Satcom Amplifiers - Comparison - SSPAs Vs TWTAs

Introduction

Since their debut back in 1948, microwave RF amplifiers based on solid state circuitry, have served as direct competition to amplifiers powered by traveling wave tubes (TWTs). At the time of their introduction, solid state power amplifiers (SSPAs) were limited in the frequency bands they could address and the amount of RF power they could generate. Fast forward to the present – transistor technology has evolved to the point that today’s SSPAs have become formidable competition to TWT amplifiers (TWTAs) in virtually every application. In some cases, the value of SSPAs greatly exceeds that of TWTAs in the areas of RF performance, reliability, resilience to shake and vibration and serviceability.

During the first few decades following the introduction of solid-state power amplifiers, field effect transistors (FETs) were based on Gallium Arsenide (GaAs) substrates. GaAs FETs by design, were limited in their ability to deliver the levels of output power necessary to compete ‘head-to-head’ with TWTAs and Klystron amplifiers. In the early 2000’s, a new FET structure based on Gallium Nitride was released to the market. Gallium Nitride (GaN) devices proved to be superior to GaAs devices in several ways.

The improved reliability of the GaN transistor marked a milestone in SSPA production, largely due to its resilience to heat. Efficiency was improved and over twice the power could be generated in a package of similar size. Though the popularity of GaAs FETs has waned over the years, it should be noted that there are applications, particularly in C-band, where GaAs FETs actually deliver superior performance. To be complete, an amplifier manufacturer’s portfolio should include products based on both technologies.

The following discussion examines some of the factors that are driving the proliferation of solid-state technology, particularly in the field of RF transmitters used for satcom ground stations. The information contained herein will compare the design characteristics and operational differences between SSPAs and TWTAs.

Non-Linearities in Amplifiers

Amplifiers used for communications must process complex signals accurately, and in many cases, there are multiple signals being passed through the amplifier simultaneously. Since multiple signals interact with one another, it is important to characterize an amplifier’s design to determine the degree of interaction - or intermodulation distortion (IMD), as too much will degrade its performance.

Typically, a standard two-tone IMD test is used to benchmark the amplifier’s performance in a multi-signal environment. Two similar signals spaced 5 MHz apart, are passed through the amplifier and analyzed at the output. This includes the original signals plus undesired artifacts (intermods) that are generated within the amplifier. The resulting intermods are measured at various power levels to determine how the amplifier can be operated.

The frequencies of the third order intermodulation products (IM3) are generally located close to the frequency of the main signal and are therefore difficult to filter out. In addition, digital signals (i.e. PSK or QAM) will generate sidelobes that can also interfere with adjacent signals. The results of this test indicate whether or not the amplifier is suitable for the intended application.

Linear Operation

Since the presence of IM3 products is unavoidable, the amplifier must be operated in its linear region (P-linear), which is less than its full rated (saturated) output power (P-sat). Most satellite standards require that the IM3 products be at least 25 dB below the amplitude of desired signals being transmitted (-25 dBc). The difference between P-sat and P-linear varies, depending on the design of the amplifier.

Regardless of the amplifier’s construction (solid-state or tube-based), the RF output of the amplifier must be reduced from its saturated output rating, expressed as ‘output back-off’ (OBO), to reach its linear operating range. As the output power is reduced, the IMD products will drop at a faster rate than the desired signals. As an example – for every 1dB of OBO, a TWTA will produce a drop in IM3 by 3dB. A non-linearized TWTA will require an OBO of 7dB to achieve an IM3 level of -25dBC, whereas a solid-state amplifier will require 3dB of OBO to produce the same result.

This gives the SSPA a performance advantage of 4dB. The inclusion of a linearizer will greatly improve the linearity of a TWTA, but even with a linearizer, a TWTA must produce an RF output 1dB lower than an SSPA for comparable IMD performance. It is common that a 500-watt SSPA will compete well with a linearized 750-watt TWTA.

?Amplifier Placement

The market demand for satellite services has grown exponentially in recent years, thanks in part to the proliferation of mobile applications that exploit the ubiquity of satellite signals or in geographic regions where terrestrial services are too difficult to deploy and maintain. The increase in user-demand has forced satcom architects to push satellite designs into higher portions of the frequency spectrum to access wider bandwidth and commensurately, to accommodate higher volumes of traffic. Unfortunately, higher operating frequencies bring higher insertion losses as the signals travel from the amplifier to the antenna feed. The higher the frequency, the greater the loss.

When considering the amplifier placement within the earth station architecture, it stands to reason that placing the amplifier close to the antenna feed port reduces signal losses, which can be significant, depending on the signal frequency and distance traveled. In a typical installation, an amplifier placed at the antenna can be half or less than the size of an amplifier placed indoors to achieve equivalent performance. For applications that require low to medium power amplifiers, the distinction between the SSPA’s and TWTA’s suitability for outdoor mounting is minimal, but high power applications are a different story.

Installations requiring amplifiers that must generate 1kW or greater, are better served with SSPAs, particularly when automatic redundancy is preferred. The benefits of SSPAs over TWTAs, when it comes to high-power redundancy, is discussed in detail later in this document.

