Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / The use of emerging microprocessors has become increasingly popular in packet radio communication equipment. This is mainly because of the improved performance and hardware simplicity they offer. The new generation field programmable gate arrays (FPGAs) and microcontrollers are now widely used in the development of terminal node controller (TNC) components. The aim of this thesis is to evaluate the use of these new generation FPGAs and microcontrollers in a TNC design, in order to utilise the software flexibility and hardware simplicity. The design process began with the selection of the available simple microcontroller-based modem that was just designed. Prior to its usage in a TNC, the software of the modem was modelled, in order to understand its signal processing functionality.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2008. / Modern technology is reducing the cost of electronic devices. The speed of these devices and their reliability is improving. They can be used to implement low cost systems without compromising performance. The design of two linear FM-CW radars using direct digital synthesis (DDS) is discussed. The use of the DDS simplifies the generation of the linear frequency sweep that is required. The systems are analysed mathematically and by computer simulation, before being implemented in hardware. The first system is to be used to measure the position of pedestrians, at short range. The other system is to be used as a ground penetrating radar to measure the depth of objects located close to the surface. The design of a micro-strip patch array with a corporate feed network is also discussed. The antenna is constructed, measured and used in the pedestrian measurement system. Both systems are tested and used for initial radar measurements and the results obtained are presented.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / This thesis relates to the design and characterization of a 10 kW L-band power combiner consisting of 8 input ports. The design is implemented in a non-radial planar transmission line architecture and operates between 1.2 and 1.4 GHz. Because of the ultra high power requirements for the combiner, special attention is given to the power handling capabilities of the transmission lines and the other components involved. Simulated S-parameter models of connector to stripline transitions and a one to four-way junction, as well as measured S-parameter models of high power terminations are incorporated in the final design. A 10 kW combiner was built and measured at low power only due to time constraints and the limited availability of high power sources. Satisfactory results were obtained in terms of the graceful degradation of unit amplifiers, port mismatches and power combining efficiency.

The properties of an elliptical microwave applicator are investigated. The investigation includes the analytical solution of the cutoff frequencies and electromagnetic field patterns in elliptical waveguides. This requires the solution of Mathieu Functions and becoming familiar with an orthogonal elliptical coordinate system. The study forms part of a wider investigation into the microwave heating of minerals and a cavity is designed in such a way that modes are produced at 896MHz. Extensive use is made of simulation packages. These software packages require that the user knows the dielectric properties of materials that are part of simulations. Therefore, the determination of these properties through measurement and the use of genetic algorithms is considered. A method to improve an S-band waveguide measurement system by implementing time domain gating and an offline calibration code previously written forms an integral part of this section of the project. It is found that, within limits, elliptical waveguides tend to produce a greater number of modes within a certain frequency range when compared to rectangular waveguides.

Thesis (MScEng (Electrical and Electronic Engineering))--Stellenbosch University, 2008. / The nite element method (FEM) is a powerful tool for the computational analysis of a wide range of electromagnetic problems. As the complexity of the problems is increased so are the demands in terms of the computational resources required to obtain a su ciently accurate solution. In an attempt to obtain a desired accuracy at a lower computational cost adaptive and higher order methods are often employed. These methods generally entail re ning the solution only in the areas where greater complexity is required, thus decreasing the total computational demand. The adaptive nite element method is implemented and used to analyse the transverse electric cuto eigenmodes of 2D waveguiding structures. The higher order hierarchical vector basis functions that form part of this implementation are automatically generated to very high orders, with the results presented in excellent agreement to analytical ones where applicable. Accuracy to the order of numeric precision is attained. Using these adaptive methods, it is also possible to achieve improved cost e ciency of the error metrics considered with respect to storage requirements and computational cost.

Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / Most speech recognition and language identification engines are based on hidden Markov models (HMMs). Higher-order HMMs are known to be more powerful than first-order HMMs, but have not been widely used because of their complexity and computational demands. The main objective of this dissertation was to develop a more time-efficient method of decoding high-order HMMs than the standard Viterbi decoding algorithm currently in use. We proposed, implemented and evaluated two decoders based on the Forward-Backward Search (FBS) paradigm, which incorporate information obtained from low-order HMMs. The first decoder is based on time-synchronous Viterbi-beam decoding where we wish to base our state pruning on the complete observation sequence. The second decoder is based on time-asynchronous A* search. The choice of heuristic is critical to the A* search algorithms and a novel, task-independent heuristic function is presented. The experimental results show that both these proposed decoders result in more time-efficient decoding of the fully-connected, high-order HMMs that were investigated. Three significant facts have been uncovered. The first is that conventional forward Viterbi-beam decoding of high-order HMMs is not as computationally expensive as is commonly thought. The second (and somewhat surprising) fact is that backward decoding of conventional, high-order left-context HMMs is significantly more expensive than the conventional forward decoding. By developing the right-context HMM, we showed that the backward decoding of a mathematically equivalent right-context HMM is as expensive as the forward decoding of the left-context HMM. The third fact is that the use of information obtained from low-order HMMs significantly reduces the computational expense of decoding high-order HMMs. The comparison of the two new decoders indicate that the FBS-Viterbi-beam decoder is more time-efficient than the A* decoder. The FBS-Viterbi-beam decoder is not only simpler to implement, it also requires less memory than the A* decoder. We suspect that the broader research community regards the Viterbi-beam algorithm as the most efficient method of decoding HMMs. We hope that the research presented in this dissertation will result in renewed investigation into decoding algorithms that are applicable to high-order HMMs.

Thesis (PhD (Electrical and Electronic Engioneering))--Stellenbosch University, 2008. / Unmanned Aerial Vehicle (UAV) technology is classified as being disruptive since it has the potential to radically change the utilization of airspace. Most unmanned vehicles are aimed at military applications, yet civilian applications of unmanned aerial vehicle technology could benefit South Africa considerably. At present, the lack of UAV regulations and standards precludes UAVs from being certified to operate on a file and fly basis in un-segregated civilian airspace. The inability for UAVs to be certified because of a lack of standards creates a “chicken and egg” – “stale mate” situation. If principles such as “equivalence”, initially proposed by Eurocontrol are adopted in South Africa, it then follows that equivalent standards used by manned aircraft could be used by UAVs. UAVs must therefore be tested and evaluated in order to prove compliance with equivalent existing manned aircraft regulations in the foreseeable future until UAV regulations and standards become available. It has been suggested that specific UAV missions such as maritime patrol, border control, search & rescue, and cargo transport could fulfil current requirements. Design considerations and possible concepts of UAV operations, maintenance and training that will enable UAVs to satisfy the immediate South African strategic requirements whilst complying with existing airspace and airworthiness regulations have been proposed in this document while further UAV specific standards and regulations are being developed. UAV testing is an essential part of proving the enabling technology, and part of the process of gaining acceptance into wider airspace. Fortunately, flight test methods and procedures applicable to manned aircraft are directly applicable to UAVs, while systems unique to UAVs can be adapted from existing procedures applied to missiles and military UAVs. Once UAVs are developed and tested, it will be necessary to start full scale operations. Some considerations will be necessary during mission planning. Air traffic management regulations however will prohibit some UAVs from operating in all airspace until enabling technology is developed and tested, while some existing UAVs will never be permitted to “file and fly”. This study also analyses existing airspace and UAV platforms in order to identify the airspace and platforms that will have the most chance of being successfully permitted to “file and fly” in civil airspace. For South Africa to advance as a UAV operating and manufacturing nation, it is therefore essential to compile a roadmap that will guide the process of developing, certifying and operating UAVs. The roadmap must include an interim process, as well as stating the end objective, which is “file and fly”. This South African UAV Roadmap proposal is based on international research that uses documentation and lessons learned from elsewhere to guide the process for creating UAV regulations and standards, while allowing existing UAV operations to expand into the existing airspace in order for further UAV research to take place. This roadmap proposal is the conclusion of a 3 year study, and references to the applicable literature are made throughout the document.

