosphere. One prediction was that the impact of HTSs on our daily lives would be equivalent to that of semiconductors in a relatively short time. Soon after that, when preliminary test results showed very low critical current density for some early HTS samples, the other extreme prediction emerged, and the title of a report about high [T.sub.c] superconductors published in one of the nation's most influential newspapers read "The Party Is Over."

At least we know now that the extreme pessimists were wrong. The party is not over. The latest data on critical current density for certain thin-film HTSs are in excess of 1 x [10.sup.7] A/[cm.sup.2]. Such high current density is sufficient to handle microwave power up to hundreds of kilowatts in some components. But what about the extreme optimists? You really cannot prove optimists wrong, because they can always say "not yet." But at least one thing is certain: without a room-temperature superconductor, the impact of superconductors will never be comparable to semiconductors. The reason is quite simple. Imagine asking consumers to put liquid nitrogen into their superconductive TV sets, walk-mans or wristwatches every day. If this scenario is impossible, then until room-temperature superconductors become a reality, the extreme optimists are also wrong.


The distinctive features of HTS microwave circuits are very high Q or very low loss due to very low surface resistance, extremely low noise due to quantum effects and cryogenic temperature operation and miniaturization due to high dielectric constant substrates. The most promising applications are those which can take full advantage of these features. The following is a short list of them.

The first is high Q resonators and filters. Extremely high Q-values have been achieved in some HTS resonant structures with impressive power-handling ability. Based on this technique, there are some attractive applications. One is an extremely low phase noise oscillator. By using such a high Q resonator, extremely low phase noise can be achieved. Some prototype HTS hybrid oscillators have shown very low phase noise already. Applications will be in radar, electronic warfare (EW), electronic countermeasures (ECM), instrumentation and telecommunications. The other application is narrowband filters. Using such high Q resonant structures as building blocks, the number of poles can be increased without suffering excessive loss. These filters have very sharp skirts, very high rejection in the stop band and very low insertion loss in the passing band. In fact, some low-power HTS filters and multiplexers have shown very good performance already. Now the challenge is in the high-power HTS filters, multiplexers and filter banks. As the technique progresses, both low- and high-power versions with excellent performance will become a reality. The applications will be in channelized receivers and transmitters for communications, both territorial and satellite.

The second is long delay lines with low loss. It is conceivable that broad-band, low-loss HTS delay lines will have microseconds of total delay in a compact package. The size and weight savings could be several orders of magnitude compared to conventional cable delay lines, while providing the same or even better performance levels. HTS delay lines could find use in radar, EW systems, ECM systems and certain instruments.

The third is receiver front ends. Receiver front ends have been a favorite candidate for applications since the early days of low-temperature superconducor electronics. The advantages are low noise, high gain and a compact system integration. In the case of the HTS receiver front end, the HTS-semiconductor hybrid version is already demonstrated. As the active HTS devices become more mature, even a monolithic HTS receiver front end is conceivable. Applications will be in communications, radar and EW and ECM systems. In fact, any low- noise receiver, if the sensitivity is not limited by other factors such as atmospheric background noise, will benefit from the HTS version.

The fourth is antenna systems. For certain antenna arrays with a large number of elements, the key is to reduce the insertion loss of the feeding network. HTS technology has shown promise in such applications. Moreover, by integrating the HTS phase shifter into the antenna system, the fast electronic beam steering ability will further enhance performance. In addition, by integrating part of the receiver front end, such as a detector or mixer, into the HTS antenna system, the receiver sensitivity will be improved enormously through the combination of noise reduction and antenna gain. The contribution of an HTS antenna system could thus greatly exceed expectations.

The fifth is analog/digital (A/D) converters. The A/D converter is the bridge from microwave to digital. Its importance cannot be overestimated. State-of-the-art conventional A/D converters have limitations in both frequency and resolution. The heart of the A/D converter is a fast switch used for sampling the analog signal. Both the laser-controlled HTS switch and the Josephson junction are excellent candidates for such a purpose. In addition, there are other roles that HTS components can play to improve the A/D converter's performance. HTS hybrid or monolithic A/D converters with orders of magnitude performance advantage over conventional versions could be seen within a few years.

The sixth is high-density ultrafast interconnections. Statistics show that for the past several decades the speed of the computer increased exponentially, with no sign of saturation yet. If this trend continues, the bottleneck in hardware will be the intermodule connections. For a multigigahertz clock rate, the pulse transient time is in the low picoseconds. For such ultrafast signals, even a short connection line has to be treated as a microwave transmission line. In a multichip-module environment, traveling delay, crosstalk and signal distortion become very crucial. HTS transmission lines will play an important role in this area.


The performance-to-price ratio is an important index for high-technology products. For HTS microwave circuits, the performance is high, but so is the price. Therefore, the way for the market to develop is to concentrate on segments that are very sensitive to performance and relatively insensitive to price. At least three markets fit these criteria.

Advanced military equipment: Historically, new technologies in the microwave industry always started from the military. There is no reason to expect HTS microwave circuits to be different. In fact, since the discovery of HTS, most microwave R&D expenditures were funded by the US government and, to a large degree, by the Department of Defense. Advanced military equipment is technology driven and high performance is the key. Even without further breakthroughs, HTS microwave circuits are ready for some very important military applications.

Space communications: Space communications include satellite and deep space communications. Both are attractive markets for HTS microwave circuits. Size, weight and power consumption are very crucial for satellite communications, and by using HTS microwave circuits, all three considerations can be greatly improved. For example, consider the filter bank. The current version is made of metal waveguides that are heavy and bulky. In some communications satellites, the weight of filters, filter banks and accessories accounts for one-eighth of the total weight of the entire satellite. Planar HTS filters and the high Q HTS-sapphire filters are potential replacements for filter banks in both the receiver and the transmitter, respectively. In addition, other HTS micro-wave circuits can also improve the performance of the satellite; for example, a hybrid or monolithic receiver front end coupled with an HTS antenna array would greatly improve system performance. Furthermore, for HTS circuits in the satellite, using passive cooling to eliminate the cryostat might eventually be possible, especially if thallium HTS materials operating at around 100 [degrees] K are used.

For deep-space communications, aside from the common features of satellite communications such as size, weight and power consumption, receiver sensitivity is extremely important because it determines the maximum communication distance. Therefore, the hybrid or monolithic HTS front end and the HTS antenna array are key subsystems for improving performance. In addition, the low ambient temperature of deep space is a plus for cryogenics. NASA has programs in place to expedite progress in this field.

The main hurdle for HTS microwave circuit installation in space communications is the generally conservative attitude of the industry. Because of durability concerns, any new technology to be installed in space vehicles must go through extensive tests that take years, if not decades.

The last category includes ultrafast interconnections, ultrafast A/D converters and possibly other A/D interface circuits. The world is going digital and microwave technology is no exception. The key component for a digitized microwave system is the A/D converter, which serves as a bridge from microwave to digital processing.

In this segment of the market, the first entry would be HTS interconnections. As mentioned previously, the HTS transmission line is the solution for high-density ultrafast electrical interconnections. In an MCM environment, in general, there are only two ways for sending ultrafast (low picoseconds transient time) signals without severe distortion: using supercon-ducting transmission lines that require cryogenics and optical transmission lines that require electrical-optical converting and reverse converting. As the clock rate of computers approaches multigigahertz, this ultrafast MCM market is bound to take off. Which way the technology will go depends on the scheme of the computer.
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Author:Shen, Zhi-Yuan
Publication:Journal of Electronic Defense
Date:Jan 1, 1995
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