Energy Cost

For satellite earth stations, the high-power amplifiers represent one of the highest power-consuming components in the system. Though the cost of utility power varies by region, it is always an important element when considering the overall cost of operation. In the case of power amplifiers, there are two principal drivers of energy cost. The first is the utility AC that powers the amplifiers. SSPAs offer features that can greatly reduce the power consumption compared to comparable TWTAs. Since TWTAs operate in a Class-A mode, the amplifiers draw maximum power regardless of how much transmit power is being produced – even if it’s not in service (standby). TWTAs must be powered up and down using a process that prevents damage to the tube itself. Instant start up from power-off is not possible.

SSPAs that utilize GaN power devices operate in a Class-AB, which means that the current draw is reduced when operating in P-linear range. Since solid state amplifiers are not susceptible to damage from powering up or down, it is possible to have standby amplifiers powered down, thus eliminating 50% of the power consumed by a redundant pair. Since tubes have a finite life span, requiring the standby amplifier to be powered 100% of the time means that both tubes (online and standby) are aging at the same rate.

The second point when considering the power budget, is the amount of power in BTUs that is necessary to cool the amplifiers while in operation. This takes us back to the Amplifier Placement discussion. Mounting amplifiers outside at the antenna removes their contribution to the heat load calculation for the HVAC system inside the equipment shelter or building.

Availability

‘Availability’ is a metric expressed in hours, that estimates how long a component or system will be functional to provide service, considering potential interruptions that result from failures. Unlike TWTAs that rely on a single device for signal amplification, SSPAs that produce medium to high output power levels, utilize transistors arranged in a parallel combining structure. This helps to mitigate the likelihood of a service interruption from a single device failure. In most cases, a single transistor failure will result in a drop in output power (as little as 0.5dB), which may give the operator some time to schedule a repair, as opposed to dealing with a catastrophic service interruption.

When considering the purchase of high-power amplifier systems, two important metrics include - 'Mean-Time-Between-Failures' (MTBF) and 'Mean-Time-To-Repair' (MTTR). Important, because together they largely determine the availability of the system - the total number of hours it will be usable over its projected lifespan - in simpler terms, ROI.

Where MTBF is more or less a reflection of a product's design quality, like how well it's able to extract heat from the transistors under high ambient conditions, or in a TWTA’s case – the lifespan of the tube, MTTR is more of a reflection of how quickly and efficiently a failed component can be removed from the system, repaired and reinstalled. In other words, how long will the system be down if a component fails.

The steps may include removal of the failed product, packing, freight-time back to the factory, Customs-clearance, repair-time, test-time, repacking, freight-time back to the site, Customs-clearance and re-installation. For systems that employ conventional redundancy, the MTTR can be greatly reduced if there is a spare component at the site that can be placed into service while the failed component is off being repaired.

In this case, a lower MTTR can come at a significant expense, particularly for high-power systems. You now have two high-dollar components sitting in stasis - the offline backup plus the shelf spare. This could equate to hundreds of thousands of dollars in exchange for peace of mind (or job security). Another option is to employ soft-fail redundancy.

Regardless of the components or technology used in an amplifier’s construction, by far – the greatest guarantee of high-availability comes with the incorporation of ‘system redundancy’. This is an area where the differentiation between TWTAs and SSPAs is particularly significant.

Soft-Fail versus Conventional Redundancy

Redundancy, in the context of earth station electronics, refers to subsystems that are designed to limit service-interruption following the failure of a major component (a 'component' being any significant device, such as a power amplifier, RF converter, LNA/B, modem etc.)

The need for redundancy is a given for 'mission critical’ applications. There are certainly instances where an interruption of service can cause great harm, like the launch of spacecraft, telemedicine, emergency broadcast or even major sporting events. Redundancy subsystems are comprised of a controller that monitors the health of the components in that subsystem and, upon detection of the failure, will send commands to a switching system that will reroute the signal to an identical standby that is sitting in a quiescent state waiting to be called into service.

This architecture, referred to as 'conventional redundancy', is a concept that has been in play since the beginning of the industry. It is a very simple approach, though not necessarily the best approach for all cases. Systems based on conventional redundancy are commonly available on both SSPA and TWTA amplifier systems, but SSPAs offer a higher degree of protection at a significant cost savings as detailed later in this document.

Polarization

When it comes to earth station components commonly used to transmit and receive satcom links, we typically think in terms of 'single pol' and 'dual pol', which refers to the two orthogonally separated polarization fields (similar for circular polarized satellites) that exist between the ground station and the satellite. If the antenna feed is configured for single-band, dual pol access (full frequency reuse), four discrete ports are available - two for transmit and two for receive. Redundancy systems can be architected to provide a dedicated backup for each polarization (two backup amplifiers) or a single backup that is shared by both pols.

A few downsides of conventional redundancy are that the dedicated backup can't be used to carry additional services and still serve as a backup (without the addition of some priority switching logic). A dedicated backup is okay if the components are relatively inexpensive (cheap LNBs or low-power amplifiers). But if you're talking about more expensive products, like very high-power HPAs, you inherently tie up a lot of capital. And if the backup amplifier sits in hot-standby (certainly the case for TWTAs), it's aging along with the online amplifier and serving no benefit until a failure occurs.

