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The work presented here provides a comprehensive coverage in the development of an emerging breed of tuneable filters in the millimetre- wave range. It involves the application of nematic liquid crystal (LC) material as substrate to the device. The first focus of the research is accurate characterisation of LC in order to obtain its dielectric properties at millimetre-wave frequencies. Based on the findings, new tuneable filter designs are presented in this thesis. In this thesis, a planar patch resonator is developed to measure the dielectric constants of nematic LC at millimetre-wave frequencies. The resonator is excited using microstrip lines fed by 50D coplanar waveguides (CPWs) which allows for on-wafer measurements. The characterisation technique is achieved using a resonator operating at 40GHz. The switching of the LC molecules is performed using an external electric field by the direct application of a bias voltage. Ac- curate measurements for E~ and Ell are attained for a particular LC mixture, E7. A second resonator is established, operating in the 30 - 60GHz frequency band. Therefore, precise measurements of LC in the millimetre-wave range (up to 60 GHz) indicate a maximum tuning range of 3.58GHz. The first LC based filter presented in this thesis is the third order tuneable parallel-coup led-line bandpass filter. It has three poles with a centre frequency of 33GHz. The bandwidth of the filter is around 3.5GHz and is fixed over the tuning range. The tuneability range is 2GHz achieved for bias volt ages of OV and 10V. The biasing is directly applied to the individual resonators within the filter structure. A detailed investigation on coupled-line resonators based on LC is also provided. The insertion loss of the filter is about 4.5dB, attributed to the microstrip line to CPW transitions used for measurement of the filter by a probe-station. The second tuneable LC bandpass filter is realised using a microstrip line periodical structure and is presented for applications at millimeter- wave frequencies. The filter is designed to operate at a centre fre- quency of 49GHz and has about 20% fractional bandwidth (FBW) (9GHz). A tuneability range of 3.2GHz is achieved over bias volt ages OV to 10V as applied to the LC substrate. The insertion loss of the filter is about 4.7dB, which is comparable or better than the inser- tion losses of tuneable filters based on the semiconductor or MEMS technology, currently available. The dimensions of the filter can be scaled down in order to operate it at the 60GHz band covering vari- ous bandwidth standards for applications in different countries under voltage control.

This research investigates application issues of the active power filter to three-phase three-wire systems, with unbalanced, distorted loads where the line impedance gives rise to voltage distortion at the point of common coupling. Five shunt active power filters are investigated: two-level, wye connected cascaded three and five level, and diode-clamped three and five level. Capabilities of the different shunt active power filter topologies to mitigate voltage/current distortion, compensate reactive power, and balance real power from the source are investigated. Results indicate that neutral point clamped multilevel inverters have advantages over wye cascaded multilevel inverters when operating under asymmetrical load conditions.

This research investigates a digital filter architecture called Multiplicative Finite Impulse Response (MFIR) filters, for implementation on Field Programmable Gate Array (FPGA) technology, MFIR filters are a class of filter structures that can be used to replace recursive Infinite Impulse Response (IIR) filters with Finite Impulse Response (FIR) equivalents, requiring significantly less hardware than classical FIR architectures that fulfil the same specifications. A theoretical analysis of the performance, coefficient quantization effects and round off error behaviour is presented. Optimal FPGA-based implementations are suggested and analyzed. The performance of the MFIR structures is checked based on real-life applications. MFIR structures cannot compete with the IIR structures in terms of hardware requirements. Therefore, the MFIR approximation is mainly appropriate when potentially unstable IIR filters (with poles close to the unit circle) must be implemented in hardware or when linear phase filters with narrow transition bands are required . In general, it can be concluded that MFIR structures, when implemented on FPGA technology, are very good structures for the realization of demanding, possibly linear phase, stable digital filters.

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A new control method for active power filters in conjunction with a passive filter circuit are presented and analysed in this thesis. A new technique for load modelling is introduced in order to enable the design of compensators to improve the power factor and to reduce harmonic levels in electrical power systems. The principles of analysis, design, operation and control of the new circuit equipped with IGBTs are presented. This enables the compensation of rapidly changing loads and reactive power. A special circuit equipped with IGBTs is able to compensate for the reactive power and harmonic currents of different orders. The important aspect of the present work is based on the compensator control circuit for power factor correction and harmonic elimination, and its application. This new configuration improves the rating of the active power filter, reducing power losses in the switches compared to existing and newly developed active filters. Furthermore, it is very stable in operation and much faster by a factor of 20. The thesis also presents a detailed mathematical modelling of the proposed system with frequency and time domain equations. The frequency response of the proposed system is also discussed. This new proposal has been checked using a dedicated software simulation program, which was specifically developed for this purpose. An experimental set-up has been designed and implemented in order to apply the new method using IGBTs as well as some other devices. This thesis also presents a critical literature survey, which provides a critical overview of previous work relevant to the power quality improvement reactive power compensation and active filtering.

For the first time, the new class of filter transfer functions, called Chained Functions is described, in detail. With Chained functions, one may define a new polynomial generating function that is given by the product of a combination of low order functions, called seed functions. The chained function concept provides with a variety of transfer functions, having the same order but different frequency-domain, time-domain and implementation characteristics. When compared to the conventional Chebyshev approximation, reduced sensitivity to manufacturing errors, lower resonator unloaded-Q requirements and, consequently, lower filter losses can be achieved by selecting the appropriate seed function combination for a given implementation technology. This can be achieved with out-of-band rejection levels ranging from those associated with Butterworth to pseudo-Chebyshev. Theoretical and experimental comparisons with conventional Chebyshev filter characteristics, presented in this thesis, demonstrate the advantages and disadvantages of this new family of filter transfer functions.