Adaptive techniques to improve the efficiency of GaN based SSPAs (up to C Band) for geo synchronous satellites

Abstract

In this thesis, various adaptive techniques to improve the efficiency of Solid State Power Amplifier (SSPA) for Geo Synchronous Satellite are presented. The Microwave Power Amplifiers (MPAs) being the most important sub-system determining payload weight, volume, cost and many important link parameters, their performance is improved by novel techniques. Out of various techniques available, the important techniques like flexible (adaptive) output power, flexible frequency and bandwidth, flexible thermal management, adaptive temperature compensation, replacement of Travelling Wave Tube Amplifiers (TWTAs) by Solid State Power Amplifiers (SSPAs) and improvement of reliability of SSPAs are presented in detail. Literature survey on reported techniques to reduce the cost of the satellite services, making the satellite payload flexible and development of newly emerging technology, Gallium Nitride (GaN) based SSPAs to replace the TWTAs are included. The recent development on a new technique so called flexible payload or adaptive payload is described. This new architecture known as of Generic Flexible Payload (GFP) has been proposed by European Space Agency and Astrium, which allows to meet various requirements like evolving business and political landscapes, the emergence of new technologies and applications, or even a change of orbital location or owner. The adaptive or flexible payload allows the user to redefine the frequency plan, redistribute the transmit power between service regions and change the EIRP according to the specific need without scarifying the satellite performance. Recent development on GaN based SSPAs by various authors are also included. SSPA being the complex sub-system, especially for Geo-Synchronous Satellite, major specifications including Reliability and Quality Assurance criteria are described in detail. The nominal value of each parameter for 200 watt GaN based SSPA is also given. The Power Added Efficiency (PAE) is the critical parameter for selection of high power amplifier for space segment. TWTA has been selected so far due to it's higher efficiency than the SSPA. Moreover, the efficiency decreases drastically for both type of amplifiers under back off operation, so it is necessary to improve the efficiency of the amplifier under back off. The state of the art device technology device, GaN HHET has been selected to demonstrate the concept of flexible output power SSPA, also called "SSPA with Dynamic biasing technique". Theoretical analysis of efficiency of SSPA at saturation and at back off has been presented and shown that the efficiency degrades at back off. To improve the efficiency, the drain voltage can be varied according to the RF input drive level and the data has been programmed in the EEPROM controller. Non-linear simulation including Harmonic balance simulation and load pull characterization has been presented using non-linear model of GaN device on Agilent's Advance Design Software (ADS) platform. The proposed work is compared and discussed with the reported work. The results for 200 Watt and 20 Watt SSPA flexible SSPA at various back off including EEPROM controller are discussed. Another novel technique to change the frequency of operation and bandwidth of SSPA on-board using tele command from ground is described using all microwave amplifying semiconductor devices. Theoretical background behind the concept of adaptive frequency amplifier is discussed in detail. Non-linear simulation, load pull counters and measured results of 100 Watt UHF SSPA, using Si RF MOSFET are presented in the thesis. The simplest technique to change the frequency of the SSPA from the ground is also described with the measured results. In addition to meeting The important aspect of this research is to improve the efficiency of the SSPA in addition to provide the flexibility in terms of frequency. Hence, the results of efficiency improvement in case of narrow band SSPA as compared to wide band SSPA are also presented. In addition to above two techniques, another new technique to improve the efficiency of SSPA by providing flexible thermal management system. The present balance configuration of final power amplifier is modified with single ended device configuration and reliability calculations are proposed using real time temperature data. Finite Element Model (FEM) analysis is carried out for both the options and thermal counters are generated using IDEAS software. Detailed study on degradation of performance of material used for thermal management system, design challenges and efficiency improvement results are described. The similar concept has been applied to make the temperature compensation adaptive by which the gain and output power variation over the specified temperature range can be adjusted as per the requirement after launching the satellite. For satellite communication payload, the power amplifiers generate third order intermodulation distortion (non-linearity) under multi-carrier operation so it's accurate characterization is necessary. The usual practice of measuring this non-linearity is not accurate so the results are non-consistent and hence, the SSPA has to be operated in linear region, which degrades the efficiency. A novel technique is presented in this thesis, which gives accurate measurement so that the SSPA can be operated near saturation resulting in improvement of efficiency. The TWTAs have been used in the satellite payloads since the beginning of the satellite technology and found to be most reliable. Now when the SSPA developers are putting efforts to replace them, it is very important and necessary that the newly developed SSPAs are also having the reliability at par with the TWTAs. Many failures have been reported during the development of GaN device and GaAs FET devices were also failed during initial phase, so it is mandatory for SSPA designer to launch the SSPA with highest reliability. A new approach is presented to provide highest reliability with various techniques implemented. These techniques will help to understand the failure if any, and to modify the operating condition of the GaN transistor.