In this thesis work, a new distributed amplifier topology is discussed and applied to three different cases. The topology is based on dividing the frequency spectrum into channels and amplifying afterwards. The channelized and amplified signals are then combined at the output for broadband amplification. This topology is used in the design of a three channel 0.1-1 GHz amplifier with a gain of 14.5&plusmn / 0.6 dB. The design is fabricated, and then the measured and simulated results are compared. A second 0.1-1 GHz amplifier with 21&plusmn / 1 dB is designed in simulation environment with five channels. This five channel amplifier is fabricated and measured results are compared with the simulated ones. A 1-6 GHz three channel amplifier is also designed with a gain of 10.5&plusmn / 0.5 dB. Application of the proposed topology to three different designs shows promising results for future amplifier designs.

This thesis presents design, fabrication and testing of a number of multi-frequency band microstrip-fed re-configurable microstrip patch antennas. All re-configurable antennas are designed to change from one resonance frequency to another by an electronic control of RF MEMS switches, one at a time. Besides a fixed size slot on the patch, switches are placed in insets for satisfying better input match at each resonance frequency individually. Also some switches are placed into the slot for adding another resonance frequency to change the effective slot-length like effective inset length changing.To actuate the RF MEMS switches in the configured way, DC-stubs are also designed to apply required potential difference between switch ports and the carrier. These stubs exhibit RF-open at switch side to prevent any RF leakage, and DCground on the other side. That RF short-to-open conversion is accomplished together with feed structure / with a taper depending on the feed network selected. All devices introduced here are built by Microwave Research Group in Electrical and Electronics Department, Middle East Technical University. Depending on the sensitivity of structure, some devices are built by RF MEMS group in Microelectronic Production Plant for MEMS (METU &amp / #8211 / MET) during the thesis study. Therefore this study is the continuation of the first national work on fabrication of RF MEMS devices.

In this thesis, the neutral point potential drift/fluctuation of the three-level neutral point clamped inverter is analyzed and a novel control algorithm, the two-parameter PWM method is proposed to confine the neutral point potential variation to a very small range. The two-parameter PWM method provides superior neutral point potential control performance even with small DC bus capacitors. The method is based on PWM pulse pattern modification and requires no additional hardware. Detailed analytical models of the neutral point current and potential as a function of the modulation parameters are established and the neutral point potential behavior is thoroughly investigated. Based on the study, the deficiency of the known methods is illustrated and the two-parameter PWM method is developed and its superior performance demonstrated. The performance of the two-parameter PWM method is verified by means of computer simulations utilizing both the per-PWM-cycle average model and the detailed model of the inverter. The results are supported by laboratory experiments involving both an R-L load and an induction motor drive.

Recently, the use of permanent magnets as magnetic field sources in biomedical applications has become widespread. However, usage of permanent magnets in magnetic resonance imaging (MRI) is limited due to their inhomogeneous magnetic field distributions. In this thesis, shape and geometry optimization of a magnet is performed. Moreover, placement of more than one magnet is optimized to obtain desired magnetic field distribution in specific region of space. However, obtained magnetic field distribution can not be used in the conventional MRI image reconstruction techniques. Consequently, an image reconstruction technique for MRI in inhomogeneous magnetic fields is developed. Apart from these, since any reconstruction technique requires signal data, an MRI simulator in inhomogeneous magnetic fields is constructed as a part of this thesis. Obtained results show that the theory developed in this thesis is valid. Consequently, new MRI devices that have permanent magnets as magnetic field sources can be constructed in the future.

In this thesis / analysis, design and implementation of a DC-DC converter with active clamp forward topology is presented. The main objective of this thesis is generating a rectified sinusoidal voltage at the output of the converter. This is accomplished by changing the reference signal of the converter. The converter output is applied to an inverter circuit in order to obtain sinusoidal waveform. The zero crossing points of the converter is detected and the inverter drive signals are generated in order to obtain sinusoidal waveform from the output of the converter. Next, the operation of the DC-DC converter and sinusoidal output inverter coupled performance is investigated with resistive and inductive loads to find out how the proposed topology performs. The design is implemented with an experimental set-up and steady state and dynamic performance of the designed power supply is tested. Finally an evaluation of how better performance can be obtained from this kind of arrangement to obtain a sinusoidal output inverted is thoroughly discussed

In this study, a model is developed to represent doubly-salient magnetic circuits and to fit finite element analysis for the aim of obtaining a set of normalized normal force, tangential force, and permeance variation data. To obtain the desired data FE field solution method is used. The reliability of finite element results have been verified by three steps / first, comparing the numerical results with analytically calculated permeance, second, by solving two switch reluctance motors and comparing the results with the measurements of static torque and flux linkage. The third step is by using the normalized data obtained by solving the model with the aid of an available software that is capable to predict the behavior of switched reluctance motors. Once the reliability of the data is assured, the desired data set is produced and presented in tables.

