Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / Higher-order Markov models are more powerful than first-order models, but suffer from an exponential increase in model parameters with order, which leads to data scarcity problems during training. A more efficient approach is to use mixed-order Markov models, which model data sequences with contexts of different lengths. This study proposes two algorithms for inferring mixed-order Markov chains and hidden Markov models (HMMs), respectively. The basis of these algorithms is the prediction suffix tree (PST), an efficient representation of a mixed-order Markov chain. The smallest encoded context tree (SECT) algorithm constructs PSTs from data, based on the minimum description length principle. It has no user-specifiable parameters to tune, and will expand the depth of the resulting PST as far as the data set allows it, making it a self-bounded algorithm. It is also faster than the original PST inference algorithm. The hidden SECT algorithm replaces the underlying Markov chain of an HMM with a prediction suffix tree, which is inferred using SECT. The algorithm is efficient and integrates well with standard techniques. The properties of the SECT and hidden SECT algorithms are verified on synthetic data. The hidden SECT algorithm is also compared with a fixed-order HMM training algorithm on an automatic language recognition task, where the resulting mixed-order HMMs are shown to be smaller and train faster than the fixed-order models, for similar classification accuracies.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / This thesis focuses on the coaxial-microstrip T-resonator measurement technique to determine the aperture plane capacitance and radiation resistance of electrically short monopoles above a finite ground plane at Radio Frequencies. The determination of these impedance parameters is of interest as they are needed in the Thévenin equivalent circuit of the monopole to establish the relationship between the monopole, the electromagnetic field within which it is immersed, and its surrounding medium properties. Electromagnetically short monopoles are used in various applications such as in permittivity and biomedical measurement techniques and near-field metrology. The coaxial-microstrip device allows the measurement of transmission coefficient nulls when connected to the network analyser. By measuring the frequency shift in transmission null points and analysing the results, the capacitance of short monopoles of various lengths can be obtained. The radiation resistance of the monopole can also be determined by comparing the depth of transmission coefficient nulls points at various resonant frequencies. Monopoles ranging from flush-mounted to various sizes of protruding centre conductors, both bare and insulated, are investigated in the frequencies ranging from 45 MHz to 3 GHz. The experimental results compare well with FEKO numerical predictions at the ranges where this technique is applicable. The technique is however not applicable near and beyond the resonant frequency of the monopole where it is considered to be electrically long. Measurements to obtain monopole impedance parameters were conducted by connecting the device to the HP 8510C Network Analyser. To enhance accuracy, the Network Analyser was calibrated using a high quality calibration kit with precisely defined standards. The technique used was found to accurately yield capacitances in the range of 0.0211 to 1 pF and radiation resistances in the range of 0.9245 to 12.1 Ohms. These results were obtained for monopoles of length 0.0062 < h/l < 0.24. Results from literature, viz. W.R Scott, R.W.P. King and L.J. Cooper, show radiation resistance in the ranges: 2.5 to 35 Ohms (0.079 < h/l < 0.22), 1.26 to 74.08 Ohms (0.026 < h/l < 0.28) and 5.59 to 74.98 Ohms (0.1 < 0.28) for electrically short monopoles. The results of this work are compared with those in the literature and show variations of less than 6%. A technique capable of accurately measuring capacitance and radiation resistance of the monopole at various lengths of protruding centre conductor has been developed. The capacitance for a flush probe compares very well (agreement is less than 6%) when compared with those obtained by the previous researchers. The technique is capable of yielding accurate results radio frequencies in the challenging low radiation resistance range around 1 Ohm.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009. / The nanosatellite is build-up of subsystems and payloads (defined as satellite nodes) connected together into the OBC using CAN bus as the main communication protocol. The flight software application is required to run within the eCos environment on the OBC to monitor and control satellite nodes. The ground station must generate commands and send them to the satellite in space. The application is developed to validate, schedule and dispatch the commands to the satellite nodes at appropriate times. Each node manager, in the flight software, is required to execute the response messages from its respective satellite node. The housekeeping and error recovery data files are defined to convey useful information about satellite status to the user and can be downloaded to the ground station. The flight software is developed using POSIX functions supported by eCos. Although it is not yet ready for real operation in space, the algorithm that can be used for full development is examined and approved.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2004. / The use of ground penetrating radar, and especially borehole radar in underground mines, is becoming increasingly attractive. This is due to the improvements in the user friendliness of this system. Borehole radar is able to sense accurately geological faults and structures hidden inside the rock body and this information is of value to geologists. In the quest to increase the accuracy of the radar, it was realized that the borehole trajectory and surrounding rock properties are needed. This thesis discusses a rock permittivity and loss sensor which is designed to be deployed in cylindrical hard rock boreholes with diameter of about 50 mm. The sensor consists of electrodes to produce capacitance that is dependant on the rock properties, and electronics to measure this dependency. The biggest problem foreseen is that the probe will not be in direct contact with the rock body. Cylindrical Electrodes were designed using numerical simulations and physical models. Sensitivity and noise attributes received attention. Electronic components were used to sense the small capacitance produced by the electrodes. The resulting signals are slow changing "DC• voltages from which an indication of the needed properties could be extracted. The system was integrated and tested in both laboratory and mine conditions. Test-result-b,ased improvements were introduced and led to satisfactory working of the probe. However, the deployment method will need attention before this probe can be used in field conditions.

