Current trends in microelectronic technology, and the rapid increase in medium and large-scale integration, have motivated the design of digital systems at the sub-system level, and made traditional discrete gate techniques obsolete in many situations. The use of devices such as read-only memories (ROMs), programmable logic arrays (PLAs) and universal logic modules (ULMs), has considerably influenced the design philosophy. The designer may therefore find himself confronted by entirely different problems from those encountered in the past, some of which will be tackled in this thesis.

The object of this thesis is to present firstly a new design technique for combinational logic circuits, and secondly a new state assignment method for the design of sequential machines. The combinational design technique to be presented has been based upon two concepts. One of them is minterm-interchange by which it is possible to decompose a Boolean function into the exclusive-OR of two lower cost Boolean functions. The second concept concerns If simplest-threshold functions which are logically simple. The cost of realisation of such functions should also be minimal in comparison with a wider class of functions. The techniques based upon these concepts are implemented in the spectrum domain, which is briefly surveyed in Chapter 1. Chapter 2 details the mathematics of the minterm-interchange operation in the spectrum domain. Chapter 3 is concerned with the design algorithm and also with the conditions of applicability of this technique to the Boolean function realization. Also in Chapter 3, the combination of the already known spectral translation technique and the new minterm-interchange technique has been considered. Finally in Chapter 4 two basic concepts used in combinational logic design are applied in the proposed new state assignment technique for sequential machine design. This new state assignment is implemented by means of a binary tree.

The likely growth in the importance of private land mobile radio in the future presents particular problems in achieving the necessary crystal oscillator reference frequency stability. The high power consumption and slow warm up time of oven controlled crystal oscillators is undesirable for mobile operation, so that frequency changes with temperature must be minimised by other means. This thesis describes a technique for temperature compensation of crystal oscillators which is primarily digital in nature. The system is capable of high stability and offers advantages not present in conventional designs. The use of a digital memory as the compensation law governing element affords great versatility, the same hardware being appropriate in a variety of applications. Automatic calibration of the device is also possible, further improving its performance and reducing the likely cost of production. Of particular importance in the realisation of the scheme is the method employed to adjust the output frequency. The requirement for a digitally controlled very high resolution frequency source of simple construction has led to the development of a new class of digital frequency synthesiser, a detailed discussion of which is included. In order to improve upon conventional methods of thermometry and to maximise the use of digital circuitry a Y cut crystal is used as the temperature sensing element. This crystal is placed in close thermal contact with the primary crystal and its linearly temperature dependent frequency is counted digitally to afford temperature information. The early chapters of the thesis discuss the underlying theory of precision frequency sources, quartz crystals and oscillators, and digital frequency synthesis, leading to a general discussion of the proposed system. Details of the design and performance of a prototype unit are then given, and some techniques of automatic programming of the device are considered. Some relevant mathematical derivations and experimental results are included among a series of appendices.

This project was dedicated to the development of solution-processed nanomaterials-based high-performance field-effect transistors (FETs) suitable for a new application area of printed reconfigurable antennas. The focus of research was on implementing solution processed high electron mobility InAs nanowires (NWs) as semiconducting channel in field effect transistors. The key direction of this work was the development of InAs NWs FETs with a designated high frequency waveguide geometry to enable they operation as microwave switch elements. Initially, InAs NW FETs were developed and tested in direct – current mode to allow evaluation and extraction of key transistor performance parameters such charge carrier mobility, threshold, on/off ratio, transconductance, subthreshold swing, and on-channel resistance. The InAs NW were assembled from nanowire ‘inks’ in the FETs channel via electric -field assisted assembly technique, dielectrophoresis. Nanowires were directly incorporated in FETs with bottom-gate architecture on Si/SiO2 substrates, and with top-gate architecture on quartz substrates with polymeric gate dielectrics. Current-voltage characteristics were measured both in controlled dry nitrogen atmosphere and ambient environment, and demonstrated an instability of unprotected InAs NW in ambient air. Protection of nanowire channel with Al2O3 layers has resulted in significant improvement of device stability. Optimised InAs NW FET devices demonstrated electron mobility over 1000 cm2/Vs and on-off current ratios up to 1000. Finally, a proof of principle for solution processed InAs NW field-effect transistors operating as microwave switches in 5-33GHz frequency range have been demonstrated. FET devices were implemented in co-planar waveguide (CPW) microwave transmission line geometry, providing efficient transmission or reflection of microwave signal. The FETs demonstrated high performance with transistor ON-state resistance as small as ≈50 Ω providing an excellent impedance match to that of microwave waveguide. Bringing FETs to the OFF state provided 1000 times resistance increase, resulting in FET microwave switch behaviour, characterised by ~10 dB change in scattering (S)-parameters, such as difference in transmission coefficient S21 between on/off switching states.