In this thesis, a wideband dual polarized microstrip antenna is designed, manufactured and measured. Slot coupled patch antenna structure is considered in order to achieve the wideband characteristic. Although rectangular shaped slot coupled patch antennas are widely used in most of the applications, their utilization in dual polarized antenna structures is not feasible due to space limitations regarding the positioning of two separate coupling slots for each polarization. For a rectangular slot, the parameter that affects the amount of coupling is the slot length. On the other hand when a H-shaped slot is considered, both the length of the center arm and the length of the side legs determine the coupling efficiency. This flexibility about the optimization parameters of the H-shaped slot makes it possible to position the two coupling slots within the boundaries of the patch antenna. Therefore, H-shaped slot coupled patch antennas are studied in this thesis. In order to investigate the effects of slot and antenna dimensions on the radiation characteristics of the antenna, a parametric study is performed by analyzing the antenna structure with a planar electromagnetic field simulation software (Ansoft Designer). By the help of the experience gained through this parametric study, a dual polarized patch antenna that can be used at the base station of a cellular system (DCS: 1710&ndash / 1880 MHz) is designed. Before manufacturing the antenna, dimensions of the antenna are re-tuned by considering a finite sized ground plane in the simulations. Finally, the antenna is manufactured and measured. An acceptable agreement is obtained between the measurement and the simulation results.

This dissertation is focused on the design, fabrication, and characterization of novel metamaterials in microwave and terahertz regions with the following outcomes: A planar &micro / -negative metamaterial structure, called double-sided SRR (DSRR), is proposed in the first part of this study. DSRR combines the features of a conventional split ring resonator (SRR) and a broadside-coupled SRR (BC-SRR) to obtain much better miniaturization at microwave frequencies for a given physical cell size. In addition to DSRR, double-sided multiple SRR (DMSRR), double-sided spiral resonator (DSR), and double-sided U-spiral resonator (DUSR) have been shown to provide smaller electrical sizes than their single-sided versions under magnetic excitation. In the second part of this dissertation, a novel multi-band tunable metamaterial topology, called micro-split SRR (MSSRR), is proposed. In addition to that, a novel magnetic resonator structure named single loop resonator (SLR) is suggested to provide two separate magnetic resonance frequencies in addition to an electric resonance in microwave region. In the third part, two different frequency tunable metamaterial topologies called BC-SRR and gap-to-gap SRR are designed, fabricated and characterized at terahertz frequencies with electrical excitation for the first time. In those designs, frequency tuning based on variations in near field coupling is obtained by in-plane horizontal or vertical displacements of the two SRR layers. The values of frequency shifts obtained for these tunable metamaterial structures are reported to be the highest values obtained in literature so far. Finally, in the last part of this dissertation, novel double-sided metamaterial based sensor topologies are suggested and their feasibility studies are presented.

Accelerometer sensors fabricated with micromachining technologies started to take place of yesterday&rsquo / s bulky sensors in many application areas. The application areas include a wide range from consumer electronics and health systems to military and aerospace applications. Therefore, the performance requirements extend form 1 &mu / g&rsquo / s to 100 thousand g&rsquo / s. However, high performance strategic grade MEMS accelerometer sensors still do not exist in the literature. Smart designs utilizing the MEMS technology is necessary in order to acquire high performance specifications. This thesis reports a high performance accelerometer with a new process by making the use of bulk micromachining technology. The new process includes the utilization of Silicon-on-Insulator (SOI) wafer and its buried oxide (BOX) layer. The BOX layer helps to realize interdigitated finger structures, which commonly find place in surface micromachined CMOS-MEMS capacitive accelerometers. The multi-metal layered CMOS-MEMS devices inherently incorporate interdigitated finger structures. Interdigitated finger structures are highly sensitive to acceleration in comparison with comb-finger structures, which generally find usage in bulk-micromachined devices, due to absence of anti-gap. The designed sensors based on this fabrication process is sought to form a fully-differential signal interfaced sensor with incorporation of the advantages of high sensitive interdigitated finger electrodes and high aspect ratio SOI wafer&rsquo / s bulk single crystal silicon device. Under the light of the envisaged process, sensor designs were made, and verified using a computing environment, MATLAB, and a finite element analysis simulator, CoventorWARE. The verified two designs were fabricated, and all the tests, except the centrifuge test, were made at METU-MEMS Research Center. Among the fabricated sensors, the one designed for the high performance achieves a capacitance sensitivity of 178 fF with a rest capacitance of 8.1 pF by employing interdigitated finger electrodes, while its comb-finger implementation can only achieve a capacitance sensitivity of 75 fF with a rest capacitance of 10 pF.

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