Thesis (MScEng (Electrical and Electronic Engineering))--Stellenbosch University, 2008. / This thesis relates to the analysis and design of a SQUID microscope. Superconductor theory is discussed in depth to provide a thorough understanding of Josephson junctions and of dc SQUID magnetometers. The behaviour and suitability of different types of single-layer dc SQUIDs are looked at. The quality of the superconducting material patterned onto a substrate and the Josephson junction design used affect the behaviour of a practical dc SQUID. Noise and cooling play an integral part in the design and operation of a dc SQUID. The source of noise is looked at in an effort to minimize its effect. Cryocooling is essential to real world operation so different cooling strategies and their consequences are analyzed. This thesis focuses on modeling the behaviour of the dc SQUID to creating a practical system for use inside a SQUID microscope. Operating the dc SQUID with the appropriate electronics will linearize the device, reduce the effect of noise, and create a device with wide bandwidth. Each step in creating a practical system is discussed in detail. Simulations are used to create models predicting the behaviour of the dc SQUID and the electronics. They are then used to design and create practical electronic systems. Measurements are performed on Josephson junctions and dc SQUID magnetometers using the designed electronics. The Josephson junctions behave as predicted and were successfully tested. The dc SQUIDs did not behave as predicted and were not successfully tested. The SQUIDs were damaged, either by a malfunction in the cryocooler or through age related deterioration. A full test of the flux-locked loop was not possible and the dc SQUID was not linearized.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / The African Speech Technology (AST) project ran from 2000 to 2004 and involved collecting speech data for five South African languages, transcribing the data and building automatic speech recognition systems in these languages. The work described here form part of this project and involved implementing methods for automatic boundary placement in manually labelled files and for determining errors made by transcribers during the labelling process.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / In an ideal software defined radio (SDR), all parameters are defined in software, which means the radio can be reconfigured to handle any communications standard. A major technical challenge that needs to be overcome before this SDR can be realised, is the design of an RF front end that can convert any digital signal to an analogue signal at any carrier frequency and vice versa. Quadrature mixing (QM) can be used to implement and analogue front end, that performs up and down conversion between the complex baseband centred around 0 Hz and the carrier frequency. By separating the tasks of frequency conversion and digital-to-analogue conversion, the latter can be performed at a much lower sample rate, greatly reducing the demands on the hardware. Furthermore, as QM can handle variable carrier frequency and signal bandwidth, this can be done without sacrificing reconfigurability. Using QM as an analogue front end may therefore be the solution to implementing SDR handsets.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2001. / Over the last decade, as the mining industry of South Africa is moving to ever deeper mines, the borehole radar is becoming an increasingly important field of research. In December 2000, Burger completed his thesis on Electromagnetic Modelling of a Borehole Radar Environment with the FDTD Method. The goal of this thesis is to extend the research presented in Burger's thesis, considering how more advanced modelling techniques can be applied to the FDTD analysis of the borehole radar environment. Some of these techniques include implementation of dispersive and conductive material models, and developing Uniaxial Perfectly Matched Layer boundary conditions for matching these model. Simulations were run to measure the performance of these boundary condition for matching dispersive and conductive materials. The thesis also includes the implementation of a parallel version of the FDTD algorithm using the Message Passing Interface library. Finally several realistic borehole models where simulated to test the accuracy of the code and to show how the code can be used to model real world problems.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2008. / The design of an aperture-coupled coaxial diplexer for R-band, is presented. To improve the ease of tuning, a tuning procedure for the diplexer with the aid of a MATLAB application with graphical user interface, is developed. Final experimental results show good agreement between the circuit model and the physical structure. Final measurements of the diplexer structure achieved 18.83 dB and 21.52 dB return loss in the lower and upper frequency band respectively and insertion loss of 0.58 dB and 0.61 dB was measured for the two frequency bands. Isolation were measured as 74 dB at 2.01 GHz and 84 dB at 2.17 GHz The accuracy of two techniques for determining coupling coefficients in coaxial and waveguide resonators are investigated. One method is the Eigenmode Method for determining the coupling coefficients in a physical resonator and the other the circuit model representation, utilising inverters to represent the coupling between resonators. Results showed that marked differences occur when using the three different inverter configurations to enable filter dimensioning for a given coupling coefficient. Four waveguide filters, utilising posts and irises respectively, are designed using dimensions obtained from the three inverter configurations as well as the Eigenmode method for a certain coupling coefficient.

