Design of X-band SSPA based on GaN HEMT for telemetry subsystem of near-earth space missions

Abstract

In this paper, the design, and implementation of an X-band, 10 W, Solid-State Power Amplifier (SSPA) based on Hybrid Microwave Integrated Circuit (HMIC) technology using Gallium Nitride (GaN) High-Electron Mobility Transistor (HEMT) dies for near-earth space applications is presented. Designed SSPA is customized for use in suborbital missions and has two stages. We have used narrowband Transmission Lines (TL) class E architecture for designing the proposed SSPA. We have applied the transconductance tuning technique for the first time to a GaN HMIC for achieving the required linearity without any additional circuits. Hybrid optimization helps us for obtaining the best output power, efficiency, and linearity in comparison to other X-band GaN HMICs. We implemented our design with a Rogers 6010 laminate printed circuit board (PCB) on a Cu plate for improving the thermal heat transfer. All the on-chip pads were bonded to PCB using gold bonding wires with 25-µm diameter directly. We obtained 40 dBm and 37 dBm maximum output power in the saturation and 1-dB compression point, respectively. The central frequency of the proposed SSPA is 10,640 MHz with 150 MHz Band Width (BW). It works in EB-class with a 40 V voltage power supply and has more than 40% efficiency in saturation output power (Psat). We obtained two tones Intermodulation Distortion Third Harmonic (IMD₃) better than −17 dBc in-band. The active area of the implemented SSPA is 40 mm × 50 mm.

Introduction

One of the most significant issues in space applications is the development of small, lightweight, low power consumption systems due to very limited area, weight, and power budget in spacecraft and satellites, and onboard telecommunication systems are no exception. Fig. 1 shows the Radio Frequency (RF) frontend of a typical transmitter for a space telemetry subsystem. The Intermediate Frequency (IF) signal after upconverted to RF signal must be amplified before transmission. Usually, there are two types of amplifiers in this step, first a Driver Power Amplifier (DPA), and second a High Power Amplifier (HPA). As exhibited in Fig. 1, the final block before the antenna is an HPA. In this block, the power of the RF signal reaches the maximum value. Because of high power consumption, the efficiency of this stage is the most important block in determining the efficiency of the transmitter. Therefore, the design of a highly efficient HPA is a very important step in this regard [1]. Table 1 shows the specifications of X-band HPAs for several space programs [2], [3], [4], [5], [6], [7], [8], [9], [10]. As mentioned in Table 1, the Traveling-Wave Tube Amplifier (TWTA) and the Solid State Power Amplifier (SSPA) technologies are both used by the Japan Aerospace Exploration Agency (JAXA), the European Space Agency (ESA) and the U.S. National Aeronautics and Space Administration (NASA) for different programs and missions. As shown in Table 1, TWTAs have higher efficiency compared to SSPAs and so they are preferred for high power transmitters because they can reduce power consumption and heat dissipation of system [2]. In contrast, SSPAs are superior to TWTAs in terms of mechanical environment tolerance, lifetime, footprint, area, weight, and discharge risk, because TWTAs use a fragile sculpted-glass tube and require high voltage (kV) operation [11], [12]. Therefore, despite their relatively lower efficiency, SSPAs are used especially in missions where weight and volume budget is very limited. It is also worth mentioning that the difference between the efficiency of TWTAs and SSPAs is very slight when low output power (for instance less than 50 W) is required. Hence, because of the less power requirement in the low earth orbit missions, SSPAs are the preferred choice.