X-ray investigation of defects in III-nitrides and their alloys

In this work structural defects in III-nitrides were investigated mainly using X-ray diffraction method. Williamson-Hall plots and Monte Carlo simulations were used to determine the density of threading dislocations in epilayers with different layer thicknesses. The density of basal plane stacking faults in semipolar (10-11) and (11-22) GaN layers grown on prepatterned sapphire substrates were derived and the dependence of crystallographic orientation of the layers and SiN mask intercalation on structure perfection, was examined

Complex-valued Adaptive Digital Signal Enhancement For Applications In Wireless Communication Systems

In recent decades, the wireless communication industry has attracted a great deal of research efforts to satisfy rigorous performance requirements and preserve high spectral efficiency. Along with this trend, I/Q modulation is frequently applied in modern wireless communications to develop high performance and high data rate systems. This has necessitated the need for applying efficient complex-valued signal processing techniques to highly-integrated, multi-standard receiver devices. In this dissertation, novel techniques for complex-valued digital signal enhancement are presented and analyzed for various applications in wireless communications. The first technique is a unified block processing approach to generate the complex-valued conjugate gradient Least Mean Square (LMS) techniques with optimal adaptations. The proposed algorithms exploit the concept of the complex conjugate gradients to find the orthogonal directions for updating the adaptive filter coefficients at each iteration. Along each orthogonal direction, the presented algorithms employ the complex Taylor series expansion to calculate time-varying convergence factors tailored for the adaptive filter coefficients. The performance of the developed technique is tested in the applications of channel estimation, channel equalization, and adaptive array beamforming. Comparing with the state of the art methods, the proposed techniques demonstrate improved performance and exhibit desirable characteristics for practical use. The second complex-valued signal processing technique is a novel Optimal Block Adaptive algorithm based on Circularity, OBA-C. The proposed OBA-C method compensates for a complex imbalanced signal by restoring its circularity. In addition, by utilizing the complex iv Taylor series expansion, the OBA-C method optimally updates the adaptive filter coefficients at each iteration. This algorithm can be applied to mitigate the frequency-dependent I/Q mismatch effects in analog front-end. Simulation results indicate that comparing with the existing methods, OBA-C exhibits superior convergence speed while maintaining excellent accuracy. The third technique is regarding interference rejection in communication systems. The research on both LMS and Independent Component Analysis (ICA) based techniques continues to receive significant attention in the area of interference cancellation. The performance of the LMS and ICA based approaches is studied for signals with different probabilistic distributions. Our research indicates that the ICA-based approach works better for super-Gaussian signals, while the LMS-based method is preferable for sub-Gaussian signals. Therefore, an appropriate choice of interference suppression algorithms can be made to satisfy the ever-increasing demand for better performance in modern receiver design

Energy-Efficient Distributed Estimation by Utilizing a Nonlinear Amplifier

abstract: Distributed estimation uses many inexpensive sensors to compose an accurate estimate of a given parameter. It is frequently implemented using wireless sensor networks. There have been several studies on optimizing power allocation in wireless sensor networks used for distributed estimation, the vast majority of which assume linear radio-frequency amplifiers. Linear amplifiers are inherently inefficient, so in this dissertation nonlinear amplifiers are examined to gain efficiency while operating distributed sensor networks. This research presents a method to boost efficiency by operating the amplifiers in the nonlinear region of operation. Operating amplifiers nonlinearly presents new challenges. First, nonlinear amplifier characteristics change across manufacturing process variation, temperature, operating voltage, and aging. Secondly, the equations conventionally used for estimators and performance expectations in linear amplify-and-forward systems fail. To compensate for the first challenge, predistortion is utilized not to linearize amplifiers but rather to force them to fit a common nonlinear limiting amplifier model close to the inherent amplifier performance. This minimizes the power impact and the training requirements for predistortion. Second, new estimators are required that account for transmitter nonlinearity. This research derives analytically and confirms via simulation new estimators and performance expectation equations for use in nonlinear distributed estimation. An additional complication when operating nonlinear amplifiers in a wireless environment is the influence of varied and potentially unknown channel gains. The impact of these varied gains and both measurement and channel noise sources on estimation performance are analyzed in this paper. Techniques for minimizing the estimate variance are developed. It is shown that optimizing transmitter power allocation to minimize estimate variance for the most-compressed parameter measurement is equivalent to the problem for linear sensors. Finally, a method for operating distributed estimation in a multipath environment is presented that is capable of developing robust estimates for a wide range of Rician K-factors. This dissertation demonstrates that implementing distributed estimation using nonlinear sensors can boost system efficiency and is compatible with existing techniques from the literature for boosting efficiency at the system level via sensor power allocation. Nonlinear transmitters work best when channel gains are known and channel noise and receiver noise levels are low.Dissertation/ThesisPh.D. Electrical Engineering 201

