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SSPA versus TWTA: Is There Room for Both?

By | October 31, 2014


      RF amplifiers are the pulmonary systems of satellite networks pumping out RF energy, the electromagnetic lifeblood that allows earth stations to communicate with satellites. Today’s satellites are larger and more powerful than their predecessors and communicate at higher frequencies. Likewise, satellite terminals are capable of transmitting much higher data rates. Without a cumulative improvement in amplifier technology, High Throughput Satellites (HTS) and other advanced networks wouldn’t have become reality.

      The Next Frontier

      Gallium nitride (GaN) transistors have continued to improve, allowing amplifier manufacturers to bring to market Solid State Power Amplifiers (SSPA) that operate at higher frequencies and produce higher output power. The trickle of GaN-based amplifiers a few years ago has tuned into a full-fledged torrent.

      “The transition to GaN-based products is huge,” says Steve Turner, VP of engineering at Teledyne Paradise Datacom (Paradise Datacom). “SSPAs were always bigger, clunkier and less prime-power efficient than TWTAs, but SSPAs had the benefit of reliability on their side. Customers have been willing to put up with the size and weight disadvantage of SSPAs in exchange for greater reliability, but GaN amplifiers are smaller and much more efficient than SSPAs in the past.”

      Bill Vassilakis, CTO at Wavestream, admits GaN is a relatively new technology compared to other semiconductors, such as Si and GaAs, and that initial reliability issues and high costs limited its appeal. “But these initial hurdles for the most part have been overcome,” he says. “GaN’s standout properties of high breakdown field, high saturation velocity and outstanding thermal properties make it the power amplifier semiconductor of choice for demanding VSAT applications.”

      Cristi Damian, VP of business development at Advantech Wireless, agrees but says there is even more to GaN. “It is now generally accepted that current GaN SSPA technology provides higher power and better linearity and efficiency than all other previous generations of solid-state amplifiers,” he says. “However, we believe we have not reached the full potential that they can offer, and that there is room for improvement. Our target is to be able to run any SSPA as close as possible to saturation, without degradation in satellite link performance.”

      CPI is already looking ahead as almost all future developments in its solid-state amplifiers will be focused on GaN because they are smaller and lighter, which is important for hub and mobile applications, such as mobility, according to Colin Eastment, VP of business development at CPI. “The relatively high efficiency of GaN vs. GaAs amplifiers makes them an attractive choice in many systems, such as ones under radomes where the removal of waste heat is a challenge. We also see major advantages for GaN devices in the airborne market, both commercial and military,” he says.

      Comtech Xicom is another company that has been quite active in the development of GaN-based SSPAs, according to Heidi Thelander, director of business development. Just as CPI, Comtech Xicom is also working on targeting the airborne market, which is very demanding, she adds. “Amplifiers that are smaller in size and weight are attractive, and perhaps that is where GaN amplifiers offer their biggest advantage: their efficiency and thermal characteristics. Amplifiers operate in extreme heat while an aircraft is parked on the tarmac and in extreme cold when at altitude.”

      The Product Race

      Each company is working on releasing their top product for the industry. Wavestream has its 12 W Ka-band block BUC, which ensures the maximum available power at the feed flange, according to Vassilakis. “This amplifier provides the ability to cover multiple frequency bands with a switchable up-converter in an Industry-leading small, rugged, outdoor package. … [it] measures 9 by 3.5 by 2.5 inches, weighs 3.25 lbs., and consumes a typical 85 Watts of DC Power.”

      Paradise Datacom will be introducing 100 through 500 Watt Ka-band SSPAs, according to Turner. And CPI is introducing a number of new products including a new smaller, lighter 750-Watt SuperLinear TWTA. The first version will be Ku-band and other frequencies will follow soon after. Additionally, CPI has increased the number of amplifiers that feature embedded web browsers, which eliminates the need to write customer drivers for Monitor & Control (M&C) systems.

      CPI is now shipping TWTAs that incorporate recently patented LifeExtender technology. “The limiting factor in the life of a tube is the amount of barium left on the cathode,” Eastment explained. “By monitoring the temperature of the cathode, you can effectively monitor the evaporation rate. Adjusting the heater voltage downward minimizes the amount of barium that boils off the cathode. This significantly decreases the ‘wear out factor,’ and a companion product called LifePredictor also helps customers reports how much life is left in the tube.

      Advantech’s Damian notes that the company is working on a new generation of GaN SSPAs that will cover the DBS bands (17.3-18.4 GHz) with power levels exceeding 2KW. “This will complete our current DTH offering, for operators that do not use standard 14 GHz band,” Damian says.

      Comtech Xicom’s new high efficiency 800-Watt Linear Ku-band HPA is one of the highest power TWTA in its class on the market today, according to Thelander. “This amplifier gives you significant multi-carrier power,” she says. It is available in an outdoor model, allowing installations close to the antenna’s feed. The company has also introduced two new X-band SSPAs: a 50-Watt Linear amplifier and a 100 Linear Watt Model.

      Thelander adds that Comtech Xicom is integrating space-proven Life Extension technology into a range of TWTAs to enhance HPA lifetime, reduce maintenance and provide end-of-life indications for replacement planning. Design changes to the tubes were required to fully implement this feature, which is superior to other life extension products.