Phase-Combining

The following section will describe 'phase-combining' and its impact on redundancy architecture. As noted previously, solid state power amplifiers depend on Field-Effect Transistors (FETs) to generate RF power. FETs come in all shapes and sizes with power levels that range from a few watts to somewhere around 130 watts at some frequencies. To generate respectable power levels, they must be cascaded, or phase-combined such that their individual contributions to the amplifier’s total RF output power can be summed. Phase-combining can be performed inside the amplifier with a break point of around 1kW or so at some frequencies. Beyond that, the physical size and weight of the amplifier becomes prohibitively impractical.

For higher power systems, one can choose to either externally phase combine these larger amplifiers, or one can choose to distribute the load over a larger number of smaller amplifiers. When it comes to redundancy in high-power systems, an alternative approach to consider is a modular system based on 'Soft-Fail Redundancy'.

Operationally, a soft-fail system isn't so different from a system that uses conventional redundancy. But behind the curtain, soft-fail systems are considerably more sophisticated, carry a host of additional benefits and cost savings that might not be readily apparent at a casual glance. But we'll get into the intimate details later. When Advantech Wireless brought the first, second and third generations of Summit (Summit I in 2011, Summit II in 2019 and Summit III in 2022), great care was taken to exploit the benefits of soft-fail system architecture, including individual amplifier/modules, each capable of generating up to 1kW of RF power and the introduction of CANBus as an operating platform, due to its high processing speed and component-level diagnostics capability.

When CAN-Bus became integral to each amplifier in the system, the need for outboard controllers was eliminated thus allowing any amplifier in the system to take over as the master in the event of a module failure. In soft-fail, unlike in a conventional redundancy system, all of the amplifiers are in service, sharing the load and with the health of each (down to the device level) being constantly monitored and reported to the master module in real time. And any module in the system can serve as the master, a feature that is handed off instantly if the amplifier serving as the master should fail.

Since the total system output power can be distributed over a larger number of smaller amplifiers, the loss from a single amplifier failure is reduced by 1.2dB for an eight amplifier system and 0.6dB for a 16-amplifier system. The soft-fail system is sized such that the necessary RF output power can be maintained with the loss of a single amplifier. If a failure occurs, the gain of the remaining amplifiers will be instantaneously increased to compensate for the loss, resulting in a consistent total system RF output. In a modular system, the amplifiers are smaller and easier to handle, are less expensive to spare and the return freight costs for service are much lower.

Back to the MTBF and MTTR metrics. In soft-fail systems, switching is not required to facilitate redundancy, which increases the MTBF - and at no point is the signal severed during the backup process (unlike the temporary break in a conventional redundancy system). As is always the case, one size doesn't fit all. Both conventional and soft-fail redundancy platforms have their respective 'perfect fit' scenarios.

Amplifier Management and Control

Most, if not all of the major components in today’s earth station designs, feature the ability to monitor and control the products remotely via a monitor & control (M&C) platform. What began as simple TTL and contact closure interfaces, has evolved into far more sophisticated communications protocols, including Web page, Ethernet, RS232, RS485 and even some of the original interfaces for reverse compatibility.

With the introduction and evolution of soft-fail redundancy, it was necessary to develop a new M&C protocol that could monitor and execute commands to a sizeable cache of products simultaneously. CANBus was selected due to its ability to make instantaneous decisions when a problem within a product is detected. CANBus was developed by the automotive industry to monitor and control the multitude of activities that are constantly occurring in the modern automobile.

Another benefit of CANBus is its ability to monitor amplifier circuits down to the transistor level. The incorporation of CANBus allows an earth station operator to monitor network integrity – to diagnose alarm sources down to the transistor itself. With that level of granularity, operators can run diagnostic routines that include the current-draw and baseplate temperature of the device while service is being carried.

In today’s world of cyber hacking, entering the system’s domain from the outside is an invitation to bad players. SNMPv3 is a secure Ethernet protocol that is becoming more and more popular, particularly with military and government network operators. Not all amplifier products carry this feature.

Conclusion

Thanks to the evolution of satellite communications technologies and the explosive growth in demand for space-based services, the number of connectivity options available to users has grown exponentially in recent years. The increase in capacity that is supported by GEO, MEO and now – LEO constellations is now able to compete head-on with terrestrial-based services in many parts of the world. Satellite ground station components have had to evolve as well. Being able to carry large volumes of data via satellite is necessary if communications via satellite is to be an economically viable alternative to terrestrial-based services.

?Increasing data throughput has been a major challenge to the industry for years - a challenge largely met by driving an increase in signal efficiency with modems that can operate with higher modulation schemes (bits-per-hertz). Transmitting high-order modulation signals requires, not just increased levels of radiated power, but also the ability to amplify the signals without producing adverse effects. Sidelobes and IMD interference levels increase with higher-order modulation schemes. With their superior AM to PM and AM to AM performance, SSPAs have proven to be a better option than TWTAs in these applications.