Electrical & Computer Engineering Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2765

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Start date: 1990End date: 1998

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    Microwave Nonlinearities in Photodiodes
    (1994) Williams, Keith Jake; Dagenais, Mario; Electrical & Computer Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
    The nonlinearities in p-i-n photodiodes have been measured and numerically modeled. Harmonic distortion, response reduction, and sinusoidal output distortion measurements were made with two singlefrequency offset-phased-locked Nd: YAG lasers, which provided a source dynamic range greater than 130 dB, a 1 MHz to 50 GHz frequency range, and optical powers up to 10 mW. A semi-classical approach was used to solve the carrier transport in a one-dimensional p-i-n photodiode structure. This required the simultaneous solution of three coupled nonlinear differential equations: Poisson's equation and the hole and electron continuity equations. Space-charge electric fields, loading in the external circuit, and absorption in undepleted regions next to the intrinsic region all contributed to the nonlinear behavior described by these equations. Numerical simulations were performed to investigate and isolate the various nonlinear mechanisms. It was found that for intrinsic region electric fields below 50 kV/cm, the nonlinearities were influenced primarily by the space-charge electric-field-induced change in hole and electron velocities. Between 50 and 100kV/cm, the nonlinearities were found to be influenced primarily by changes in electron velocity for frequencies above 5 GHz and by p-region absorption below 1 GHz. Above 100 kV/cm, only p-region absorption could explain the observed nonlinear behavior, where only 8 to 14 nm of undepleted absorbing material next to the intrinsic region was necessary to model the observed second harmonic distortions of -60 dBc at 1 mA. Simulations were performed at high power densities to explain the observed response reductions and time distortions. A radially inward component of electron velocity was discovered, and under certain conditions, was estimated to have the same magnitude as the axial velocity. The model was extended to predict that maximum photodiode currents of 50 mA should be possible before a sharp increase in nonlinear output occurs. For capacitively-limited devices, the space-charge-induced nonlinearities were found to be independent of the intrinsic region length, while external circuit loading was determined to cause higher nonlinearities in shorter devices. Simulations indicate that second harmonic improvements of 40 to 60 dB may be possible if the photodiode can be fabricated without undepleted absorbing regions next to the intrinsic region.
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    Data Acquisition Interface of a VLSI Cochlea Model
    (1993) Edwards, Thomas G.; Shamma, Shihab; Electrical Engineering; Digital Repository at the University; University of Maryland (College Park, Md)
    Computer models of cochlear processing take exceedingly long times to run, even for short data sets. A data acquisition system was developed for a new switched-capacitor VLSI cochlea model chip, in order to rapidly perform cochleaI processing on digitzed speech samples. The system is capable of processing very long speech samples. Processing is in near-real-time, though it, takes about 2 minutes per second of speech to write the large amount of data to a hard drive. Software has also been developed to convert the output data into a form readable by the ESPS digital signal processing package from Entropic Speech, Inc.
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    Networks for Fast and Efficient Unicast and Multicast Communications
    (1992) Lee, Ching-Yi; Oruç, A. Yavuz; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
    This dissertation presents new results on networks for high-speed unicast and multicast communications which play key roles in communication networks and parallel computer systems. Specifically, (1) we present past parallel algorithms for routing any one-to-one assignment over Beneš network, we propose new multicasting networks that can efficiently realize any one-to-many assignments, and we give an explicit construction of linear-size expanders with very large expansion coefficients. Our parallel routing algorithms for Beneš networks are realized on two different topologies. Using these algorisms, we show that any unicast assignment that involves )(k) pairs of inputs and outputs can be routed through and n-input Beneš network in O(log2 k+lg n) time without pipelining and O(lg k) time with pipelining if the topology is complete, and in O(lg4k+lg2k lg n) time without pipelining and O(lg3 k+lg k lg n) time with pipelining if the topology is extended perfect shuffle. These improve the best-known routing time complexities of parallel algorithms of Lev et al. and Nassimi and Sahni by a factor of O(lg n). Our multicasting networks uses a very simple self-routing scheme which requires no separate computer model for routing. Including the routing cost, it can be constructed with O(n lg2 n) bit-level constant fanin logic gates, O(lg2 n) bit-level depth, and can realize any multicast assignment in O(lg3 n) bit-level time. These complexities match or are better than those of multicasting networks with the same cost that were reported in the literature. In addition to its attractive routing scheme, our multicasting network is input-initiated and can pipeline multicast assignments through itself. With pipelining, the average routing time for O(lg2 n) multicast assignments can be reduced to O(lg n) which is the best among those of the multicasting networks previously reported in the literature. Our linear-size expanders are explicitly constructed by following a traditional design and analysis technique. We construct a family of linear-size with density 33 and expansion coefficient 0.868. This expansion coefficient is the larges among the linear-size expanders that were similarly constructed. Using these expanders, we also report a family of explicitly constructed superconcentrators with density 208.
