Clear signals: Why Power Linearity Matters for Satellite Uplink Amplifiers
Transmitting strong and undistorted signals from the ground to orbiting satellites requires amplifiers with high output power, but also excellent linearity. Power saturation (Psat) is often seen as a key performance specification, but amplifiers will never actually operate at this maximum power level in satellite communications systems. What really matters is power linearity (Plin) – the usable, distortion-free output power of the amplifier.
Specifying the correct amplifiers for multi-channel satellite uplink applications requires an understanding of both power saturation (Psat) and power linearity (Plin) – and how they affect digitally modulated signals. Plin is the critical factor because it denotes the power level at which an amplifier can boost signals without distortion. Psat is simply the maximum output power the amplifier can produce. At the saturation point, linearity is severely compromised and output signals are highly distorted.
Why linearity matters in satcom power amplifiers
With digital signal modulation, it’s vital to keep signals as linear as possible throughout the transmission chain. That means ensuring amplifiers or any other devices in the chain are not operated under compression. To avoid compressing signals, amplifiers must be operated at a level below their power saturation point (backed off) so that linearity is maintained and signals are not distorted.
The extent to which power needs to be backed off from the saturation point to achieve linearity differs between the two main amplifier technologies – travelling wave tube amplifiers (TWTAs) and solid-state power amplifiers (SSPAs).
TWTAs were the original amplifier technology used in RF communications. They are structurally complex, vacuum-based devices that are time-consuming and expensive to manufacture. The challenger technology is the SSPA. These semiconductor-based devices can be produced quickly and in high volumes, at dramatically lower cost than TWTAs.
How does linearity vary between amplifier technologies?
Broadly speaking, SSPAs offer better power linearity than TWTAs. Tube-based amplifiers were originally recognised as having excellent amplification qualities with very low distortion. But as the number or power of signals passing through the amplifier increases, TWTAs perform less reliably and need to be backed off considerably from their power saturation point. SSPAs, by contrast, can be operated much closer to their Psat point while maintaining linearity.
The first mmWave SSPAs, using gallium arsenide (GaAs) semiconductors, delivered much better like-for-like linear performance than TWTAs. However, linearisers were then developed for TWTAs, which improved their linear power. Although they still couldn’t quite match the linear performance of SSPAs, TWTAs remained the amplifier technology of choice for high-power, high-frequency applications due to their superior energy efficiency. GaAs amplifiers consume around three times more power than TWTAs when used for high-power satellite communications.
Gallium nitride SSPAs level the playing field
The advent of gallium nitride (GaN) semiconductors for SSPAs changed the equation. GaN-based systems offer more energy-efficient and reliable operation at higher frequencies, since they use higher-voltage, lower-current power supplies compared with GaAs systems.
However, unlike GaAs amplifiers, the first GaN amplifiers were not as linear. That was until a new type of corrective lineariser was developed which enabled GaN amplifiers to produce very good linear performance. As a result, GaN high-power amplifiers are becoming the first choice for high-frequency, high-date rate satellite communication uplinks, since they combine efficiencies approaching those of TWTAs with significantly better linear performance. These amplifiers are now making inroads into the higher frequency mmWave bands such as Ka, Q, V and E-band where TWTAs had previously dominated.
Make like-for-like comparisons between amplifiers
While it’s widely recognised that linearity is vital for high-quality satellite communications, it’s less well understood that amplifiers must be operated below their Psat point to achieve optimum linearity. This is particularly true for TWTAs, which cannot be operated anywhere near their maximum power levels when used for communications applications.
Satcom specifiers should be aware of which power level is being advertised on any amplifier, and should understand the relationship between Psat and useable ‘linear’ power (Plin) for any amplifier – so that like-for-like comparisons can be made.
Many amplifier specifications refer to Psat only. Furthermore, TWTA specifications traditionally describe the Psat level of the tube within the device, not the final packaged device itself. The actual saturated output power of the fully packaged TWTA will be below the Psat level of the tube within it. By contrast, the saturated output power stated for SSPAs is the Psat of the device as a whole, not any of its constituent parts. And, of course, SSPAs require considerably less ‘backing off’ from Psat to achieve linearity.
Extending the capabilities of satcom power amplifiers
At Filtronic, to avoid misunderstandings, we are moving towards using power linearity (Plin) in the specification of our SSPAs. We believe that transmitters, amplifiers and other RF devices used in satcoms should be marketed according to their useable linear power, which is the critical characteristic for undistorted signal transmission.
Filtronic is leading the push to develop GaN SSPAs for the satellite market. We are using our proprietary chip designs and proven design and manufacturing expertise to develop SSPAs that are now displacing TWTAs at higher power levels in the higher frequency bands such as Ka, V and E -band. Our GaN-based SSPAs achieve much closer alignment between Psat and Plin, maintaining linearity almost up to the saturation point.
The emergence of SSPAs that can now outperform TWTAs for both efficiency and power linearity is enabling satellite operators to expand their networks rapidly – helping them keep pace with accelerating demand for more and faster data.