      “There has always been a big competition between TWTAs and SSPAs,” says Turner. “The development of GaN devices has normalized all of the competitive issues, plus you get a tremendous increase in efficiency. For the first time in my career there is a 400 Watt SSPA equal in size and weight and slightly more efficient than a TWTA in C-band. … Tubes have raw power advantages over SSPAs beyond 20 GHz. Tubes are able to process higher power levels with greater efficiencies than SSPAs at millimeter wave frequencies, but that is changing quickly. For a few more years, tubes will enjoy an advantage in Ka-band but that will go away rapidly. The tide has really turned. This is why all the excitement [is] over GaN.”

      “I think that we have definitely reached a symbolic milestone,” Damian stated. “Today we deliver SSPAs that offer more power than any TWT, and it seems to me this is the point of no return. We see massive Klystrons and TWTs replacement programs, as customers learn about GaN technology and understand the full benefits.”

      But while some manufacturers tout the “sky’s the limit” vision for SSPAs, others are not so enthused.

      “There seems to be a lot of progressive disinformation in the satellite amplifier market,” Eastment says. “Very often, both tube and solid-state amplifiers will work for the same application. It comes down to the environment the customer will operate in and what is the best solution for them. The truth is that no one technology is suitable for all applications. Unfortunately, when companies have only one solution to sell it leads to a lot of misinformation.”

      CPI sells both TWTAs and SSPAs and Eastment is quick to point out the benefits of each in specific applications, but he disagrees with some of his competitors regarding the power level at where it makes sense to choose a tube amplifier over solid-state. “Ten years ago it was 100 Watts. Now it is around 200 Watts. Some of our competitors are saying that the dividing line is 2 KW. That is just not true. It doesn’t make sense to use an SSPA at such high power levels because they are too heavy and inefficient,” Eastment adds. “We can prove that it will cost $50,000 per year to operate a particular type of amplifier but only $10,000 per year for another. Earth station operators care about this. Unfortunately, system integrators may have been tasked to do the job for the lowest capital cost. It’s not about what you can do technically, it’s really about whether your decision is practical.”

      Thelander says there is a certain power level where combining transistors doesn’t make sense. “There are different ways to combine but every method has limits. At some point, the combining loss begins to overwhelm the benefits of additional transistors,” she adds. “In many applications both tube and solid-state amplifiers will work. The customer needs to make some trade-offs based on what is most important. Over the years, the overlap between SSPAs and TWTAs has been fairly consistent, but the power levels at which SSPAs compete has been steadily moving up, even as the highest power available from TWTAs has moved up.”


      The HTS Play

      Grandiose plans for HTS that deliver terabytes of throughput are now in discussion and network designers feel that they need more spectrum if their visions are to come true. V-band (47-51 GHz) offers almost 5 MHz of available real estate in contrast with Ka-band, which is only 3 GHz wide. Q-band (38-40GHz) is another possibility. Both frequencies are uninhabited and are possibilities for future satellite communications.

      “The deployment of HTS is one of the biggest drivers in the satellite market,” says Brendan Murray, an analyst with the Washington, D.C. consultancy agency Avascent. “It’s not just Ka-band, services but Ku-band as well. As these new HTS systems are deployed around the world, they flood the market with capacity. Satellite is no longer the connectivity option of last resort.”

      Murray points out that HTS systems have an expanded gateway architecture and require more gateways than FSS networks. He highlights the NBN system in Australia as an example: it has only two high throughput satellites but 10 gateways, which address only 3 percent of the underserved population in the country. To reach more subscribers, more gateways will be needed, driving future demand for RF amplifiers.

      “Contrast the HTS paradigm with an FSS constellation, such as the one SES operates,” Murray adds. “They have 56 satellites but only 16 gateways. The construction of new HTS constellations will have a positive impact on the RF amplifier market.”

      Additionally, countries such as Congo, Nigeria, and Sri Lanka have launched national space programs and now rely on their own satellites to distribute programming. Murray notes that each new space program increases the market size for amplifiers.

      “The aeronautical market is very interesting, particularly the commercial airline segment,” Murray says. “There is intense competition to deliver in-flight Internet services, but the final business plans haven’t been fully fleshed out yet. Customers state that they want to stay connected while they are travelling but their willingness to pay isn’t totally understood yet. Will the Internet services be free to travellers? Or will they be free in business class? Or will they be offered at a flat rate? The real answer is that there is a lot of experimenting going on.”

      It should be noted that aeronautical satellite systems are like every other contested satellite network with lots of people using the same resource at the same time. When that happens, everyone’s connection slows down. Airlines can’t afford to give the services away and need to recoup their investment somehow, but will the travelling public be willing to pay for services? Or will they expect service to come at no perceived extra cost, like hotel Wi-Fi? The aeronautical market could be an important growth area for RF amplifiers and the rest of the satellite market, but the jury is still out on how in-flight services will be monetized.

      Satellite communications is a small part of the overall market for RF amplifiers. Electronic warfare, radars, surveillance, and deep space research all use RF amplifiers. Many of the research and development benefits intended for these other market segments spill over into the satcom market. Look for continued improvement in both tube and solid-state amplifiers, both in the short term and well into the future. VS