The achievement of relatively low-cost rocket launches has enabled the commercialization of low earth orbit (LEO) satellite constellations, aiming to provide broadband internet connectivity anywhere on the planet. Starlink, OneWeb and Project Kuiper are three of the many systems vying to capture business in consumer, industrial, government and military markets. A network comprising hundreds to thousands of LEO satellites and millions of ground terminals faces significant technical challenges for the high data rate links that constantly switch among the satellites and terminals for both users and gateways. The most practical architecture for these links is a beam-steered antenna array, enabling the satellite or ground terminal to follow the receiver or transmitter as the satellite traverses the sky. The satellite-to-earth downlink and earth-to-satellite uplink use different frequency bands. With the current generation of systems, the gateway uplinks use the 27.5 to 30 GHz band and the downlinks use the 17.7 to 20.2 GHz band. Project Kuiper plans to use these same bands for their users, while OneWeb and Starlink use Ku-Band for the user links.
FAMILY OF MMICS FOR LEO SYSTEMS
Figure 1 (a) TMC261 measured output power. (b) TMC261 measured PAE.
To serve these LEO systems, mmTron is developing a family of MMIC power amplifiers (PAs) and low noise amplifiers (LNAs) for both the satellites and gateway terminals. The TMC261, mmTron’s latest PA MMIC, was developed for the gateway downlink and covers 17.3 to 21.2 GHz. The performance and availability of this family is shown in Table 1.
Based on customer link budgets, mmTron designed the TMC261MMIC to provide 31 dBm output power at 1 dB compression with high linearity and power-added efficiency (PAE). High PAE is essential to minimize a satellite’s DC power and thermal load. Figure 1a shows the PA output power and Figure 1b shows PAE versus input drive at 17, 19 and 21 GHz.
Linearity is also a key requirement for transmitting high data rates and satellite system engineers typically use noise power ratio (NPR) to specify the required performance. At an NPR of 13 dB, the TMC261 provides 1.1 W output power and 35 percent PAE. The small-signal gain of the TMC261 is greater than 20 dB across the band, typically 24 dB mid-band as shown in Figure 2, with input and output return loss better than 10 dB. The PA was designed for an operating bandwidth from 17.3 to 21.2 GHz.
Figure 2 TMC261 measured versus modeled small-signal gain.
The TMC261 was designed to be biased at +18 V on the drain and draws 74 mA, set with a negative gate supply. The drain bias can be increased to +24 V to increase the output power to 2 W while maintaining good PAE and NPR performance. The TMC261 is available as a 3.5 × 2.5 mm, 0.004 in. thick die or in a 6 × 6 mm air-cavity ceramic QFN package. Production quantities of the die are available for immediate delivery.
PA DESIGN
To ensure sufficient gain with satellite temperature variations, the TMC261 is a three-stage, class AB design. While either GaAs or GaN would achieve the design goals, GaN is the more attractive option. The higher supply voltage of GaN enables more efficient power distribution and higher system efficiency than would be achievable with a GaAs PA.
Simultaneously optimizing the output power, linearity and PAE of a PA is quite challenging and the process gets more challenging as the bandwidth increases. Historically, many applications have focused on achieving two of the three parameters, but not all three. One of the challenges of designing a 1 W PA with a GaN device biased at +18 V is transforming the impedance up within each stage. This makes achieving the PAE difficult over the wide bandwidth.
After choosing GaN and specifying the epitaxial structure and fabrication process steps to maximize device performance, mmTron’s designers determined the output device periphery to meet the output power and linearity specifications. Then, they performed a detailed analysis of possible transistor quiescent points and associated matching impedances to identify the best load targets that would meet the design bandwidth. The “secret sauce” was optimizing the matching networks to minimize the AM/AM and AM/PM distortion at the chosen bias points.
Given the importance of the thermal design, the designers paid close attention to minimizing the power dissipation of each device. This also included accounting for losses in the matching networks. To complete the circuit design, a thorough stability analysis was performed to ensure stability over process, temperature and loading variations.
READY FOR SPACE
Designed to fly on a LEO satellite, the TMC261 is fabricated in a foundry using GaN processes that have built other MMICs flying in space. The TMC261 was designed for maximum reliability and it includes on-chip ESD protection. Consistent with industry standards, bond pad and backside metallization are Au-based and compatible with eutectic or high conductivity epoxy die attach processes as well as ribbon and wedge bonding.
With the TMC261, mmTron has expanded its MMIC satellite amplifier portfolio. The mmTron team has designed the TMC261 to have a unique combination of power, linearity and efficiency performance over a 20 percent bandwidth at K-Band. The company has not seen a higher efficiency, higher linearity three-stage GaN PA commercially available or published in the literature.
mmTron Inc.
Redwood City, Calif.
www.mmtron.com