Sistema ativo de load-pull para micro-ondas

Atualmente, os elevados requisitos de eficiência e linearidade exigidos aos amplificadores de potência utilizados em sistemas de comunicações sem fios tornam essencial uma boa caracterização dos dispositivos ativos utilizados. Os sistemas de load-pull são uma das técnicas mais utilizadas para essa caracterização. Estes sistemas são normalmente divididos em três categorias, sistemas passivos, ativos em malha fechada e ativos em malha aberta. Nesta dissertação é apresentado o projeto de um sistema de load-pull em malha aberta baseado nos equipamentos do laboratório de rádio-frequência do Instituto de Telecomunicações, como alternativa aos sistemas comerciais. Os sistemas de load-pull em malha aberta são particularmente interessantes pois permitem apresentar ao transístor cargas com elevado VSWR (Voltage Standing Wave Ratio) sem sofrerem de problemas de oscilação. Os coeficientes de reflexão apresentados ao transístor são sintonizados a partir do método de Newton-Raphson. Para a correta medição das métricas desejadas foi aplicada uma calibração SOLT e uma calibração de potência. O sistema foi aplicado para a obtenção das curvas de nível de eficiência e potência de saída de um transístor de nitreto de gálio (GaN). À frequência de 2,1 GHz, obteve-se uma eficiência máxima de 80.5% e uma potência de saída máxima de 42.5 dBm. Por fim, projetou-se um amplificador baseado na carga de máxima eficiência obtida pela simulação de load-pull. A eficiência do amplificador comprovou os resultados obtidos através do sistema de load-pull projetado.Currently, the high efficiency and linearity requirements of power amplifiers used in wireless communication systems have made it essential to have a good characterization of the active devices used. Load-pull systems are one of the most used techniques for this characterization. These systems are normally divided into three categories, passive, closed-loop, and open-loop systems. This dissertation presents the design of an open-loop load-pull system based on the equipment of the radio frequency laboratory of the Instituto de Telecomunicações, as an alternative to commercial systems. The open-loop load-pull systems are particularly interesting because they allow to present loads with high VSWR (Voltage Standing Wave Ratio) to the transistor without suffering from oscillation problems. The reflection coefficients presented to the transistor were tuned using the Newton-Raphson method. For the correct measurement of the desired metrics, a SOLT calibration and a power calibration were applied. The system was applied to obtain the efficiency and power contours of a gallium nitride transistor. At a frequency of 2.1 GHz, a maximum efficiency of 80.5% and a maximum output power of 42.5 dBm were obtained. Finally, based on the maximum efficiency load obtained by the load-pull simulation, a power amplifier was designed. The amplifier's efficiency proved the results obtained through the designed load-pull system.Mestrado em Engenharia Eletrónica e Telecomunicaçõe

Developing a multi-tone load-pull system for the direct extraction of Cardiff behavioural model coefficients

The main objective of this thesis is to develop and utilize a high-speed measurement system based on PXIe modules for multi-tone measurements. This thesis addresses challenges that have been tackled during developing an accurate LabVIEW software to measure the multi-tone signal. Having solved these problems, the system's functionality was demonstrated by using it to extract some useful data such as stability and gain information for RF designers in, ‘real time’ during active load-pull measurements. In the final part of the thesis, the system was used to aid the development of the Cardiff behavioural model. One of the main challenges in the development of the Cardiff behavioural model is to correctly select the required mixing terms to have an accurate model. Initial work has been focused on determining the correct, phase polynomial coefficients of the Cardiff Model. The first technique presented in this thesis utilises two-tone measurements and the Fast Fourier Transform (FFT) to observe the mixing order from the resultant intermodulation (IMD) products, which are directly associated with the Cardiff Model coefficients. Employing the IFFT, the selected tones have been transferred to the time-domain travelling-wave. This allowed for b-wave analysis and load modulation coverage to be seen on the Smith Chart. The resultant load-modulation from the two-tone measurements has been used as a target for CW impedance measurements to verify whether the identification holds for the CW domain. The result shows there is an VII offset between the CW measurements and the two-tone measurements for larger annuli as th