Thesis (MScIng)--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet.

The majority of fundamental research on wave-driven sediment transport has been based on regular waves and regular oscillatory flows. Systematic research into isolating the effect of irregularity on oscillatory boundary layer hydrodynamics and sand transport are not presently available. This research reports on large-scale oscillatory flow tunnel experiments designed to isolate the effect of flow irregularity (specifically amplitude-modulated flows) on oscillatory flow boundary layer hydrodynamics and net sand transport rates. Two main series of experiments - one focussing on the hydrodynamics of amplitude-modulated flows over a fixed (immobile) bed and a second focussing on the sediment transport over mobile sand beds – were conducted. Detailed boundary layer velocity measurements are presented for regular and amplitude-modulated oscillatory flows over two fixed rough beds, coarse sand and gravel. The results show that amplitude-modulation of the flow has a variable effect on the hydrodynamic behaviour of the oscillatory boundary layer: the broad conclusion is that time-history effects are not significant near the bed but become more significant higher in the flow. Net transport rate measurements are presented for regular and amplitudemodulated flows for two mobile sand beds, medium and fine sand. The mobile bed experiments show that: for medium sand, net transport rates are similar for the amplitude-modulated and equivalent regular; for fine sand, net transport rates are very different for the amplitude-modulated and equivalent regular flows. The SANTOSS model-predicted sand transport rates shows that the model correctly predicts the net transport for the present experimental conditions, indicating that the major unsteady transport processes are captured in the model. The SANTOSS model is used to compare net transport rates for equivalent regular and amplitude-modulated flows across a wide range of flow conditions and two sand sizes. The results show good agreement in net transport rate between equivalent regular and amplitude-modulated flows, for conditions in which phase lag effects are weak and poor agreement when phase lags effects are strong.

Experiments were carried out to determine the characteristics of a reflection-type modulator. The modulator consisted of an E-plane T-junction that was terminated on the side-arm by a reflex klystron operating in the passive region. Switching was accomplished by applying a pulse to the repeller of the passive klystron. It was found that the switching characteristics were strongly dependent on the power level at the input to the modulator. Good switching characteristics were obtained at low power levels hut these deteriorated as the input power became comparable to that produced by the klystron when operating as an oscillator. For the particular klystron used, satisfactory switching was possible for input power levels approximately 5 to 10 dB less than the output of the switching klystron. It was also found that the modulator characteristics depended on the operating mode of the passive klystron. The desired operating mode was a compromise between power-handling capability and the switching rate. An attempt was made to explain the decrease in switching range at high input power levels by relating the observed results to an increase in the bunching parameter at the boundaries of the passive region. However, this did not fully account for the observed results. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Two types of devices employing ferroelectric modulation of a semiconductor thin-film have been realized and studied. The first consists of a cadmium selenide film with electrodes deposited on a barium titanate substrate together with a switching electrode on the other side of the substrate. This gives a two-valued resistor; in effect, a nondestructive readout of the state of the ferroelectric crystal which is regarded as a storage element. The second device is a thin-film transistor (TFT) deposited on a barium titanate crystal. A fourth counterelectrode - on the other side of the crystal allows changing between two opposite polarization directions in the crystal, thus giving a TFT with two sets of characteristics, roughly equivalent to a two-valued built-in gate bias. The read-in, or switching time, of the device is substantially determined by the barium titanate crystal and can be in the microsecond range for high switching fields. Readout of the devices can be continuous or not, as desired. Characteristics of the TFT, which is considered equivalent to a two-gate device, are analyzed in terms of the gradual channel approximation. Experimental results of the two devices are presented and discussed in relation to the predicted behaviour. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

M.Ing. (Electrical & Electronic Engineering) / Please refer to full text to view abstract

M.Ing. (Electrical & Electronic Engineering) / Please refer to full text to view abstract

Abstract This dissertation presents new incoherent scatter radar measurement techniques and data analysis methods. The measurements used in the study were collected by connecting a computer-based receiver to the EISCAT (European Incoherent SCATter) radar on Svalbard. This hardware consists of a spectrum analyzer, a PCI-bus-based programmable digital I/O card and a desktop computer with a large-capacity hard disk. It takes in the 70-MHz signal from the ESR (Eiscat Svalbard Radar) signal path and carries out down-conversion, AD conversion, quadrature detection, and finally stores the output samples effective sampling rate is 1 MHz, large enough to span all the frequency channels used in the experiment. Hence the total multichannel signal was stored instead of separate lagged products for each frequency channel, which is the procedure in the standard hardware. This solution has some benefits including elimination of ground clutter with only a small loss in statistical accuracy. The capability of our hardware in storing the incoherent scatter radar signals directly allows us to use very flexible and versatile signal processing methods, which include clutter suppression, filtering, decoding, lag prole calculation, inversion and optimal height integration. The performance of these incoherent scatter radar measurement techniques and data analysis methods are demonstrated by employing an incoherent scatter experiment that applies a new binary phase code. Each bit of this code has been further coded by a 5-bit Barker code. In the analysis, stochastic inversion has been used for the first time in decoding Barker-coded incoherent scatter measurements, and this method takes care of the ambiguity problems associated with the measurements. Finally, we present new binary phase codes with corresponding sidelobe-free decoding filters that maximize the signal-to-noise ratio (SNR) and at the same time eliminate unwanted sidelobes completely. / Original papers The original papers are not included in the electronic version of the dissertation. Lehtinen, M., Markkanen, J., Väänänen, A., Huuskonen, A., Damtie, B., Nygrén, T., & Rahkola, J. (2002). A new incoherent scatter technique in the EISCAT Svalbard Radar. Radio Science, 37(4), 3-1-3–14. https://doi.org/10.1029/2001rs002518 Damtie, B., Nygrén, T., Lehtinen, M. S., & Huuskonen, A. (2002). High resolution observations of sporadic-E layers within the polar cap ionosphere using a new incoherent scatter radar experiment. Annales Geophysicae, 20(9), 1429–1438. https://doi.org/10.5194/angeo-20-1429-2002 Damtie, B., Lehtinen, M. S., & Nygrén, T. (2004). Decoding of Barker-coded incoherent scatter measurements by means of mathematical inversion. Annales Geophysicae, 22(1), 3–13. https://doi.org/10.5194/angeo-22-3-2004 Lehtinen, M. S., Damtie, B., & Nygrén, T. (2004). Optimal binary phase codes and sidelobe-free decoding filters with application to incoherent scatter radar. Annales Geophysicae, 22(5), 1623–1632. https://doi.org/10.5194/angeo-22-1623-2004

