Pulse time modulation (PTM) techniques have drawn considerable attention over the years as suitable schemes for transmission of information over optical fibres. PTM schemes are known to utilise the vast optical bandwidth to provide efficient transmission characteristics. Pulse code modulation is one such modulation scheme that has been used widely in various communication systems. In this thesis digital pulse interval modulation (DPIM), a form of PTM, is proposed as a suitable modulation scheme for optical communication systems. In this scheme the information is represented by means of varying the anisochronous frame interval between two successive pulses. Each pulse fulfils the dual role of representing the frame boundaries and initiation of the next sampling event within the modulator or sample reconstruction in the demodulator. In this study DPIM frame structure is proposed and sampling criteria, information capacity, bandwidth requirements are discussed in depth. The spectral behaviour of the scheme is investigated and a mathematical model is developed to represent the spectra. The model was numerically evaluated and verified with the practical measurements to prove its validity. Spectral predictions were made for random as well as periodic information signals showing the existence of the distinct slot frequency component, which is used for slot synchronisation. Frame synchronisation is not required as DPIM has self synchronised frame structure. For random signals, slot component is found to be about 15 dB and for periodic single tone sinusoidal signal this was at about 5 dB. Dependency of this component on the various system parameters such as bit resolution, pulse width, pulse shape are discussed. A detailed account of the receiver performance is given. Receiver analysis was carried out for narrow band as well as for wide band channels. Possible error sources are presented and the system performance degradation with these error sources is discussed and a comparison is made with isochronous DPPM and PCM. For performance evaluation of analogue systems, signal-to-noise ratio was mathematically modelled and compared with PCM. This analysis showed that DPIM SNRs shows three important regions as compared to PCM. That is when the SNR is inferior, superior and identical to PCM performance. Threshold levels corresponding to the above regions vary depending on the system bit resolution. A prototype DPIM system was designed and implemented to transmit low speed analogue signal (≈15 kHz) at bit resolutions of 4, 5 and 6 where the slot frequencies are at 510, 990 and 1950 kHz, respectively. Measurements were carried out in order to verify the predicted performance. This results showed close agreement with the predicted. Receiver sensitivity of the prototype at bit error rate of 10^-9 was found to be about -45.5 dBm at all three cases with transmitted average power of -28.8 dBm allowing high optical power budget. The signal-to-noise ratio threshold level of the system was at -47 dBm. Finally, linearity measurements of the overall system were made at the above bit resolutions and the quantitative and qualitative results are presented.

Over the last 20 years, advances in our understanding of the pathophysiology of severe traumatic brain injury (TBI) and in particular the contribution of secondary injury to poor outcome, has served to improve clinical management and reduce the mortality in these patients. However despite many promising preclinical studies there has been a failure to introduce a specific therapeutic intervention to further improve outcome. Inflammation, with cytokine release and leucocyte infiltration, is a significant secondary injury processes. However the inflammatory response to brain injury, its control and modulation remain incompletely described. Chemotactic cytokines, known as chemokines, are mediators of leucocyte recruitment and activation. Expression of chemokines and the resultant recruitment of leucocytes into the brain are generally thought to be integral to the enlargement of cerebral contusions which accompany clinical deterioration following severe TBI. Previous studies indicated that the main neutrophil chemokine, macrophage inflammatory protein-2 (MIP-2/CXCL2) and the monocytic chemokine, monocyte chemoattractant protein-1 (MCP-1/CCL2) are derived from glial. However the origin of these chemokines following TBI, has not been established. Furthermore, little is known about the modulation of these chemokines: The relationship of serum levels of pro-inflammatory mediators such as the human neutrophil chemokine, interleukin-8 (IL-8/CXCL8; a functional homologue of MIP-2/CXCL2), MCP-1/CCL2 and soluble interleukin-6 receptor (sIL-6R) to contusion enlargement has not been investigated. In this thesis, I investigated chemokine expression and modulation both in in-vitro, in-vivo models and in a clinical study. Initially, I compared chemokine expression in rodent and human glial cell cultures and investigated the modulation of chemokine expression by interleukin-6, the glucocorticoid dexamethasone and the immunosuppressant agent FK506. To investigate chemokine expression in-vivo I established the rat lateral fluid percussion injury (LFPI) model of TBI and measured MIP-2/CXCL2 and MCP-1/CCL2 expression in the brain following injury. Inhibition of this expression by dexamethasone and FK506 was then investigated. To identify the cellular source of chemokine expression I developed an immunohistochemical protocol for MIP-2/CXCL2 and MCP-1/CCL2. Finally, in a clinical study of serum chemokine and sIL-6R concentrations after severe TBI, I examined the relationship between these inflammatory mediators and clinical deterioration. Rat glia (microglia and astrocytes) produce chemokines with a response profile that was qualitatively similar to that of human derived cells. These chemokines were increased in the ipsilateral hemisphere following TBI. Surprisingly, immunohistochemical studies identified marked chemokine expression localised to cells with the morphology of degenerating neurones in contused tissue, rather than in glia. Furthermore, while dexamethasone significantly inhibited both MIP-2/CXCL2 and MCP-1/CCL2 expression in a rat astrocyte derived cell line, only MCP-1/CCL2 expression was reduced by steroid treatment in-vivo. Clinically, serum IL-8/CXCL8, MCP-1/CCL2 or sIL-6R were not significantly different in patients that deteriorated due to contusion enlargement from those that remained stable. However these inflammatory mediators were significantly increased in those patients that died. These studies indicate that astrocytes may not be the major source of chemokines following TBI and highlight the need for caution when inferring pathophysiological mechanisms from in-vitro studies.