Thesis (MScEng (Electrical and Electronic Engineering))--Stellenbosch University, 2008. / This thesis is aimed at testing commercially available CMOS amplifier ICs at 4 K. Super Conducting Electronics (SCE) will also be used to amplify RSFQ signals for easier detection by CMOS technology and better signal-to-noise ratios. The SCE comprises of a Suzuki stack amplifier, a 250 μA JTL and a DC-to-SFQ converter. The Suzuki stack amplifier is simulated in WRSPICE. It is able to amplify an SFQ signal synchronised with an external clock signal. The amplified signal can then be detected by a normal commercially available CMOS amplifier IC. To keep the noise in the signal to a minimum, the commercial amplifier must be be situated as close as possible to the SCE. The amplifier must therefore be able to operate at 4 K. Ten different amplifier ICs were tested and three was found that worked down to 4 K.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / A major challenge in the design and operation of High Temperature Superconducting (HTS) devices is the fabrication of reproducible Josephson junctions with good IcRn products. One objective of this thesis was to fabricate successfully HTS step-edge junctions. This objective necessitated a critical evaluation of the available facilities to provide much needed improvements. These improvements included a newly optimised photolithography process, the incorporation of a three-gridded extraction system into the in-house argon ion mill as well as alterations to the Pulsed Laser Deposition (PLD) system to improve thin film quality. These process modifications finally allowed for the fabrication of novel PrBa2Cu3O7−δ buffered step-edge junctions. These junctions were tested for dc and ac Josephson effects and displayed IcRn products of 1.5 mV at 55 K as well as well-defined Shapiro steps. A second objective was to introduce a high quality thin film deposition system that could produce smooth superconducting films for use in filters and multilayer technology. An Inverted Cylindrical Magnetron system was built and optimised to grow YBa2Cu3O7−δ thin films on MgO (001) substrates. A complete optimisation process of these films are presented by utilising several growth and electrical characterisation methods such as XRD, RBS and AFM.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / As secondary objective an introduction to the quantification, theory and measurement of phase noise is presented to make this field of study more accessible for the novice to the field. Available phase noise theory is evaluated at the hand of its application to the design of a low phase noise quartz crystal oscillator. A low phase noise crystal oscillator was designed by application of the presented theory. This oscillator was constructed and measured yielding phase noise low enough to compare favourably with commercially available ultra-low phase noise crystal oscillators. Within the sensitivity of the phase noise measurement equipment good agreement between the theoretically predicted and the measured phase noise was achieved.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / The Gaussian mixture model (GMM) performs very effectively in applications such as speech and speaker recognition. However, evaluation speed is greatly reduced when the GMM has a large number of mixture components. Various techniques improve the evaluation speed by reducing the number of required Gaussian evaluations.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / This thesis investigates to the design, analysis and testing of a Superconducting Programmable Gate Array (SPGA). The objective was to apply existing programmable logic concepts to RSFQ circuits and in the process develop a working prototype of a superconducting programmable logic device. Various programmable logic technologies and architectures were examined and compared to find the best solution. Using Rapid Single Flux Quantum (RSFQ) circuits as building blocks, a complete functional design was assembled incorporating a routing architecture and logic blocks. The Large-Scale Integrated circuit (LSI) layout of the final chip is presented and discussed followed by a discussion on testing. This thesis demonstrates the successful implementation of a fully functional reprogrammable logic device using RSFQ circuitry.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2003. / Ground penetrating radar is currently proving itself as an invaluable tool for the prediction of geological structures in the mining environment. Borehole radar is a specific application of this technology, useful in the prediction of the course of rock layers between boreholes ahead of mining. Establishing borehole radar as an industrial tool requires a system that is more userfriendly and easier to deploy than the bi-static radar systems currently available. The development of a monostatic radar system is discussed. It is an ultra wide band, pulse radar system that operates at VHF (10-100 MHz). The system is required to detect reflections from objects 5 m away and further. This translates to a total electromagnetic propagation time of approximately 100 ns or more in the rock medium. The complete design process - from fundamental requirements, through a conceptual design, to a final electronic circuit - is discussed. The design is also built, measured in the laboratory and taken for initial field trails. The following aspects are considered: • Pulse generation by means of an original circuit based on power MOSFETs. • Routing of the transmitted- and received signal between transmitter, receiver and antenna. This is done using a novel, active quasi circulator topology. • Methods of increasing isolation (actively and passively) between transmitter and receiver. • Interfacing with a specific receiver, antenna and data acquisition system.