La demande en bande passante des systèmes de communication optique ne cesse de croitre. Des débits de données de l’ordre de plusieurs centaines de TBit/s sont attendus dans un futur proche. La photonique silicium est une technologie majeure pour faire face à ces besoins croissants. Sa compatibilité avec les technologies CMOS permet naturellement une co-intégration photonique/électronique sur les mêmes circuits. A court terme, l’augmentation des débits de données dans les générations futures de système de communication optique passe par l’utilisation de formats de modulation avancés, et l’augmentation du nombre de bits par symbole transmis. A plus long terme, de nouvelles techniques de multiplexage sont nécessaires. Le multiplexage de modes est actuellement une solution attractive à l’étude dans ce but.Dans ce travail de thèse, différents moyens pour implémenter ces nouveaux systèmes de communication optiques sont étudiés au niveau de l’émetteur. Ces travaux incluent dans une première partie la modélisation, conception et caractérisation des modulateurs silicium. Dans une seconde partie, de nouveaux composants pour manipuler les modes sur circuits intégrés photoniques sont proposés, conçus et caractérisés, avec pour application le multiplexage de modes.Une nouvelle méthode a été proposée pour la modélisation des modulateurs optiques silicium. Cette méthode permet de réduire le temps de simulation de 2 ordres de grandeur, en maintenant un bon niveau de précision. En utilisant ce modèle, des modulateurs basés sur des diodes PN latérales et interdigitées ont été conçus pour fonctionner en bande O des communications optiques. Les résultats expérimentaux ont permis la mise en évidence de diagrammes de l’œil avec des taux d’extinction de 10 dB pour des modulations de type OOK (ON-OFF Keying) à 10Gbit/s. De plus des modulations de type BPSK (Binary Phase Shift Keying) ont également été démontrées à 10Gbit/s.De nouveaux convertisseurs de modes et multiplexeurs ont été proposés, conçus, fabriqués et caractérisés, pour être utilisés dans des systèmes de multiplexage modal. Les résultats expérimentaux ont permis de mettre en évidence des fonctionnements large bande passante avec de grands taux d’extinction. / Bandwidth demand in optical communication systems is continually growing. Data rate values in the order of several hundreds of TBps are expected in the near future. In order to cope with those expectations silicon based technologies are believed to be the best suited. Its naturally compatibility with CMOS easily enables the electronics and photonics co-integration. In the short-term the way increase data rates in next generation optical communication systems goes through using advanced modulation format and increase symbol rates. In the long-term view, new multiplexing techniques will be required. In this sense, mode division multiplexing is nowadays an attractive approach under consideration.In this Thesis work, the way to implement these new optical communication schemes is studied from the transmitter point of view. It includes, on a first part the modeling, design and characterization of silicon modulators. And in a second part, it includes the proposition, design and characterization of novel mode handling devices for mode division multiplexing.A new way of modeling silicon modulators has been developed. This new model permits to reduce the computation time of modulator analysis up to two orders of magnitude, while maintaining a good level of accuracy. Using the model, modulators based on lateral PN junctions and interdigitated PN junctions were designed to work in the O-Band of optical communications. Characterization work has been performed on these modulators with good results. Wide-open OOK (On Off Keying) eye diagrams with 10 dB extinction ratio were obtained at 10GBps. Furthermore, BPSK (Binary Phase Shift Keying) modulation was also demonstrated at 10GBps.New kind of mode converters and multiplexers, intended to work as mode division multiplexing subsystems have been proposed, designed, fabricated and characterized. Measured results show broad bandwidth operation with high extinction ratio.

"When the subject of the use of FM radio in the public schools was suggested by the writer as a subject for a research paper one of the first comments made by a fellow student was, 'Radio is a dead issue. Television has already made it out of date.' This comment immediately made the subject a challenge. Had we in America, in less than a generation, advanced to rapidly that one of the world's greatest media of communication was already out of date? The writing of this paper is an effort to answer that question, and in answering it to present a picture of the status of frequency modulation broadcasting in the public schools throughout the nation today"--Introduction. / Typescript. / "August, 1953." / "Submitted to the Graduate Council of Florida State University in partial fulfillment of the requirements for the degree of Master of Arts." / Advisor: John W. Mitchell, Professor Directing Paper. / Includes bibliographical references (leaves 67-68).

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