Amplifiers Technology Focus

It has been said that circuit board design is an art, but RF design is a black art. Indeed, the developers of RF amplifiers are part scientist and part sorcerer, coaxing incremental improvements in power and efficiency from tubes and transistors, while at the same time minimizing distortion created by the amplification process.

Radio frequency (RF) amplifiers are at the heart of every satellite terminal. They take in electrical signals and convert them into radio signals, significantly boosting the signal strength along the way. Without amplification, high frequency radio waves would have limited range and satellite communications wouldn’t be possible.

The quest for smaller, more powerful, and more efficient RF amplifiers never stops. What drives RF amplifier manufacturers to constantly push the technology curve? What new products are hitting the market now, what’s around the corner, and what might the sorcerers be seeing in their crystal balls?

The two primary criteria satellite engineers must take into account when designing a satellite terminal are the size of the antenna and RF amplifier, respectively. The cost, performance, and operating expense of each must be considered. If there is enough real estate, and the project timeline is long enough, the installation of a large antenna and small amplifier is usually the best choice. If space is limited, a smaller antenna fitted with a larger amplifier will get the job done as well.

Keep in mind that the “available space” could be real estate or inside a soldier’s backpack. The inter-relationship between antenna size and amplifier power is an ongoing ballet that must be re-performed for every new application and with every improvement in amplifier technology.

Why the quest for more power? To increase the amount of information that is transmitted via a radio wave, you must use a higher frequency and transmit that frequency at a higher power. Network architects never ask for slower data circuit speeds, so the trend is always upward.

Pressure
Internet growth, particularly the increase in video traffic, keeps the pressure on RF amplifier manufacturers to constantly seek higher outputs. “Satcom has always had a lot of video distribution and broadcast because of the one-way nature of it, but now there is much more real-time, on-demand and even two-way video services being carried,” says Heidi Thelander, director, business development at Comtech Xicom.

“It could be a military general who wants to see HD video of a target before he orders an attack; or it could be a gamer that wants to play a game with friends around the globe and have the same experience as if they were sitting in the same room; or it could be a family that wants to download a movie in two minutes, not 30 minutes. End users are demanding more video and that demand ultimately translates into amplifiers that can deliver higher power at higher frequencies.”

Although some are further along in the production of gallium nitride (GaN)-based amplifiers, every manufacturer points out their benefits, including higher power, higher efficiency, and smaller form factors.
“Gallium Nitride allows single transistors to reach higher powers than those based Gallium Arsenide,” says Michael DeLisio, CTO of Wavestream. “The improvement is roughly a factor of five and Gallium Nitride allows solid state amplifiers to go much higher in power. We are early in the adoption curve but increased commercialization of this new technology will have a huge impact on the market. We can now reach hundreds of Watts at Ka-band.”

DeLisio notes that Ka-band chipsets were just becoming available. In addition, fabrication techniques have now been standardized, allowing repeatability and reliability. He expected GaN technology to continue to evolve and improve and that 500-Watt solid-state Ka-band amplifiers would be available within five years.

Advantech Wireless’ CEO, David Gelerman, explains that “GaN-based amplifiers close the performance gap between solid state amplifiers and travelling wave tube amplifiers (TWTA). GaN-based amplifiers offer the lowest cost of ownership, with the lowest capital cost, and the lowest operating cost. It might be another 20 or 30 years before there is a breakthrough of this magnitude. GaN technology continues to progress and improve. There maybe something that comes along better than GaN but I doubt it will be in our professional lives,” he says.

Key Advantage
One key advantage that GaN-based amplifiers can offer is their smaller size. Manufacturers can now offer the same power amplifier in smaller footprint, or they build an amplifier of the same size, which produces more power. Either way, the market has responded positively to GaN amplifiers. C-band SSPAs are now available delivering 2 kilowatts of power and require only seven rack units (7 RU) of space compared to the full rack required by a klystron amplifier. Broadcasters in metropolitan areas are beginning to warm to these high power SSPAs as the space savings are significant.

With apologies to Mark Twain, the death of tube-based amplifiers is greatly exaggerated. TWTAs aren’t going away anytime soon. While it is true that improvements in SSPA power have made them a credible substitute for TWTAs, at lower powers and in some frequency bands, tube-based amplifiers still get the nod in all types of applications including those that require high power, especially at higher frequency bands, like Ka.

CPI manufactures both SSPAs and TWTAs and the company is a strong proponent of both types of amplifiers. “GaN amplifiers at low powers have the potential of becoming as efficient as traditional linearized TWTAs; however, in most medium and all higher power applications, especially at high frequencies, TWTAs excel and are a much better choice,” says Andy Tafler, president of CPI Satcom Division. “Also, TWTA technology has not stood still. There continues to be significant advances in improving efficiency and increasing Mean Time Before Failure (MTBF) of TWTs so the relative technical differentiation between SSPAs and TWTAs has not changed. GaN technology is a good solution for use in mobile and transportable terminals where smaller, lighter high power BUCs can be an advantage.”

Tafler pointed to the company’s state-of-the-art 500 Watt Superlinear Ka-band amplifiers that are used in a number of Ka-band gateways around the world. CPI’s current generation of Ka-band TWTAs are based on helix technology but Tafler notes that their research and development team is also working on future commercial products at even higher power levels, which use coupled cavity technology. This technology, previously used only in military products, will allow CPI to bring to market a 700 Watt Ka-band amplifier in the future.