Active Control of Pressure Pulsation in a Switched Inertance Hydraulic System

The nature of digital hydraulic systems may cause pressure pulsation problems. For example, switched inertance hydraulic systems (SIHS), which are applied to adjust or control flow and pressure by a means that does not rely on dissipation of power, have noise problems due to the pulsed nature of the flow. An effective method to reduce the pulsation is important to improve system performance and increase efficiency. Although passive systems to reduce the noise have been shown to be effective in many situations, their attenuation frequency range is limited and they may be bulky. Furthermore, attenuation devices based on expansion chambers, accumulators or hoses are likely to be unsuitable for SIHS as they add compliance to the system and would impair the dynamic response. This thesis is concerned with issues relating to the development of an active noise canceller for attenuating the pressure pulsation which is caused primarily by pulsed flow from high-speed valves in SIHS. Active control methods are widely and successfully applied in the area of structureborne noise (SBN) and air-borne noise (ABN) cancellation. The idea is using the intentional superposition of waves to create a destructive interference pattern such that a reduction of the unwanted noise occurs. However, applications for fluid-borne noise (FBN) attenuation based on the ‘Active noise control (ANC) principle’ are rare due to the restriction of the hardware and experimental apparatus in previous researches. In this thesis, an adaptive controller has been developed for active control of pressure pulsation in hydraulic system. The principle of the adaptive LMS filter and details of the controller design are described and the implementation was carried out through simulation. The designed controller was applied on a vibration test rig initially prior to the hydraulic testing in order to investigate its advantages and limitations in practice. Extensive testing on a switched inertance hydraulic rig proved that the controller, which used a piezoelectric valve with fast response and good bandwidth, is effective and that it has several advantages over previous methods, being effective for low frequency cancellation, with a quick response, and is robust and versatile. A novel method for the accurate measurement of unsteady flowrate in a pipe was proposed. This was applied and validated on a pipe, and was shown to give good results. This method solves the difficulty for measuring the unsteady flowrate currently by using easy-measured signals, such as pressures. It can be used widely for predicting the unsteady flowrate along the pipe.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Investigation of Performance Evaluation and Design Techniques for Large Industrial Mufflers

This document is the culmination of an investigation into the techniques used to design and evaluate the acoustic performance of large industrial mufflers using three-dimensional acoustic theory. Three performance criteria were considered. These criteria were noise reduction (NR), insertion loss (IL), and transmission loss (TL). Three different large sized mufflers were considered in this study. Each muffler was experimentally evaluated through field measurements. These field measurements were then used to validate finite element analysis (FEA) software that is designed to predict muffler performance. A method to corroborate in situ measurements with idealistic FEA simulations was developed. Once the software was validated, it was used to perform parametric studies that investigated the effects of certain muffler design elements on the TL performance of the muffler. The parametric studies yielded a significantly better understanding of the effects of individual muffler features, and their role in a complex geometry

