Thesis (MScIng)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

Thesis (M.S.)--Virginia Polytechnic Institute and State University, 1988. / Vita. Abstract. Includes bibliographical references (leaves 48-49). Also available via the Internet.

Novel feedback signal interference concept for bandpass filter (BPF) design is proposed in this thesis. This new concept was utilized to design wide stopband BPF with superior performance for WLAN applications. The proposed filtering structure consists of two simple coupled-line couplers. The first coupler was employed within the main signal path and the second coupler which is open circuited at the opposite ends was used for the feedback circuit. This new filtering structure was fabricated using low temperature co-fired ceramic technology. The fabricated BPF exhibits an insertion loss (IL) of -1.3 dB with a 3dB fractional bandwidth of 13% at a centre frequency of 2.4 GHz. Furthermore, an attenuation level of -15 dB is achieved up to 4.7fo. Using stepped impedance coupled-lines in the feedback; the stopband performance of the proposed structure can be significantly improved while keeping passband performance intact. Furthermore, the feedback signal interference concept proved to be versatile and can be used to design high selectivity microstrip BPF using composite right/left handed transmission line unit cell in the feedback circuit. The measured results show that roll-up/down rate of more than 300 dB/GHz can be achieved with low IL.The spectrum based quality factor for CRLH TL based BPF is explored and thoroughly studied in this thesis to design a low phase noise oscillator. The proposed metamaterial BPF has higher spectrum based quality factor within the left-handed region due to the slow-wave propagation. This intriguing feature enables the design of a free-running oscillator with excellent phase noise performance operating at a frequency of 2.05 GHz. The fabricated oscillator demonstrates a phase noise of -126.7 dBc/Hz at 100 kHz frequency offset and a FOM of -207.2 dBc/Hz at a 1 MHz frequency offset, being one of the very best reported so far. Many microstrip multi-band BPF design techniques are presented in the literature that offers superior performance in terms of IL, 3dB FBW, and high selectivity. These methods mainly lack the ability to obtain high performance with compact size. Coupled slotted open stubs are used to design a miniaturized dual-wideband BPF. Interdigital capacitor and inner open stubs are used to improve rejection level within the stopbands and increase selectivity. The measurement results reveal that the fabricated dual-wideband BPF has two passbands with a 3dB FBW of 117% and 36%, at respective centre frequencies of 1 GHz and 6.65 GHz. The filter has a super compact size (0.09 Gammag × 0.05 Gammag) where Gammag denotes the guided wavelength at the centre frequency of the first passband and exhibits an attenuation level greater than 20 dB up to 12 GHz.

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One strain of oleaginous yeasts, Cryptococcus curvatus (ATCC 20509) has been studied to grow on several substrates including biodiesel production byproduct crude glycerol and sweet sorghum juice. After cultivation, yeast cells were heated under microwave radiation to extract lipid and produce biodiesel through in-situ transesterification. Firstly, the yeast growth with crude glycerol was studied. When cultured in a one-stage fed-batch process wherein crude glycerol and nitrogen source were fed intermittently for 12 days, the final biomass density and lipid content were 31.2 g/L and 44.2%, respectively. When cultured in a two-stage fed-batch operation wherein crude glycerol was supplemented at different time points while nitrogen source addition was discontinued at the middle of the experiment, the biomass density was 32.9 g/L and the lipid content was 52% at the end of 12 days. On the second step, an optimization of yeast fermentation with crude glycerol was conducted. Through Box-Behnken design and response surface methodology, the optimal temperature, pH, and glycerol concentration for yeast growth on pretreated crude glycerol was identified as 30.2 deg C, 6.0, and 19.8 g/L, respectively. Adopting these optimal parameters, the biomass density and lipid concentration obtained were 7.11 ± 0.36 g/L and 38.53 ± 1.88%, respectively, which matched well with the model predicted values of 6.98 g/L and 41.31%.The resulting parameters of the response surface method optimization were used in a fed-batch fermentation where crude glycerol was automatically pumped in responding to exhausted oxygen levels in the fermentor. At the end of 12 days, the biomass density and lipid content were 44.53 g/L and 49%,respectively. Compared with our fed-batch experiment which was conducted under un-optimized condition, the yield of biomass and lipid increased 35.26% and 25.29%. When cultured in a fed batch process where sorghum juice syrup was supplemented at different time points for 3 days, the final biomass density was 23.6 g/L with a lipid content of 51%. To extract lipids from cells in an effective and fast fashion, a domestic microwave oven was used with different solvents. With only methanol, a lipid yield of 33.2% of yeast cells was obtained in 4 min. This was comparable with a lipid content of 51% attained through using a traditional solvent extraction approach. In the end, to convert yeast lipids to biodiesel directly without the step of lipid extraction, the in-situ transesterification method used microwave irradiation on the simultaneous extraction and transesterification of wet yeast biomass to biodiesel. Response surface methodology was used to analyze the influence of the process variables (solvent to biomass (v:w) ratio, catalyst concentration, and reaction time) on the fatty acid methyl ester conversion. Based on the experimental results and RSM analysis, the optimal conditions for this process were determined as: methanol to yeast biomass (v:w) ratio of around 50:1, catalyst concentration about 5 wt.%, and reaction time of 2 min. The biodiesel samples were analyzed with GC and the FAME content in biodiesel is about 50%.