Switched mode, Class-D power amplification allows for high efficiency power amplification of an audio signal. This thesis investigates its application to high power car audio systems where there is a demand for efficient high power amplification. Examination of the present car audio power amplifiers, which comprise a switched mode power supply combined with a linear output stage, has shown that there is significant scope for improvement in efficiency and power density. A novel power stage in which the attributes of a switched-mode power supply and full bridge output stage is presented. It is demonstrated that elimination of the intermedjate DC supply results in an amplifier which has a significantly lower part count, size and cost compared to conventional designs. Two different modulation schemes are explored (PWM and PDM) with a view to finding the most suitable for the new power stage. The theoretical performances of the modulators are verified by practical measurements. The design of high order DeltaSigma modulators is difficult as they show unstable behaviour and an alternative design methodology has been presented to ease this task. The mechanisms which introduce distortion in a practical amplifier are discussed, and for the case of a PWM driven output stage, a new model is presented to predict the effect of dead time on harmonic distortion. This form of distortion is shown to be the dominant cause of open loop non-linearity. The use of feedback is also investigated and yields a factor of 20 improvement in amplifier total harmonic distortion . The design throughout has been supported with practical results and these have illustrated the importance for careful circuit layout in high frequency switching systems.

International Telemetering Conference Proceedings / October 17-20, 1994 / Town & Country Hotel and Conference Center, San Diego, California / With the development of the airborne telemetry technique, it will be demanded that the transmitting sets on the missiles are more reliable and smaller. A frequency modulation (FM) oscillator stabilized with a dielectric resonator (DR), which can operates in the S-band directly, is presented. The FM oscillator is of simple circuit, reliable operation in the stabilization, small size, light weight and low cost. It will have a certain prospect of application in the airborne telemetry transmitting sets.

In metozoans, most of the key events that lead to cell activation and inhibition are controlled by tyrosine phosphorylation. Extracellular signals are transmitted by membrane bound receptors, which have intrinsic kinase activity or themselves recruit intracellular kinases to specialised inhibitory or activating phosphorylation motifs. In this way, the pattern of kinase activation creates its own turnover and can rapidly generate amplified signals by positive feedback, or recruit inhibitory proteins to counteract the signals. This process of inhibition is also constitutive since it requires continuous counter-inhibition by phosphatases at the cell surface and intracellularly even in the absence of ligands. The absence of phosphatase activity results in unbridled protein phosphorylation and form this and other data it has been proposed that the triggering of the T cell receptor and other co-receptors may result simply by physical exclusion of the large phosphatases such as CD45 from the vicinity of the receptors. Superagonist monoclonal antibodies may work in a similar way, by binding receptors close to the plasma membrane and excluding extracellular phosphatases. The work described in this thesis seeks to discover if antibody superagonists can be generated against the T cell inhibitory cell surface receptor PD-1 and test if this approach can attenuate the immune response. Using in vitro assays of lymphocyte activation and a mouse model expressing human PD-1, this study characterises a series of anti-PD-1 antibodies and shows how patterns of inhibitory activity varying according to binding sites. The inhibitory effects of the anti-PD1 antibodies are seen in the humoral, cellular and transplant immune responses. Agonistic anti-PD1 antibodies induce regulatory T cells and may have role in suppression of autoimmune disease. The thesis suggests that superagonism may be harnessed clinically to dampen the immune response, through activation of inhibitory receptors.

La lumière comprimée est un état du champ électromagnétique pour lequel le bruit, mesuré selon une certaine quadrature est inférieur au bruit du vide. Dans cet ouvrage, nous étudions la possibilité de générer de la lumière micro-onde à partir d’un transistor à effet de champ commercial. D’une part, nous observons le bruit de grenaille du canal drain-source à basse fréquence, ce qui suggère que le canal est cohérent. Ensuite, nous exploitons cette cohérence et procédons à une expérience standard de compression par le bruit de grenaille. D’autre part, nous prédisons, à l’aide d’un modèle simple, la possibilité de comprimer le bruit par modulation de la résistance du canal drain-source pour des mesures de bruit à hautes fréquences. Nous concluons en proposant une mise en œuvre de cette méthode.