Thesis (MEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2002. / An efficient radiating strucllire was needed for borehole pulsed radar applications in the 10-100 MHz frequency band. Both resistively loaded and insulated wire antennas were investigated and an active antenna is proposed as a fmal solution. The study proceeded from the characterization of the origin of radiation on a conductive dipole wire antenna when excited with a transient. Different radiation mechanisms were identified when the antenna was excited with a current or voltage source. The wire antenna in insulated surroundings was modelled using transmission line theory to simulate the antenna in the borehole environment. The transmission line model proved to be useful for investigating conducting and resistively loaded antennas for the dimensions associated with borehole surveys. From the modelling results, it became apparent that the asymmetric resistively loaded antenna might provide the best practical solution. This antenna displays reasonably stable input impedance and low far-field variations for different theta angles across the desired frequency band. Different percentage Wu-King resistive profiles were studied to show that a 50% reduction in the normal Wu-King resistor values will add only a little ringing but have better amplitude response than the 100% Wu-King loading. The asymmetric resistively loaded antenna has better sensitivity to receiving transients when combined with a high impedance source load than when symmetrically loading the antenna. An active antenna incorporating the asymmetric resistively loaded antenna and a low noise current feedback front end amplifier was built and measured in an air environment. The results show that the active antenna has a flat transfer function and reacts as a wide band electric field probe with better resolution than in the conventional 50 Q loaded case

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / Modern day digital modulation techniques in communication systems produce large peak-to-average ratios. To maintain linearity, power amplifiers have to operate at backed-off levels. This results in low efficiency with consequences such as high power consumption, short battery life and excessive heat in power amplifiers. A Doherty amplifier is an efficiency enhancement technique which increases an amplifier’s efficiency at backed-off levels. This thesis presents a design procedure for a Classical Doherty amplifier. A method where Sparameter measurements from a transistor are used to predict the transistor’s transmission phase response for varying input power is presented. This method is found to be accurate by comparing it to measurements done on a non-linear network analyser. The measured S-parameters are also used to design the Doherty amplifier at its predicted peak output power. Two Classical Doherty amplifiers are designed, manufactured and characterised. The measurements are performed on a custom measurement setup using in-house developed Matlab code to automate the measurements. The first Doherty amplifier used small-signal Siemens CFY30 GaAs FETs and the second Doherty amplifier used 10W Motorola MRF282 LDMOS transistors. The performance of both amplifiers is compared to similar balanced amplifiers and shows improvements in their efficiency. The improvement in efficiency for the 10W Doherty power amplifier in relation to a balanced amplifier is compared to results found in the literature and a good correspondence between the measured and published results were obtained.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / A low earth orbit satellite is to provide the telecommunications link to facilitate email services to rural areas, where the infrastructure necessary for e-mail is lacking (e.g. no telephone lines). Communication time with this satellite from any particular point on the ground is less than one hour per day. It is thus of utmost importance to maximise the data throughput rate for the system. The contribution of this thesis is to improve the performance of CSMA/CA by adapting and optimising it for the above application. This improved protocol is used to regulate data flow through the system. Specific attention is given to the comparison of various random variable distributions for use as the back-off random variable. Two pieces of software are further contributed. First, a set of MATLAB scripts which are used for comparing various back-off random variable distributions and optimising each of these distributions. Secondly, an extensive (more than 2500 lines of code) OMNeT++ simulation of the improved CSMA/CA protocol, complete with MATLAB scripts for setting up multiple simulation runs and plotting the results. Both pieces of software accept the system constraints as parameters, and are thus easily adaptable for a similar system which may use the same protocol, but has different parameters. It is concluded that the set of MATLAB scripts are a fairly accurate tool for optimising throughput, as is confirmed by the OMNeT++ simulations, and that OMNeT++ has merit for simulating the given type of system and protocol.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2007. / The work presented in this Thesis relates to the increased capabilities of the mobile Internet and the possible use of cellphones as an enhancement to video surveillance systems. The focus of the Thesis is on the delivery of live video content to Java enabled cellphones. The various characteristics, capabilities and limitations of the mobile networks- and phones are investigated. Various options for streaming video content to cellphones are also explored. The design and implementation of a digital surveillance system with the ability to stream live video to a cellphone is presented. Two versions of the streaming protocol are developed and implemented in cellphone applications, with which the video stream can be viewed. An evaluation and real-life testing of the applications are presented. Recommendations regarding further enhancements are provided.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2006. / This thesis investigates a computational model of an electronically steered antenna array. It focuses on a simple element comprising an S-shaped, crossed, cylindrical cavity-backed slot antenna. An unusual aspect of the element is the addition of a top hat, which contributes to the low scanning ability of the array. The objective was to confirm this contribution of the top hat. During the investigation, the computation code Feko was validated for slot analysis through consideration of the basic properties of a dipole and slot and how they related to each other. Various alternative models were evaluated, before a final accurate model was modelled in Feko, to find possible equivalent models. The final model, which implemented the use of Feko as release 5, was the only one that achieved successful results. The results showed that the top hat does play an important role in the steering of the beam. The mechanism through which this occurs was identified. The results that where obtained indicated that there may be more effective elements than the cylindrical element and the identification of this is proposed as a field for further study.