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Theoretical and experimental studies of real-time interrogation of optical sensors based on wavelength-to-time (WTT) mapping are presented. The sensing information is encoded in the spectrum of an optical sensor, and transferred to the time domain by using WTT mapping. Utilizing digital electronics for post processing, the sensing information can be interrogated at an ultra-high speed and resolution. Two sensors based on WTT mapping are proposed and demonstrated. First, a random grating sensor for simultaneous measurement of the temperature and strain is investigated. An ultra-short pulse from a mode-lock laser is spectrum shaped by a high-birefringence random grating to generate two orthogonally polarized spectrums, which are then fed to an optical loop in which a linearly chirped fiber Bragg grating is incorporated. Linear WTT mapping is implemented, and two temporally separated optical pulses are generated, and then converted to two electrical waveforms at a photodetector. Pulse compression is then employed. By measuring the temporal intervals of the temporally compressed pulses, the strain and temperature information can be retrieved. Conventional fiber based sensors are not sensitive to the refractive index change of the environment. In the second sensor, a silicon photonic microdisk resonator (MDR) for temperature and liquid refractive index sensing is proposed and demonstrated. By using the notches in the spectrum of the MDR, a microwave photonic filter (MPF) is implemented. By feeding a linearly chirped microwave signal to the MPF, a filtered signal with its temporal location representing the spectrum is generated. By monitoring the time location of the filtered signal, the temperature or the refractive index information is revealed.

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Abstract Ionic liquids and microwave heating have been studied in four different reactions namely esterifications, etherifications and ene and sulfonylation reactions. These techniques revealed several advantages over conventional methods. In esterification of alcohols with anhydrides in ionic liquid solvents, the low boiling acid by-product could be removed before product recovery. The acid by-product could be regenerated back to the anhydride. Similar or higher yields were observed from esterifications with acetic anhydride of carbohydrates than with conventional methods. Even cellulose and starch could be esterificated in ionic liquids in the homogenous phase. The etherification reaction in ionic liquid was challenging, due to the basic reaction conditions needed. 1-Methyl-3-butylimidazolium -cations ([BMIM]) could not be used in basic conditions. The new information was that [BMIM]-cation could not be used with epichlorohydrin. The ene reaction was carried out with microwave heating in various solvents. Ionic liquids could be used as a solvent, but the ene reaction between allyl benzene. The best results were obtained without additional solvent. The yields by using microwave heating were high and reaction times were relatively short. The sulfonylation reaction of aromatics could be catalyzed by metal bistriflimide complexes. Even chlorobenzene could be sulfonylated when Bi-complex was used as a catalyst. Ionic liquids could be used as a solvent, but sulfonylations were best carried out without additional solvents.