In RF communications with a rotorcraft such as a helicopter, the rotor blades can impart a modulation onto the received signal called Rotor Blade Modulation (RBM). This modulation is caused by the reflection of a signal off the rotating blades. The reflected signal is Doppler shifted based on where the signal is reflected along the length of the blade as well as the angle between the axis of rotation and the emitter. RBM is known to degrade the performance of RF communications on rotorcraft and can be used in radar applications to detect and classify aircraft, but there is little on its usefulness in other areas. This thesis looks at the ability to utilize the RBM phenomenon on the rotorcraft itself to geo-locate and track a signal emitter on the ground. To do this a 3D RF ray tracing program was developed in C++ to produce simulations of RBM signals. The developed program is based on optical ray tracing algorithms with modified physical propagation effects for RF signals, and swapping lights and cameras for RF transmitters and receivers respectively. The ray tracer was then run over a realistic set of physical parameters to determine their effects on the received signal; this includes transmitter azimuth and elevation angle, receiver position, blade pitch, etc. along with their combinations. The simulations of the azimuth and elevation angle produce predictable modulations on the received signal. Based on the trends in the signal's modulation, a DSP algorithm was distilled down that accurately determines the azimuth and elevation angle of the transmitter from simulated signal data.

The viability of any electric vehicle is critically dependent on it having an acceptable range between charges, a feature which is ultimately dictated by the capacity of the battery energy store. Considerable improvements in vehicle range are possible, however, by ensuring the most effective use of this limited energy resource through the minimisation of the losses in the electric drive-train, i.e. the combined machine and power electronic controller. A particular consideration is that, for the majority of the time, the electric drive-train will be operating at part load. The thesis investigates the operation of induction motor based electric traction drive-trains, with a view to minimising the system loss over typical driving cycles. The study is based around a 26kW induction motor and IGBT inverter drive, which is typical of the technology used to power a small urban vehicle. A potential advantage of an induction motor based drive-train is the ability to vary the level of excitation field in the motor, and therefore the balance of iron and copper loss. The control of the supply voltage magnitude necessitates the use of some form of modulation on the output of the power converter. The method of modulation employed will influence the harmonic content of the supply to the motor, the level of parasitic harmonic loss in the machine and the switching losses of the power semiconductors. A theoretical study supported by experimental work on a DSP controlled drive is presented and used to determine the most appropriate modulation strategy at a given operating point to achieve an optimal balance between the motor copper, iron and harmonic loss and inverter switching and conduction loss. It is shown that compared to the established method of constant flux and fixed inverter switching frequency control, a significant reduction in the traction system loss can be achieved. Some different modulation schemes involve varying amounts of computational overhead in a DSP, the implementation of candidate modulation and control schemes has also been investigated to ensure the defined scheme is practically realisable.

QC 351 A7 no. 53 / The optical performance of three stereomicroscopes commonly used for photo - interpretation is described in terms of magnification, field of view, and square wave response (SqWR). For a given field size, the SqWR can be used to compare the amount of information in the image of each microscope. The SqWR was determined by scanning a square wave test target at four field positions for each of four field sizes (corresponding to magnifications of 6, 12, 25, and 50X). The measured SqWR was used to evaluate the relative ability of each microscope to perform two typical photointerpretation tasks: (1) detailed viewing of a target, and (2) scanning or searching for a target. For these tasks the Wild M -5 stereomicroscope was found to be somewhat better than the Bausch and Lomb 240 stereomicroscope, and both were found to be superior to the Olympus SZ III stereomicroscope. The SqWR curves are included and can be used to compare the stereo - microscopes for various photointerpretation tasks.

A digital modulation method using Chirp-Slope Keying (CSK) is developed for coherent underwater acoustic communications. Effective signal detection is a critical stage in the implementation of any communications system; we will see that CSK solves some significant challenges to reliable detection. This thesis is primarily based on analyzing the effectiveness of CSK through simulations using Matlab's Simulink for underwater communications. The procedure begins with modulating a chirp's slope by random binary data with a linear-down-slope chirp representing a 0, and a linear-up-slope chirp representing a 1. Each received symbol is demodulated by multiplying it with the exact linear-up-slope chirp and then integrating over a whole period (i.e., integrate and dump). This slope-detection technique reduces the need for the extensive recognition of the magnitude and/or the frequencies of the signal. Simulations demonstrate that CSK offers sturdy performance in the modeled ocean environment, even at very low signal-to-noise ratio (SNR). CSK is first tested using the fundamental communication channel, Additive White Gaussian Noise (AWGN) channel. Simulation results show excellent BER vs. SNR performance, implying CSK is a promising method. Further extensive analysis and simulations are performed to evaluate the quality of CSK in more realistic channels including Rayleigh amplitude fading channel and multipath.