Management tools for satellite networks continue to evolve and amplifier manufacturers have embraced this paradigm shift. Gone are the days of satellite engineers being cloistered in teleports with their RF infrastructure. Today’s satellite engineer wants to manage remote facilities over the Internet. In addition to laptop connectivity, engineers want instant access to amplifier performance statistics via smartphones and tablets. To accommodate their needs, amplifier manufacturers have embedded web GUIs into their products and have developed applications that allow engineers to query and amplifiers from mobile devices.

Amplifier efficiency is just as important as output power as it affects the total cost of ownership. Green initiatives are picking up momentum and European broadcasters are leading the industry in the adoption of more energy efficient amplifiers. As energy prices climb, the cost of electric utilities always follow suit. The combined cost of electricity to power the amplifier and to dissipate excess heat must be factored into the overall cost of ownership. Expect green initiatives to cross the Atlantic and be adopted by broadcasters in the United States in the near future.

Military
Military actions in Iraq and Afghanistan accelerated the development of man-pack VSATs. Sometimes referred to as “disadvantaged terminals,” man-packs are engineered to remove as much weight and bulk as possible. They are designed to operate on batteries should they need to be deployed in areas without grid power. Amplifier manufacturers have already begun to address ways to maximize battery life. A good example is Comtech Xicom, which has integrated a “clear sky” setting into some of their amplifiers. Since link budgets are designed for worst-case scenarios, amplifiers are sized to overcome any atmospheric losses. When the weather isn’t bad, the extra output power is wasted. The “clear sky” setting lowers the output power to what is actually needed and drastically reduces amplifier power consumption, thereby maximizing battery life.

Paradise Datacom, which is owned by Teledyne, manufactures a broad range of SSPAs and recently announced the introduction of a 500-Watt Ka TWTA. Paradise also supplies very high power modular GaN SSPAs to the U.S. and European governments using a proprietary combining method. Mike Towner, senior director of sales & marketing at Paradise Datacom, comments on this development as well as the increasing need for power efficient amplifiers for man-pack applications. “The goal is to maximize the battery life of the man-pack,” he says. “We are seeing highly integrated products coming to market. Our OEMs are integrating our block upconverters (BUC) and modem modules into man-packs. The integrated terminals have far less weight and take up less space. Because the components are highly integrated, the terminal is much more efficient than a collection of individual components. As a result, the integrated terminal draws significantly less battery power.”

The quest for higher speed satellite circuits will ultimately lead us to higher frequencies. Gelerman notes that we need to start thinking about what comes after Ka-band. “Twenty years ago, no one believed in Ka-band,” he says. “It wasn’t believed to be viable. Now, Ka-band is a workhorse. In five years, Ka-band will be built out and the frequency band will be saturated. We need to explore new frontiers and start thinking about higher frequencies than Ka-band.”

The Next Steps
Tafler says there is work being done in higher frequencies. “There are already some Q-band products, around 40 GHz, being manufactured today. These are gateway products used for the U.S. Government’s AEHF constellation,” Tafler says. “There is also research work being done in V-band, which is around 50 GHz. Currently, there is an experimental satellite with one V-band transponder in orbit and there are products being shipped for experimental purposes.” The enemy of all electronics is heat. Thermal density becomes a serious challenge as the output power of RF amplifiers increase but their form factors shrink. In addition to advancements in transistor materials, cooling techniques will become more important to the success of new amplifier designs.

“Lots of people are working on all sorts of cooling technologies to help deal with the thermal density issues,” says DeLisio. “One area under investigation involves thermal dissipating materials. For example, artificially-grown diamond is better than many metals at dissipating heat. The big question that remains is whether they can grow the material inexpensively enough to make it commercially viable.”

Although it won’t happen anytime in the near future, DeLisio postulated that micro-machined materials could one day make their way into RF amplifiers. The materials would contain miniature cooling pipes filled with fluid that would be built into a microchip and would act like a tiny cooling system.

Dustin Kaiser, senior analyst at Futron, shared the company’s insight on the RF amplifier market. The Bethesda, Md.-based consultancy firm is in the process of finalizing the release of their latest forecast of global satellite services demand. “Ka-band is now being commercialized,” Kaiser says. “Previously, it was exclusively a military frequency but now ViaSat, Hughes, Eutelsat, and others have developed commercial satellites based on the frequency. This is creating new opportunities in the RF amplifier market for large numbers of gateway amplifiers.”

Kaiser also expects the market for Ka- and Ku-band amplifiers leveraging new High Throughput Satellite (HTS) beams to grow in the maritime market. Futron sees higher bandwidths needed by ships as they turn more to expert systems. As older mariners retire, ship lines are relying more on expert systems and less on skilled sailors to operate ships.

Oil and gas is another growth segment for Ka-band amplifiers, particularly the offshore segment. Small stabilized terminals using powerful HTS spot beams will allow oil and gas vessels to support data-intensive activities such as seismic surveys, real-time drilling data management, and other remote surveillance and control activities.

Futron expects the military market for amplifiers to be strong in the future. “The U.S. Government’s sequester is the great unknown. The satcoms market supports many strategic programs, such as special forces, and those programs continue to get funding since they are considered force multipliers,” Kaiser adds.

Outlook
The outlook for the RF amplifier market is healthy. Manufacturers are able to spread research and development dollars across products aimed at the satellite, cellular, and electronic warfare markets. GaN-based amplifiers will continue to make important strides but the manufacturers of tube-based amplifiers aren’t sitting still. They will continue to improve their products as well.

Look for robust competition between SSPAs and TWTAs in the future. In the past, decisions about which type of amplifier to use were black and white, but as the power output of SSPAs in some frequency bands increased, clear choices became blurred and a grey area evolved in which either type of amplifier would work. Expect this grey area to increase in size, promoting spirited competition between tubes and transistors. The future looks bright for RF wizards.