Browsing by Subject "Modulators (Electronics)"
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Item A high performance class-D amplifier with cascaded sigma-delta modulators(Texas Tech University, 2004-05) Trehan, ChintanThe focus of this thesis is on analysis, simulation and board level implementation of the proposed Class-D power amplifier architecture. The structural design consists of two Sigma-Delta Modulator (SDM) stages in cascade with an intermediate decimation-filter between them. Noise and high tone introduced at the first- stage is filter out through the decimation filter. The signal is converted to a 1-bit Pulse Duration Modulation (PDM) signal by the second stage SDM. The H-Bridge is made part of the SD loop, which enables not only the noise shaping of the quantization noise but also stabilizes the output power switching stage. Output of the H-Bridge is converted to a digital signal using a comparator and latch circuitry and is fed back. To further increase the linearity and performance, high frequency ripples introduced at the H-Bridge is quantized by using a 4-bit SD Analog-to-Digital Converter (ADC) in the feedback loop. Due to the intermediate digital stage and the feedback control at the output stage, the proposed structure has high efficiency and linearity and still is very compact making it possible for wide range of applications.Item A multibit cascaded sigma-delta modulator with DAC error cancellation techniques(Texas Tech University, 2004-05) Su, Chun-hsienNoise reduction techniques are developed for a multibit cascaded sigma-delta (ÓÄ) modulator used in the analog interface of a digital signal processing system to improve its performance by reducing the errors introduced by digital-to-analog converters (DACs). The idea of the proposed architecture is to create extra feedback paths around the modulator to reduce the DAC errors further by properly designing the error cancellation logic. Transfer functions show that the DAC error at the final stage of the proposed architecture is totally cancelled, while DAC errors from other internal stages are shaped by an order higher than those in a conventional cascaded modulator. The difficulty in circuit implementation of modulators with high resolution and bandwidth increases due to the imperfection of analog components in VLSI processes. Structural and circuit-level compensation techniques are generally used in developing such modulators. Major analog nonideal effects in a multibit cascaded ÓÄ modulator include coefficient mismatches, DAC nonlinearity errors, and integrator leakages. While providing solutions for each of these nonidealities, this dissertation focuses on the minimization of the DAC error since it causes the most performance deterioration. A configurable fourth-order (2-1-1) ÓÄ modulator is implemented for architecture verification. This modulator can be configured as the proposed architecture as well as a conventional cascaded structure with various modulator orders. The design of the system's parameters and analog blocks are fully described in this dissertation. The system is fabricated by the AMI Semiconductor (AMIS) 0.5ìm double-poly triple-metal mixed- signal process through the MOSIS service. Measurement results show that with on-chip error of ±0.15 LSB for each DAC and an oversampling ratio (OSR) of 32, an improvement of 8dB of the proposed architecture over the conventional structure is observed.Item Bilinear processing with acousto-optical modulators(Texas Tech University, 1986-08) Spillman, Gerald EdwardNot availableItem Electro-optic modulators based on polymeric Y-fed directional couplers(2004) Zhou, Qingjun; Chen, Ray T.The performance of current commercially available electro-optical (EO) intensity modulators is adversely affected by the distortions caused by the nonlinearity of their modulation curves. Furthermore, the direct-current biases, required in order to bring these modulators to their best working points, result in more complicated circuit fabrications than are desirable. This current research investigated the feasibility of fabricating bias-free and/or high-linear EO modulators by incorporating the merits of the Y-fed directional couplers (YCMs) with those of the EO polymer materials. To this end, EO polymer-based conventional YCMs were employed to fabricate bias-free EO modulators. A low switching voltage of 3.6V and a high extinction ratio of 26dB were obtained, at a 1.34µm wavelength, with a conventional polymeric YCM having lumped electrodes. A high-speed intrinsic 3dB-biased EO modulator based on a conventional polymeric YCM operating at a 1.55µm wavelength was successfully demonstrated to provide a 22-GHz 3-dB bandwidth, with a performance similar to that of a quadrature-biased Mach-Zehnder modulator (MZM) in terms of nonlinear distortion suppression when no direct-current bias was applied. A broadband high-linear EO modulator based on a ∆β-inverted polymeric YCM was also demonstrated for the first time. This modulator had a 3-dB bandwidth of 20GHz, and a high nonlinear distortion suppression, which was approximately 20dB larger than that of a quadrature-biased MZM in the range between 0 and 8 GHz. The linearity properties of the high-speed ∆β-inverted YCMs were also investigated theoretically. The results show that broadband high-linear modulators can be achieved by taking advantage of the small dispersion in EO polymers’ refractive indexes at microwave and optic frequencies. A systematic study on traveling-wave (TW) electrodes for polymer-based directional couplers was conducted for the first time in order to find the optimal electrode structure and dimensions. A conformal-mapping approach to the optimum TW electrode design of a coplanar waveguide with ground (CPWG) was developed and employed to optimize the electrode dimensions.Item High performance thermo-optic switch and electro-optic modulator based on polymeric multi-mode waveguides with high device packing density for optical network applications(2001-08) Lu, Xuejun; Chen, Ray T.In this research, a multi-mode waveguides thermo-optic switch was first developed based on the unidirectional coupling mechanism. This device has a packing density of 40 channels/cm. Simulation result shows that an extinction ratio of greater than 20dB can be achieved with the device- electrode interaction length of 30mm. The thermo-optic switch operating at wavelengths of 632.8nm and 1.3 µm has been demonstrated experimentally with extinction ratios of 21dB and 22dB, respectively. Based on the same unidirectional coupling mechanism, A multimode waveguides electro-optic modulator was also designed, fabricated, and tested. The device has a modulation depth of 91% and dynamic range of 68V. An EO modulator array was also developed based on multi-mode optical waveguides. The device has eight-multi-mode waveguide EO modulator on a size chip of 20mm by2.5mm. Each of the EO modulator has the modulation depth of 91%. The cross talk between the EO modulators was measured to be –22dB. These devices can be used for multi-mode fiber optic communications.Item Photonic crystal-based passive and active devices for optical communications(2008-08) Chen, Xiaonan, 1980-; Chen, Ray T.With the progress of microfabrication and nanofabrication technologies, there has been a reawakened interest in the possibility of controlling the propagation of light in various materials periodically structured at a scale comparable to, or slightly smaller than the wavelength. We can now engineer materials with periodic structures to implement a great variety of optical phenomena. These include well known effects, such as dispersing a variety of wavelength to form a spectrum and diffracting light and controlling its propagation directions, to new ones such as prohibiting the propagation of light in certain directions at certain wavelengths and localizing light with defects in some artificially synthesized dielectric materials. Advances in this field have had tremendous impact on modern optical and photonic technologies. This doctoral research was aimed at investigating some of the physics and applications of periodic structures for building blocks of the optical communication and interconnection system. Particular research emphasis was placed on the exploitation of innovative periodic structure-based optical and photonic devices featuring better functionality, higher performance, more compact size, and easier fabrication. Research topics extended from one-dimensional periodic-structure-based true-time delay module, to two-dimensional periodic-structure-based silicon photonic-crystal electro-optic modulators. This research was specifically targeted to seek novel and effective solutions to some long-standing technical problems, such as slow switching speed, large device size, and high power consumption of silicon optical modulators, among others. For each subtopic, research challenges were presented and followed by the proposed solutions with extensive theoretical analysis. The proposals were then verified by experimental implementations. Experimental results were carefully interpreted and the future improvements were also discussed.