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This thesis presents an investigation into advanced squirrel-cage induction machines (IMs), with a particular reference to the reduction of the total axial length without sacrificing the torque and efficiency characteristics and analysis of recently found non-sinusoidal bar current phenomenon, which occurs under some certain design and operating conditions, and affects the overall performance characteristics of the IMs. As a first step, the most convenient method is determined by utilizing a fractional-slot concentrated winding (FSCW) technique, which has advantages such as non-overlapping windings, high slot filling factor, and simple structure. After implementing this technique, it is found that due to the highly distorted magnetomotive forces (MMFs) created by the FSCWs, significant high rotor bar copper loss occurs. In order to reduce the MMF harmonics without increasing the size of the machine, a new technique titled “adapted non-overlapping winding” is developed. This technique consists of the combination of the auxiliary tooth and phase shifting techniques, resulting in a stator with concentrated windings of two-slot coil pitches but without overlapping the end-windings. Thanks to this method a large number of the MMF harmonics are cancelled. Thus, a low copper loss IM with significantly reduced total axial length is obtained. Influence of design parameters; such as stator slot, rotor slot, and pole numbers, number of turns, stack length, stator and rotor geometric parameters, etc. on the performance characteristics of the advanced IM is investigated and a comprehensive comparison of advanced and conventional IMs is presented. This thesis also covers an in-depth investigation on the non-sinusoidal bar current phenomenon. It is observed that the rotor bar current waveform, usually presumed to be sinusoidal, becomes non-sinusoidal in some operation and design conditions, such as high speed operation close to synchronous speed, or fairly high electrical loading operation, or in the IMs whose air-gap length is considerably small, etc. Influences of design and operating parameters and magnetic saturation on the rotor bar current waveform and the performance characteristics of squirrel-cage IMs are investigated. The levels of iron saturation, depending on the design and operating parameters, in different machine parts are examined and their influences are also investigated, whilst the dominant part causing the non-sinusoidal rotor bar current waveform is identified. It is revealed that the magnetic saturation, particularly in the rotor tooth, has a significant effect on the bar current waveform.

Next generation wireless communication systems are expected to support unprecedented extremely high data transfer rates. This objective requires wider bandwidths which are presently only available at the millimeter waves (mm-waves) spectrum (30-300 GHz). Due to stringent propagation impairments, mm-waves mainly rely on the line of sight communication links which require high gain and wide angle beamsteeering smart antennas to maintain their performance. Owing to the complexity and losses in array beamformers, the realization of a high gain wide angle electronic beamsteering antenna solution at mm-waves becomes a key challenge. This research provides a potentially competing novel high gain electronic beamsteering antenna solution for mm-waves in the form of a phase quantized smart reflectarray consisting of high performance reconfigurable unit cells. Novel contributions of this research are: (a) Analysis of mm-wave reflectarray unit cells including the effects of fringing fields, surface waves, finite metal conductivity and metal surface roughness. (b) New measurement techniques for mm-wave reflectarray unit cells to ease the alignment, orientation, and DC biasing issues. (c) Characterization of PIN diodes at 10 GHz and 60 GHz for their ON/OFF state models extraction from measurements. (d) Design of three state implicit phase shifter reflectarray unit cell at 60 GHz, reduction in its DC bias lines, and an optimization technique to improve polarization purity of a multi-state reconfigurable unit cell. (e) A fast algorithm to prepare the electromagnetic simulation model of large reflectarrays. (f) Conception and measurement based validation of phase quantized reflecarrays and their performance matrix. (g) Conception and measurement based analytical solution of low DC power consuming smart reflectarrays. The resulting solution is agile, simple to implement, do not necessarily require multiple RF chains, enables wide angle electronic beamsteering (+-78 degree), is scalable for any gain/frequency requirements, can be made foldable for smaller satellite platforms, is very reliable, and consumes low DC power. This smart reflectarray platform can implement any phase only synthesis technique for radiation pattern control including single/multiple pencil beams, contoured beams, and their scanning over wider angles. Findings of this research would potentially benefit next generation terrestrial/air/space communication systems and radars.

