Slogan GaN based technologies

SLOGAN is an international research project co-funded from EU 7th Framework Programme. The project Space Qualification of High-Power SSPA based on GaN technology (SLOGAN) aims at designing, developing, implementing a GaN SSPA Engineering Qualification Model (EQM) for the next generation of Galileo satellites.

The development of SLOGAN project will allow not only to show the feasibility of implementing a high output power GaN SSPA for Galileo application, but also will open the door to a wide variety of applications, such as radio broadcasting, Tx/Rx modules of Earth Observation space and airborne radars, where GaN technology, with its increased power and mass efficiency, promises a clear advantage over current solutions.

Demonstrating the Capability of GaN HEMTs for Satellite Communication

GaN HEMTs are ideal for making high-power amplifiers for satellite communication, because they are lightweight, compact, efficient and capable of delivering a high output power and uniform gain over a broad bandwidth.

Tubes are the incumbent technology, offering greater maturity and impressive output powers and efficiencies, especially in the high-frequency domain of the Ka-band and beyond. However, they suffer from several significant drawbacks. They command very high DC voltages, and they take up a large volume, especially in the lower frequency bands. What’s more, expensive realization processes have to be employed to avoid unwanted electron resonance, known as multipaction, as well as corona effects, which can lead to device breakdown via gas ionisation.

Solid-state PAs based on GaN have the upper hand over travelling-wave tube amplifiers on multiple fronts. They are smaller and lighter; they operate at lower voltages; they are less susceptible to hot electron phenomena; and there are no issues surrounding heating times. In addition, these devices degrade gracefully over time and offer a far higher degree of reconfigurability, which allows different power levels to be realised by combining a varying number of basic modules.

Within Europe, effort is being directed at demonstrating the capability of GaN technology applied to HPAs for satellite systems. While the promise of GaN is beyond doubt, devices made from this wide bandgap semiconductor cannot be deployed in space with confidence until the technology is proven at various stages, beginning with the breadboard and ending with the version that is prepared for launching into space.

Results within SLOGAN are already showcasing the feasibility of implementing a high output power GaN PA for Galileo, which is a growing collection of satellites that will provide Europeans with a navigation system delivering horizontal and vertical precision of 1m. Note, however, that the success with GaN has already shown that this technology is capable of serving many other applications, including radio broadcasting, and transmit/receive modules for earth observation and airborne radar.

Figure 1. The SLOGAN solid-state PA includes an electronic power controller (EPC), a power supply unit (PSU) and a temperature controller (TC).

The PA that has been built during SLOGAN consists of three sub-units: an RF unit, a power supply and an electronic power conditioner (see Figure 1). The role of the RF unit is to amplify the Galileo signal to up to 230 W across a 1550.42 MHz to 1600.42 MHz band, while providing a minimum gain of 65 dB. Within this section, there is a gain control unit for managing the gain of the overall power amplifier.

This has to compensate for variations in temperature, aging and operating conditions. The job of the power supply is to convert the 50 V satellite primary bus voltage to internal required DC values, which can be positive and negative. Meanwhile, the electronic power conditioner is there to implement the telemetry and telecommand functionalities; properly control PA operation, while compensating against thermal and/or lifetime variation; and to control the current and the voltage applied to the RF power stages.

To realise the required power level at the output of the RF unit, four identical PAs are employed in parallel. They are in the form of two 25.6 mm power bars (see Figure 2 for the architecture of the RF unit, and Figure 3 for an image).

Figure 3. (a) photo and (b) performance of the 80 W power module based on a pair of 25.6 mm power bars

Completion of the RF chain of the PA is via: the introduction of two cascaded driver stages, using the 12 mm power bar; forming a gain control unit from linear amplifiers (UMS CHA3801-99F); and using fixed attenuators, designed for this project, and a commercial analogue attenuator (Hittite HMC346MS8G)

Requirements for the mechanical design of the systems include appropriate accommodation of the three sub-units and shielding of RF modules. Another key requirement involved accounting for the larger thermal dissipation required behind the power bars of the final stages, in order to avoid hot spots.

This was accomplished by undertaking a thermal analysis into the thermal distribution of the PA. Results of this enabled identification of a suitable temperature reference point for monitoring the amplifier’s operating temperature.

To determine the best foundation for the amplifier, a comprehensive analysis was conducted that accounted for multipaction and thermal issues.  Emphasis was placed on the high-power final stage, and the selection of bonding and ribbon technologies for linking the different units or the external connectors (SMA at the input, TNC at the output).

These steps also prevented corona effects. The investigation led to the selection of Roger laminates with low outgassing characteristics. Roger RT/duroid 6035HTC laminates were employed for the high-power section.

The three sub-units – the RF unit, power supply and electronic power conditioner – have been united in a power amplifier box (see Figure 4) to create a system with a mean-time-before-failure of almost 600,000 hours. This level of reliability implies a probability of failure during a useful life of 12 years of only 16.2 percent. Characterisation of this unit reveals that it can deliver a CW output exceeding 300 W at a power-added efficiency that is close to 45 percent  (see Figure 5). The gain delivered by the unit is incredibly flat (see Figure 6).

Figure 5. The power amplifier built in the SLOGAN project can produce an output in excess of 300 W at an efficiency exceeding 45 percent.

Life testing also produces some impressive results. Operating in CW mode at 1575 MHz, with fixed input power of -11 dBm that correspondingly to roughly 2dB of compression, the output power is incredibly stable. It varies by less than 0.15 dBpp during 500 hours of operation. Moreover, the amplifier has been subject to vacuum tests, and no failures due to multipaction have been observed.

Figure 6. The gain of the SLOGAN power amplifier is incredibly flat. It is less than 1.6 dB in the range of -3 dB, +1 dB of input power back-off, and less than 0.2 dBpp over a 10 MHz frequency span.

The SLOGAN project has received funding from the European Union Seventh Framework Programme for research, technological  development and demonstration.

The SLOGAN Team: