3,282 research outputs found
Stability analysis of digital microwave power amplifiers
This work presents a stability analysis of microwave power amplifiers (PAs) driven by binary pulse trains, as in the case of class-S PAs. First, using a simplified digital PA test bench in class-D configuration, different qualitative behaviors are obtained when varying the pulsewidth, including subharmonic and incommensurable oscillations. The mechanisms affecting the stability properties are studied with a harmonic balance-based formulation, by means of pole-zero identification and bifurcation detection. A sufficiently high number of harmonic components must be considered, together with the Krylov decomposition for an efficient computation of the inverse of the Jacobian matrix. It is demonstrated that, when varying the pulsewidth, the distinct pairs of complex-conjugate poles may shift to the right-hand side of the complex plane and, therefore, lead to different kinds of unstable behavior. This phenomenon is related to the dependence of the critical poles on the average value of the input signal. Boundaries of the various types of unstable behavior are traced in the plane defined by the pulse repetition rate and pulsewidth, using bifurcation detection techniques. All the predicted phenomena have been confirmed experimentally. In a second step, the algorithms derived from the simple class-D circuit are transferred to study the stability of a more complex tri-band class-S amplifier. It has been analyzed versus the input bitrate, obtaining a fully stable behavior that has been validated experimentally.This work has been supported by the Spanish Government under contract TEC2014-60283-C3-1-R, the European Regional Development Fund (ERDF/FEDER) and the Parliament of Cantabria (12.JP02.64069)
Power system design for a Jupiter solar electric propulsion spacecraft
Power system design for Jupiter solar electric propulsion spacecraf
Fault studies on synchronous generators with rectifier excitation
Imperial Users onl
Study of spacecraft direct readout meteorological systems
Characteristics are defined of the next generation direct readout meteorological satellite system with particular application to Tiros N. Both space and ground systems are included. The recommended space system is composed of four geosynchronous satellites and two low altitude satellites in sun-synchronous orbit. The goesynchronous satellites transmit to direct readout ground stations via a shared S-band link, relayed FOFAX satellite cloud cover pictures (visible and infrared) and weather charts (WEFAX). Basic sensor data is transmitted to regional Data Utilization Stations via the same S-band link. Basic sensor data consists of 0.5 n.m. sub-point resolution data in the 0.55 - 0.7 micron spectral region, and 4.0 n.m. resolution data in the 10.5 - 12.6 micron spectral region. The two low altitude satellites in sun-synchronous orbit provide data to direct readout ground stations via a 137 MHz link, a 400 Mhz link, and an S-band link
Amplificadores de potência para radiofrequência insensíveis à impedância de carga
Solid state power amplifiers (SSPAs) evolved significantly over the last few
decades, mainly, due to the use of new transistor technologies, such as
gallium nitride (GaN) high-electron-mobility transistors (HEMTs), very advanced
computer-aided design (CAD) software, and very effective digital
pre-distortion (DPD) algorithms. This led to a considerable performance
improvement, in terms of energy efficiency, output power, and linearity. To
achieve this performance, power amplifier (PA) designers normally push the
used transistors very close to their physical safe operating limits, and consider
them to operate for a fixed output load. However, the designed PAs are
used for many different industrial and/or telecommunication applications,
and, in some cases, such as, for example, microwave cooking or massive
multiple-input multiple-output (MIMO) fifth generation (5G) base stations
(BSs), the output load of these amplifiers can change. Under this nonoptimal
scenario, the used transistors will operate for non-nominal loads,
and the PAs performance can be severely degraded. Moreover, in highly
optimized designs, where the transistors are operated close to their safe
limits, their reliability can be reduced or, in extreme cases, they can even be
permanently damaged. Therefore, load insensitive PA architectures, and/or
techniques that aim at reducing the load variation seen by the PA, are necessary
to improve the performance under load varying scenarios. This thesis
presents various strategies to improve load insensitiveness of PAs. The presented
techniques are based on tunable matching networks (TMNs) and on
the amplifiers’ drain supply voltage (VDS) variation. The developed TMNs
successfully reduced the load variation seen by the PA, and its performance
was greatly improved, for non-optimal loading, by also using the derived load
dependent VDS variation. These different approaches were tested and validated
on single-ended PAs and then, based on their advantages and disadvantages,
the most promising technique – the supply voltage modulation –
was selected for the design of a Doherty power amplifier (DPA), which is of
paramount importance for telecommunication applications. Moreover, since
in some applications the output load variation can be unpredictable, we also
developed a complete quasi-load insensitive (QLI) PA system that includes
an impedance tracking circuit and an automatic real-time compensation of
the amplifier performance.Os amplificadores de potência de estado sólido (SSPAs) evoluíram significativamente
nas últimas décadas, principalmente devido à utilização de
novas tecnologias de transístores, como os transístores de alta mobilidade
(HEMTs) de nitreto de gálio (GaN), de ferramentas muito avançadas de
projeto assistido por computador (CAD) e de algoritmos de pré-distorção
digital (DPD) muito evoluídos. Isto levou a uma melhoria de desempenho
considerável, em termos de eficiência energética, potência de saída e linearidade.
Normalmente, para obter estes níveis de desempenho, os engenheiros
projetam os amplificadores permitindo que os transístores utilizados operem
muito perto do seu limite físico de funcionamento seguro e considerando
que vão operar para uma carga fixa. No entanto, os amplificadores projetados
são utilizados em diversas aplicações industriais e/ou telecomunicações
e, em alguns casos, como por exemplo fornos micro-ondas ou estações base
5G, a sua carga de saída pode variar devido a várias causas, que podem
ser previsíveis ou imprevisíveis. Neste cenário não ideal, os transístores utilizados
operam para cargas não ótimas e o desempenho dos amplificadores
pode ser muito degradado. Além disso, em projetos muito otimizados, onde
os transístores são operados perto do seu limite de funcionamento seguro,
a sua durabilidade pode ser reduzida ou, em casos extremos, podem até
ser permanentemente danificados. Portanto, para melhorar o desempenho
dos amplificadores em cenários de carga variável, são necessárias novas arquiteturas
e/ou técnicas que visam reduzir a variação da carga vista pelos
transístores utilizados. Esta tese apresenta várias estratégias para melhorar
a insensibilidade dos amplificadores em relação à variação de carga.
As técnicas apresentadas são baseadas em malhas de adaptação dinâmicas
(TMNs) e na variação da tensão de alimentação dos amplificadores. As
malhas de adaptação desenvolvidas permitiram reduzir a variação de carga
vista pelo amplificador e a variação da sua tensão de alimentação permitiu
melhorar o desempenho para operação com cargas não ótimas. Estas
abordagens foram testadas e validadas em amplificadores baseados num só
transístor, e, posteriormente, com base nas suas vantagens e desvantagens,
a técnica mais promissora – a modulação da tensão de alimentação – foi
selecionada para o projeto de um amplificador Doherty, que é imprescindível
para telecomunicações. Além disso, como em algumas aplicações a variação
da carga de saída pode ser imprevisível, também desenvolvemos um sistema
completo que inclui um circuito de medida de impedância e compensação
do desempenho do amplificador em tempo real.Programa Doutoral em Engenharia Eletrotécnic
Effect of fullerene containing lubricants on wear resistance of machine components in boundary lubrication
Fullerenes, a new form of carbon nanomaterials, possess unique physical and mechanical properties that make their use as additives to liquid lubricants potentially beneficial. The goal of this study was to investigate the effect of fullerene containing lubricants on wear resistance of steel-bronze couples operating under boundary lubrication conditions. A mathematical model of deformed asperity contact was built to calculate real contact area and real contact pressure. Computer controlled wear friction testing methodology and equipment were designed, developed and implemented for obtaining reliable and objective experimental data. In addition, optical and scanning electron microscopy and standard surface texture analysis were employed. Heavy duty motor oil SAE 10 was modified by admixing fullerenes C60, a fullerene mixture of C60 and C70, fullerene containing soot, and graphite powder. The experiments showed that all of the selected fullerene additives dissolved in liquid lubricants reduce wear of the tested materials. In addition, it was found that despite improvements in wear resistance, the selected modified lubricants did not significantly change friction characteristics.
Improvement of wear resistance of contact surfaces operating with fullerene modified lubricants can be explained by the presence of fullerenes in real contact while the liquid lubricant is squeezed out. Fullerenes are considered to function as minute hard particles that do not break down under applied normal force, and tend to separate direct contact of functional surfaces of selected materials
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