X-band high efficiency class-F GaN SiC-HEMT high power amplifier design for small satellites and amplitude&phase sensitivity analysis for RF power fets

Space race is also a race about the technological development. Artificial satellite technology and research is one of the main branches at this race, which uses all facilities of the technological developments on materials, mechanics and especially electronics. Satellite communication is one of the most important topics for satellite technology. If a satellite has active communication link, it means the satellite is functioning even if some subsystems are not available at the satellite. Wireless communication system of the satellite provides data from sensors to ground station and receives commands from ground station. Nowadays, small satellites become more popular because of their low cost and short design period. Development of material, mechanical and electrical technologies makes small satellites more efficient and capable. Especially, low launch and deployment costs of small satellites make them suitable research tools for universities. A lot of scientific researches can be handled with small satellites in the universities. Additionally, these days many companies prefer to use small satellites to test their products under space conditions to get space heritage. Although small satellites have advantages in terms of cost and time, there are two important restrictions for small satellite systems: Short communication window limits the amount of data transfer, and small surface area of the satellite limits the received energy from solar panels. Small satellites are located on low earth orbit (LEO) which means 400-2000km altitude. Therefore, their speed is very high (~7.5km/second) around the Earth. Generally, ground stations can receive data above a few degree from the horizon. However, high orbital speed causes decreased communication window. Ground stations usually have a few minutes for communication. Short communication time may not be enough for large data packages with low data rate communication systems. Therefore, ground station has to wait for another suitable pass. Small satellites require high data rate communication systems to receive data such as high-resolution images, high accuracy position sensor data etc. Wide bandwidth systems with high carrier frequency are necessary to transfer large data. To increase data rate without expanding the bandwidth is to use a complex modulation such as QAM. Generally, combination of wideband signal and complex modulation is more efficient to increase data rate. On the other hand, small satellites have small surface area. Therefore, the number of the solar panel that can be placed is limited. Power generation capability of the solar panels determine the power budget of the satellite. This constraint forces designers to make high efficiency devices. In this thesis, a high efficiency and high power amplifier at X band is proposed. Designed amplifier is suitable for a small satellite transmitter. Amplifier has SMA female connectors. A commercially available GaN on SiC HEMT was used for the amplifier. GaN on SiC HEMTs have important advantages such as wider bandgap voltage, higher thermal conductivity, higher power capability, higher impedance and higher breakdown voltage than GaAs devices. These advantages also makes GaN devices very suitable for satellite applications. Efficiency of a high power amplifier can be increased by harmonics' appropriate terminations. Class F was preferred for the proposed X band amplifier. At the output, 2nd harmonic component was terminated as short circuit and 3rd harmonic component was terminated as open circuit. At the input, both 2nd and 3rd harmonic components are shorted. Proposed amplifier was completed in three steps: Nonlinear transistor model import and its verification, computer aided design, assembly and tests. After the decision of the RF power transistor, its nonlinear model was provided from the manufacturer company. Model was imported to computer aided design tool. Then imported model's small signal and DC analysis were obtained to compare with measured data of the transistor for the verification of imported model. After successful import of the nonlinear model, designing on an electronics design automation tool for Class-F amplifier started. First of all, harmonics' terminations were prepared for the input and output stages. Microstrip lines were preferred instead of discrete components at the termination stages. Because, behavior of discrete components may not be suitable at the high frequency region. DC bypass capacitors before connectors, coupling capacitors onto the bias lines and a parallel resistor-capacitor couple for the stability were inserted to the structure. A basic optimization was run to satisfy proper harmonics terminations for the Class F amplifier after the essential discrete components were added. To determine the impedance for the main signal, source and load pull analysis were carried out for the best PAE. After determination of the input and output impedances for the best efficiency, double stub tuning was preferred for the matching. Final design with microstrip lines and some additional discrete components was simulated on the EDA tool. After simulation, all microstrip lines and via holes were considered under electromagnetic simulation to determine coupling and radiation effects of the microstrip lines. In this part, a basic optimization was also run. After verification of the final optimization, layout of the circuit was prepared for the printed circuit board fabrication. Circuit was divided into two parts. Input and output printed circuit boards were fabricated separately. Golden plating was preferred for the surface finish. Brass plate was used as the carrier for the PCBs. Metal case was also made by brass. After the assembly of the circuit, it was tested to observe stability of the amplifier. At the next step, small signal performance was measured and compared with the simulation data. At the small signal measurements, a small difference was observed at the gain. EDA tool was used to understand and solve the problem. After a basic tuning at the output stage impedance matching circuit, measured small signal gain became similar to simulation result. Then; efficiency, high power gain and output power were measured at the several input frequencies. Final circuit has 52.1 % peak efficiency with 27 dBm input power at the 8.1 GHz. Output power is 36.6 dBm (~4.5W). Designed amplifier has more than 50% efficiency between 7.9 GHz and 8.2 GHz. Amplifier is able to satisfy 5W output power capability. High power amplifiers operate at the saturation region to obtain high efficiency. This situation causes nonlinear distortions on the phase and amplitude of the signal. Additionally, in this thesis, first order single parameter phase and amplitude sensitivity analysis of a RF power FET was derived. At the beginning of the study, equivalent circuit for a power FET was determined. Most dominant components were used at the equivalent circuit of a power FET. Source and load resistors were added for the circuit analysis. Circuit function of the equivalent circuit was derived to find for the sensitivity analysis. Obtained circuit function was verified at the EDA tools NI AWR and PSpice. After verification of the derived equations, a commercial GaN on SiC HEMT's model data was used to find weights of the transistor's intrinsic parameters. According the numerical analysis, output resistor and output capacitor are the most dominant components on phase sensitivity respectively. gm value and output capacitor are dominant components on the amplitude function sensitivity respectively.