M.Ing. (Electrical and Electronic Engineering) / The South African Railways covers a total distance of some 24 000 km, making use of a signalling network of 1000 interlocking systems. This comprises of the order of 1000 signalling houses each with its own unlnterruptible power supply system of which a DC·AC converter Is one of the components. Due to the requirements by Spoomet, high frequency (ultrasonic) chopper technology had to be used. This treatise describes the design and development of a 3 kVA composite DC·AC converter with an ultrasonic AC link for this application. In the primary converter (centre tapped topology) ZVS Is achieved by good circuit layout and controlled transformer capacitance. As a result the performance of the converter was improved with regard to efficiency and switching transients compared to the conventional hard switched converter. The four pulse topology with an output LC·filter was used for the secondary converter. Due to the modulation technique being used, the load perceived the switching frequency as 20kHz while the actual switching frequency was only 10kHz. In this way switching losses were kept within reasonable limns as well as satisfying the low audible noise requirement. The stability of the LC·filter was achieved with a current feedback loop. The literature describes the capacitor current being used in the feedback loop. In this case the Inductor current was used with the advantage of output short circuit protection without an additional current measurement. Apart from the advantages of small weight and volume from a maintenance point of view, the composite DC·AC converter also achieved high efficiency (> 85%) over a wide load range (10%-100% of full load).

The first part of this thesis describes the design and construction of a pulse communications system called delta modulation. A simple transistorized coder and decoder was built to convert speech signals into binary pulses, and back again. This system was tested in several ways: the circuit performance was checked electronically, and the quality of transmission through the system was judged by listening tests and by comparison with special tapes containing controlled amounts of signal and thermal noise. The second part concerns the design and construction of a compressor and expandor that can be used with a delta system in order to improve its transmission quality. A fast-acting compressor and expander was constructed using diode-bridges as non-linear elements. Tests of the two components used "back-to-back" showed a satisfactory matching of their characteristics. If these components were used with the delta modulation system, there would be a considerable increase in the range of signal levels that could be transmitted with reasonable quality. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A special purpose modem has been designed to fill the interface requirements peculiar to RCA's DatawayTmk hotel/motel data management system. The modem interface operates on dedicated telephone quality wiring over distances up to 4000 feet using frequency shift keying. A data rate of 2880 bits per second is achieved through the interface. This report discussed the requirements of the interface and details the worst case design of the modem transmitter and receiver sections. The approach used relies heavily upon the use of digital logic circuitry in the modulation and demodulation of the FSK signals.

M.Ing. (Electrical and Electronic Engineering Science) / This study presents a simple methodology developed to set up and establish a simplex communication system over a power line using load signatures. Traditional concepts such as normalization and matched filtering form the core of these algorithms. The analysis also relies extensively on the voltage and current measurements made over a fixed period of time on a single phase. Furthermore, analyses on the capacity of this kind of system are also presented, taking into consideration various disturbances.

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Thesis (Ph. D.)--University of Texas at Austin, 2003. / Vita. Includes bibliographical references. Available also from UMI Company.