Anti-reflection coating characterization using scattered polarized light

An increase of the energy conversion efficiency of solar cells can be achieved by minimizing reflection losses. A thin film coating serves this purpose when using optimum values of its refractive index and thickness. Also, the presence of surface roughness decreases the reflection of sunlight. Consequently, advanced cell concepts combine thin films with surface texturing. Research and development of these concepts require characterization methods. Ellipsometry is a fast, non-destructive, optical measurement diagnostic which enables characterization of substrates and films with high precision. The use of ellipsometry as characterization tool requires an optical model to extract physical information from the measured data. The optical models used for thin film metrology are generally based on flat surface samples. This work explores the possibilities to extend the range of applicability of ellipsometry to rough surface samples. Polished, acid etched and alkali etched silicon samples were deposited with thin silicon nitride films using a DEPx plasma enhanced chemical vapor deposition system which makes use of the expanding thermal plasma technique. The samples were studied by means of reflectometry, scanning electron microscopy and atomic force microscopy. A single wavelength ellipsometer (?=632.8 nm) was used to measure all samples resulting in ellipsometric layer thickness trajectories for each type of surface roughness. Comparison of the layer thickness trajectories indicated the influence of surface roughness on ellipsometry. Experiments and simulations showed that this influence can be attributed to light scattering and depolarization effects. These effects can be reduced by increasing the angle of incidence during the ellipsometry measurements.The influence of the acid etched surface roughness on the ellipsometry output showed similarities with the influence of an angle of incidence offset. A physical explanation of this behavior is given based on a feature size of the surface roughness in the so called short wavelength regime. This behavior can be exploited for relative layer characterizing measurements on deposited acid etched samples in which a film free sample is measured for reference. The accuracy of these results is acceptable for layer thickness determination, but unacceptable for the determination of the refractive index of the layer. The influence of the surface roughness of the alkali etched wafers on ellipsometry can be explained using the effective gradient medium approximation in addition to the 'angle of incidence offset' model. This extension of the effective medium approximation is designed for the long wavelength regime. It is shown that ellipsometry can be applied to rough surface samples. The influence of the surface roughness on ellipsometry can be explained physically. Additionally, ellipsometry can be used to characterize thin films on top of rough surface samples within the short wavelength regime. An increase of the energy conversion efficiency of solar cells can be achieved by minimizing reflection losses. A thin film coating serves this purpose when using optimum values of its refractive index and thickness. Also, the presence of surface roughness decreases the reflection of sunlight. Consequently, advanced cell concepts combine thin films with surface texturing. Research and development of these concepts require characterization methods. Ellipsometry is a fast, non-destructive, optical measurement diagnostic which enables characterization of substrates and films with high precision. The use of ellipsometry as characterization tool requires an optical model to extract physical information from the measured data. The optical models used for thin film metrology are generally based on flat surface samples. This work explores the possibilities to extend the range of applicability of ellipsometry to rough surface samples. Polished, acid etched and alkali etched silicon samples were deposited with thin silicon nitride films using a DEPx plasma enhanced chemical vapor deposition system which makes use of the expanding thermal plasma technique. The samples were studied by means of reflectometry, scanning electron microscopy and atomic force microscopy. A single wavelength ellipsometer (?=632.8 nm) was used to measure all samples resulting in ellipsometric layer thickness trajectories for each type of surface roughness. Comparison of the layer thickness trajectories indicated the influence of surface roughness on ellipsometry. Experiments and simulations showed that this influence can be attributed to light scattering and depolarization effects. These effects can be reduced by increasing the angle of incidence during the ellipsometry measurements.The influence of the acid etched surface roughness on the ellipsometry output showed similarities with the influence of an angle of incidence offset. A physical explanation of this behavior is given based on a feature size of the surface roughness in the so called short wavelength regime. This behavior can be exploited for relative layer characterizing measurements on deposited acid etched samples in which a film free sample is measured for reference. The accuracy of these results is acceptable for layer thickness determination, but unacceptable for the determination of the refractive index of the layer. The influence of the surface roughness of the alkali etched wafers on ellipsometry can be explained using the effective gradient medium approximation in addition to the 'angle of incidence offset' model. This extension of the effective medium approximation is designed for the long wavelength regime. It is shown that ellipsometry can be applied to rough surface samples. The influence of the surface roughness on ellipsometry can be explained physically. Additionally, ellipsometry can be used to characterize thin films on top of rough surface samples within the short wavelength regime

An analog approach to interference suppression in ultra-wideband receivers

Because of the huge bandwidth of Ultra-Wideband (UWB) systems, in-band narrowband interference may hinder receiver performance. In this dissertation, sources of potential narrowband interference that lie within the IEEE 802.15.3a UWB bandwidth are presented, and a solution is proposed. To combat interference in Multi-Band OFDM (MB-OFDM) UWB systems, an analog notch filter is designed to be included in the UWB receive chain. The architecture of the filter is based on feed-forward subtraction of the interference, and includes a Least Means Squared (LMS) tuning scheme to maximize attenuation. The filter uses the Fast Fourier Transform (FFT) result for interference detection and discrete center frequency tuning of the filter. It was fabricated in a 0.18 õm process, and experimental results are provided. This is the first study of potential in-band interference sources for UWB. The proposed filter offers a practical means for ensuring reliable UWB communication in the presense of such interference. The Operational Transconductance Amplifier (OTA) is the predominant building block in the design of the notch filter. In many cases, OTAs must handle input signals with large common mode swings. A new scheme for achieving rail-to-rail input to an OTA is introduced. Constant gm is obtained by using tunable level shifters and a single differential pair. Feedback circuitry controls the level shifters in a manner that fixes the common mode input of the differential pair, resulting in consistent and stable operation for rail-to-rail inputs. As the new technique avoids using complimentary input differential pairs, this method overcomes problems such as Common Mode Rejection Ratio (CMRR) and Gain Bandwidth (GBW) product degradation that exist in many other designs. The circuit was fabricated in a 0.5õm process. The resulting differential pair had a constant transconductance that varied by only ñ0.35% for rail-to-rail input common mode levels. The input common mode range extended well past the supply levels of ñ1.5V, resulting in only ñ1% fluctuation in gm for input common modes from -2V to 2V