Thesis (MScEng (Electrical and Electronic Engineering))-- University of Stellenbosch, 2006. / This thesis presents the design and implementation of a high voltage DC-DC converter using a series stacked topology. The converter’s specifications were set by Spoornet and the converter forms part of a DC-AC inverter to be installed in Spoornet substations. Different converter topologies will be considered. A high frequency, high power coaxially wound transformer will be analysed, designed and manufactured for this specific converter application. A thermal analysis of the transformer will be carried out. The merits of different control schemes, leading to the choice of an average current mode controller, will be discussed. This controller will be designed and implemented to control the converter. The converter is then simulated to test and verify the controller functionality. A two-level series stacked converter is built and tested to verify the converter design and to test the functionality of the coaxially wound transformer. Finally, the results obtained will be discussed.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / Power system stability simulations are of growing importance for studying the operational integrity of modern power systems, especially in developing economies where generating and transmission capacity lead the demand by relatively small margins. The relevant model topologies, i.e. for synchronous generators, automatic voltage regulators (AVR) and governor control systems, and the simulation software tools are well established. The MATLAB® Power System Blockset provides engineers with a versatile power system stability simulation environment, particularly where the focus is on individual units or small systems. In comparison with dedicated power system simulation tools such as DIgSILENT®, the MATLAB® environment features a superior set of advanced data processing and data analysis features. This includes features such as optimisation and parameter estimation functions. The main aim of this project is to make use of the MATLAB® package in a bid to test an alternative platform with which to estimate the synchronous machine parameters. Conditioning of field data can delay the process considerably, thus the secondary task of this thesis is to solve this issue by ensuring that only one platform is needed for the entire process starting in the field and ending in the modelling and parameter estimation environment within MATLAB®. In closing, the following points summarise the essential aims of this project: • An application using MATLAB® Script must be created that is responsible for importing and processing the data, so it is suitable for analysis purposes. The processing could include cropping, scaling and filtering of data. • Once the data has been imported it must be used with appropriate models to estimate for machine parameters. This will require the use of the Power Systems Blockset. The actual estimation process also requires the creation of an effective cost function, thus a number of different scenarios will have to be investigated before a solution can be found.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / Statistical speech recognition systems typically utilise a set of statistical models of subword units based on the set of phonemes in a target language. However, in continuous speech it is important to consider co-articulation e ects and the interactions between neighbouring sounds, as over-generalisation of the phonetic models can negatively a ect system accuracy. Traditionally co-articulation in continuous speech is handled by incorporating contextual information into the subword model by means of context-dependent models, which exponentially increase the number of subword models. In contrast, transitional models aim to handle co-articulation by modelling the interphone dynamics found in the transitions between phonemes. This research aimed to perform an objective analysis of diphones as subword units for use in hidden Markov model-based continuous-speech recognition systems, with special emphasis on a direct comparison to a context-dependent biphone-based system in terms of complexity, accuracy and computational e ciency in similar parametric conditions. To simulate practical conditions, the experiments were designed to evaluate these systems in a low resource environment { limited supply of training data, computing power and system memory { while still attempting fast, accurate phoneme recognition. Adaptation techniques designed to exploit characteristics inherent in diphones, as well as techniques used for e ective parameter estimation and state-level tying were used to reduce resource requirements while simultaneously increasing parameter reliability. These techniques include diphthong splitting, utilisation of a basic diphone grammar, diphone set completion, maximum a posteriori estimation and decision-tree based state clustering algorithms. The experiments were designed to evaluate the contribution of each adaptation technique individually and subsequently compare the optimised diphone-based recognition system to a biphone-based recognition system that received similar treatment. Results showed that diphone-based recognition systems perform better than both traditional phoneme-based systems and context-dependent biphone-based systems when evaluated in similar parametric conditions. Therefore, diphones are e ective subword units, which carry suprasegmental knowledge of speech signals and provide an excellent compromise between detailed co-articulation modelling and acceptable system performance