Recently data traffic has experienced explosive increase with the proliferation of wireless devices and the popularity of media-based free services. The academic and industry of mobile communications have predicted an estimated $1000$x increase in traffic volume for the forthcoming 5G networks. This traffic explosion stimulates the deployment of heterogeneous networks (HetNets) with small cells (SCs) underlying in the traditional macrocells, which has been considered as a promising technique to contribute to the $1000$x traffic capacity gain. Initially, licensed spectrum bands are expected to be used in SCs, thus the SC deployment introduces the cross-tier interference between SCs and macrocells, which degrades the downlink signal to interference plus noise ratio (SINR) of user equipments (UEs) severely, especially for the edge UEs in a ultra-densely deployed scenario. To alleviate this cross-tier interference between SCs and macrocells, unlicensed spectrum bands are advocated to be used in SCs. Specifically, with the aid of carrier aggregation, the $5$ gigahertz (GHz) unlicensed band has become an option for SCs in the Long Term Evolution (LTE)-Unlicensed (LTE-U) scheme, but the $5$ Ghz unlicensed band has already been used by WiFi networks. Thus downlink cross-tier interference also occurs between LTE-U and WiFi networks. Accordingly, downlink cross-tier interference is inevitable no matter licensed or unlicensed spectrum band (i.e., 5 GHz) is used in SCs, and interference coordination schemes, such as further enhanced inter-cell interference coordination (FeICIC) for macrocells and SCs, and Licensed Assisted Access (LAA) for WiFi networks and LTE-U networks, have been proposed to mitigate these cross-tier interferences. In this dissertation, we mainly focus on the modelling and performance analysis of HetNets with the aforementioned two interference coordination schemes (i.e., FeICIC and LTE-LAA) under the stochastic geometry framework. Firstly, as the configuration of reduced power subframe (RPS)-related parameters was not well investigated in a two-tier HetNet adopting RPSs and cell range expansion (CRE), we derive the analytical expressions of the downlink coverage probability and rate coverage probability in such a HetNet. The optimal settings for the area of macrocell center regions, the area of SC range expansion regions, and the transmit power of RPSs for maximizing the rate coverage probability are analysed. As compared with the rate coverage probability in the two-tier HetNet with almost blank subframes (ABSs), which is proposed in the previous version of FeICIC, i.e., the enhanced inter-cell interference coordination (eICIC), the results show that ABSs outperform RPSs in terms of the rate coverage probability in the two-tier HetNet with the optimal range expansion bias, but lead to a heavier burden on the SC backhaul. However, with static typical range expansion biases, RPSs provide better rate coverage probability than ABSs in the two-tier HetNet. Secondly, the conventional FeICIC scheme ignores the potential of RPSs being adopted in both tiers of a two-tier HetNet without CRE, which is envisioned to improve the SINR level of edge UEs in both tiers. Accordingly, we study the downlink coverage probability and rate coverage probability of a two-tier HetNet applying with our proposed scheme. The results reveal that adopting RPSs in both tiers not only improves the coverage probabilities of edge UEs, but also increases the rate coverage probability of the whole two-tier HetNet. Thirdly, in both previous works, strict subframe alignment (SA) was assumed throughout the whole network, which is difficult to maintain between neighbouring cells in reality. Consequently, we propose a novel subframe misalignment (SM) model for a two-tier HetNet adopting RPSs with SM offsets restricted within a subframe duration, and analyse the coverage probability under the effects of RPSs and SM. The numerical results indicate that the strict SA requirement can be relaxed by up to $20\%$ of the subframe duration with a loss of below $5\%$ in terms of the downlink coverage probability. Lastly, since stochastic-geometry-based analysis of the coexisting LTE-LAA and WiFi networks, which adopt the carrier-sense multiple access with collision avoidance (CSMA/CA) as the medium access control (MAC) scheme and share multiple unlicensed channels (UCs), was missing, we analyse the downlink throughput and spectral efficiency (SE) of the coexisting LTE-LAA and WiFi networks versus the network density and the number of UCs based on the Matern hard core process. The throughput and SE are obtained as functions of the downlink successful transmission probability (STP), of which analytical expressions are derived for both LTE-LAA and WiFi UEs. The results show that the throughput and SE of the whole coexisting LTE-LAA and WiFi networks can be improved significantly with an increasing number of accessible UCs. Based on the numerical results, insights into the trade-off between the throughput and SE against the number of accessible UCs are provided. All the derived results have been validated by Monte Carlo simulation in Matlab, and the conclusions observed from the results can provide guidelines for the future deployments of the FeICIC and LTE-LAA interference coordination schemes in HetNets.

X-ray detectors are an invaluable tool for healthcare diagnostics, cancer therapy, homeland security and non-destructive evaluation among many fields. However, potential uses for X-rays are limited by system cost and/or detector dimensions. Current X-ray detector sensitivities are limited by the bulk X-ray attenuation of the materials and consequently necessitates thick crystals with dimensions of ~ 1 mm - 1 cm, resulting in rigid structure, high operational voltages and high cost. Semiconducting polymer X-ray detectors are an emerging low cost technology. Their solution processable nature enables the fabrication of such detectors on flexible substrates over large areas using printing and roll-to-roll coating techniques. However, the low atomic number (Z) of organic materials results in low X-ray attenuation and hence, low X-ray sensitivities. This thesis focuses on direct detection of X-rays using organic semiconducting systems incorporating high Z Bi2O3 nanoparticles (NPs). For the work discussed, a thick organic bulk heterojunction (~10 - 20 μm) consisting of the p-type poly(3-hexylthiophene-2,5-diyl) (P3HT) and the n type [6,6]-Phenyl C71 butyric acid methyl ester (PC70BM) are utilised. The effectiveness on the utilisation of a dual carrier system is demonstrated through the fabrication of detectors in a diode architecture by varying the NP loading from 0 - ~50% (by wt.). These hybrid detectors demonstrate sensitivities of 1712 μC mGy-1 cm-3 when irradiated under 50 kV tungsten X-ray source and ~30 and 58 μC mGy-1 cm-3 under 6 and 15 MV X-rays generated from a medical linear accelerator. A flexible detector was fabricated which demonstrated a high sensitivity approaching 300 μC mGy-1 cm-3, highlighting the promise of the technology for dosimetry and imaging in non-planar architectures. These performances are discussed based on the structural and electrical characteristics of the hybrid thick film diodes. Possible mechanisms for the high sensitivity observed are proposed where photoconducting gain, impact ionisation and Mie scattering are identified as central contributors towards the detector sensitivity. Routes to increasing the detector thickness to 100 μm - 1 mm thickness range is demonstrated using a pellet based on powder sintering technique. These P3HT:PC70BM:Bi2O3 detectors operate as photoconductors showing X-ray sensitivities in the region of ~ 160 C mGy-1 cm-3 and a preliminary X-ray imager based on the pellets are fabricated. Finally, potential routes for further improvement of the detector characteristics are discussed.