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    New Methods for the Detection and Interception of Unknown, Frequency-Hopped Waveforms
    (1990) Snelling, William Edward; Geraniotis, Evaggelos; Electrical & Computer Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    Three new methods for the detection and interception of frequency-hopped waveforms are presented. The first method extends the optimal, fixed-block detection method based on the likelihood ratio to a sequential one based on the Sequential Probability Ratio Test (SPRT). The second method is structured around a compressive receiver and is highly efficient yet easily implemented. The third method is based on the new concept of Amplitude Distribution Function (ADF) and results in a detector that is an extension of the radiometer. The first method presents a detector structured to make a decision sequentially, that is, as each data element is collected. Initially, a purely sequential test is derived and shown to require fewer data for a decision. A truncated sequential method is also derived and shown to reduce the data needed for a decision while operating under poor signal-to-noise ratios (SNRs). A detailed performance analysis is presented along with numerical and Monte Carlo analyses of the detectors. The second method assumes stationary, colored Gaussian interference and presents a detailed model of the compressive receiver. A locally optimal detector is developed via the likelihood ratio theory and yields a reference to which previous ad hoc schemes are compared. A simplified, suboptimal scheme is developed that trades off duty cycle for performance, and a technique for estimating hop frequency is developed. The performance of the optimal and suboptimal detectors is quantified. For the suboptimal scheme, the trade-off with duty cycle is studied. The reliability of the hop frequency estimator is bounded and traded off against duty cycle. In the third method, a precise definition of the ADF is given, from which follows a convolutional relationship between the ADFs of signal and additive noise. A technique is given for deconvolving the ADF, with which signal and noise components can be separated. A detection statistic characterized, yielding a framework on which to synthesize a detector. The detector's performance is analyzed and compared with the radiometer.
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    Analysis of Control Strategies for a Human Skeletal System Pedaling a Bicycle
    (1995) Abbott, Scott Bradley; Levine, William S.; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md)
    The study of human locomotion has gained more attention recently with the availability of better analytic and computational tools with which to examine it. A subject under much study within the field today is the effort to model human motor control systems using control systems methods. Analytic, computational, and experimental studies of locomotion can produce models that provide further insight into the design and function of human systems, as well as provide directions for research into therapies for muscle and nerve related disorders affecting these systems. This thesis examines how computational methods can be utilized to study the functionality of these systems. Building on past research, dynamic models for a human skeletal system pedaling a bicycle are used as a basis for examining various methods of implementing inputs that will control the cycling. Two models are used – a three degree-of-freedom model implementing ideal torque inputs at the hip, knees, and feet, and a one degree-of-freedom model involving inputs at the hip and knee only. Both models are characterized by highly nonlinear dynamics, requiring the use of nonlinear analysis, optimization theory, and computational methods for examination. Control of the one degree-of­-freedom model has been addressed in previous work; here, parameterization of the control and the process of learning it is examined. Next, control strategies for the more complex three degree-of-freedom model are developed. Finally, results for upright and recumbent cycling are compared using the three degree-of-freedom model.
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    Modeling Multi-Band Effects of Hot-Electron Transport in Simulation of Small Silicon Devices by a Deterministic Solution of the Boltzmann Transport Equation Using Spherical Harmonic Expansion
    (1998) Singh, Surinder Pal; Mayergoyz, Isaak D.; Goldsman, Neil; Electrical Engineering; University of Maryland (College Park, Md.); Digital Repository at the University of Maryland
    Solution of Boltzmann equation by a spherical-harmonic expansion approach is a computationally-efficient alternative to Monte Carlo. In this dissertation we extend this technique to compute the distribution function in multiple bands of silicon, using a multi-band band-structure which is accurate for high energies. A new variable transformation is applied on the spherical harmonic equations. This transformation (a) improves the numerical properties of the quations by enhancing the diagonal dominance of the resulting equations; (b) accounts for exponential dependence of the distribution function on energy as well as electric potential; and (c) opens the possibility of using superior Poisson solvers (d) while retaining the linearity of the original equations intact. The resulting Boltzmann equations are discretized using the current-conserving control-volume approach. The discretized equation are solved using line successive-over-relaxation (SOR) method. Numerical noise in the distribution was analyzed to be originating from the absence of coupling. Noise is removed by using acoustic phonons in inelastic approximation. A novel self-adjoint easy-to-discretize formulation for the inelastic acoustic phonons is developed. A test case of thermal equilibrium for multi-band is derived and used to validate the code. Hole-continuity and Poisson equation were solved along with the multi-band Boltzmann equations. The equations are solved in a Gummel-type decoupled loop. A \nnn\ device is simulated to test the simulator. The simulator is then applied to study a one-dimensional short-base bipolar junction transistor. While these simulations are self-consistent, a two-dimensional sub-micron MOSFET is simulated in a non-self-consistent manner.