Doctoral Degrees (Electrical and Electronic Engineering)

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Browsing Doctoral Degrees (Electrical and Electronic Engineering) by browse.metadata.advisor "De Swardt, J. B."

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    Microwave dielectric heating through interference modulation with narrow band high power sources
    (Stellenbosch : Stellenbosch University, 2002-12) Meier, Ingolf; De Swardt, J. B.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: One of the most difficult problems in microwave dielectric heating is the generation and control of field and heating patterns. A technique allowing the synthesis of different, pre-determinable heating patterns by interference modulation is proposed. The proposed concept may be described by the term 'interference modulation'. Interference modulation is a technique which enables particular patterns, called features, to be obtained by signals from several sources interfering with each other. The relative phases of the signals are modulated, by which process known features may be selected. Weights are assigned to these features, which may be combined over time to form a new heating pattern. Phase changes may then be used to switch to specific, known features, with weights which will determine the contribution of each feature to the desired overall pattern. In the practical implementation described, magnetron tubes are the sources. Each of these narrow-band high-power sources was injection locked to a low power control signal. The control signals are derived from a reference source and their phase is set to select a corresponding feature. Calculation and measurement showed that reliable locking occurs with a control signal power of at least 3% of the magnetron's emitted power. Measurements of patterns were carried out with materials formed into sheets and blocks. Some were chemically prepared to reveal the overall heating pattern. The observed patters, simulations and field measurements concur, thus validating the concept and operation of the proposed topology.
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    Modelling and mitigation of specular multipath interference in a dual-frequency phase comparison FMCW radar system
    (Stellenbosch : Stellenbosch University, 2017-03) Schonken, Willem Petrus Francois; De Swardt, J. B.; Van der Merwe, Paul; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: Digital signal processing technology has improved greatly over the last two decades. Increased processing power, cheaper memory and higher sampling rates have enabled the application of Frequency-Modulated Continuous Wave (FMCW) radar to a myriad of new areas. FMCW offers a number of advantages, such as continuous coverage and low peak output power, making it an attractive technology for industrial and automotive applications. Expansion into new application environments and the use of new signal processing algorithms have created a need for new multipath interference models. This study aims to fulfil that need through rigorous mathematical modelling of both the physical multipath environment and the two-dimensional Fast Fourier Transform (FFT) signal processing method, in the context of a Dual-Frequency Phase- Comparison FMCW radar sensor. It will be shown that specular reflections can have a profound effect on the amplitude and the phase of an FMCW radar’s base-band received signals. The multipath phase error interacts with the signal processing method, resulting in new and interesting effects. Furthermore, new mitigation methods will be proposed and critically evaluated by means of simulated and real-world measurements.
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    Phase tracking electronically variable attenuators with receiver protection
    (Stellenbosch : Stellenbosch University, 2018-12) Stofberg, Anneke; De Swardt, J. B.; Van der Walt, P. W.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: This dissertation presents the development of a set of optimal phase tracking electronically variable attenuators. Secondly, a compact high power PIN diode limiter is developed and its minimum attainable resistance is extracted through high power measurements. Close range reflections cause the receiver of a multi-channel digital beamforming radar to saturate. Controlled attenuation over time, implemented with electronically variable attenuators, is used to prevent receiver saturation (sensitivity time control). An electronically variable attenuator is placed in front of the first low noise amplifier in each channel; its insertion loss directly adds to the receiver’s noise figure. A multi-channel digital beamforming radar receiver requires good phase tracking between its receiver channels to minimise direction of arrival estimation errors. The set of electronically variable attenuators used for sensitivity time control need to track in phase over the control range. In this dissertation, sensitivity analysis is used to identify a set of optimal phase tracking electronically variable attenuators. A root sum square error measure is derived from the multiple output sensitivities of an electronic network. The error measure gives the expected RMS phase error within a set of networks. Applying the error measure to several electronically variable attenuators over the control range, the cascaded parallel quarter-wave attenuator is identified as having optimal phase tracking within a set of attenuators over control range. The cascade parallel quarter-wave attenuator is developed further and optimised through the application of sensitivity analysis. The final attenuator has excellent attenuation flatness, attenuation range, phase tracking and a simple biasing scheme. A multi-channel digital beamforming radar receiver also has to be protected against large signals. These large potentially damaging signals are either due to the radar’s own transmitted signal, or from other radars transmitting large amounts of power in the same frequency band. A receiver protector (e.g. a limiter) typically supplies this function. In a multi-channel digital beamforming radar, a compact circuit based high power limiter has many advantages in terms of space and cost when it is compared to a waveguide limiter. The compact high power limiter developed in this dissertation consists of PIN diodes implemented on a multi-layer printed circuit board. The circuit is referred to as an active PIN-Schottky limiter. The maximum power handling capability of the active PIN-Schottky limiter is determined by the PIN diode at the limiter’s input. The minimum attainable resistance is not given by the manufacturers, so that the diode’s minimum attainable resistance can not be found from the datasheet information. It is difficult to estimate how much power is dissipated in the diode when a large signal is incident. Through a temperature controlled measurement, the PIN diode’s voltage decrease as a function of junction temperature increase is measured. By fitting the PIN diode’s measured and simulated junction temperature increase, it is possible to extract the resistance of the diode when large forward bias is applied. Once the resistance is known, the power dissipated in the PIN diode can be calculated for different operating conditions.