Amplificadores de envolvente para transmissores de rádio frequência

Mestrado em Engenharia Electrónica e TelecomunicaçõesNeste trabalho é feita a análise matemática de um amplificador comutado auto-oscilante, considerando a existência de um atraso de propagação e/ou histerese no comparador, por forma a garantir as condições de oscilação. Para validar este resultado teórico, diversas simulações são realizadas, e é descrito o projecto de um amplificador auto-oscilante. É montado um protótipo experimental e os seus resultados observados na prática são comparados com os obtidos quer pela simulação do circuito, quer pelo modelo matemático apresentado.In this work, the mathematical analysis of a switched self-oscillating amplifier is performed, considering a propagation delay and/or hysteresis in the relay in order to ensure the oscillation conditions. To validate this theory, several simulations are performed, and the design process of a self-oscillating amplifier is described. Then, a practical prototype is built, and its experimental data is compared against circuit simulation data and the results obtained using the proposed mathematical model

Characterisation of microbial attack on archaeological bone

As part of an EU funded project to investigate the factors influencing bone preservation in the archaeological record, more than 250 bones from 41 archaeological sites in five countries spanning four climatic regions were studied for diagenetic alteration. Sites were selected to cover a range of environmental conditions and archaeological contexts. Microscopic and physical (mercury intrusion porosimetry) analyses of these bones revealed that the majority (68%) had suffered microbial attack. Furthermore, significant differences were found between animal and human bone in both the state of preservation and the type of microbial attack present. These differences in preservation might result from differences in early taphonomy of the bones. © 2003 Elsevier Science Ltd. All rights reserved