Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
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Item Correlation of Signals, Noise, and Harmonics in Parallel Analog-to-Digital Converter Arrays(2009) Lauritzen, Keir Christian; Peckerar, Martin; Electrical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Combining M analog-to-digital converters (ADC) in parallel increases the maximum signal-to-noise ratio (SNR) by a factor of M, assuming the noise is uncorrelated from one channel to the next. This allows for a significant increase in SNR over a single ADC; however, noise and harmonic correlation degrade this improvement. ADCs have three sources of noise: thermal (and other random physical processes), sampling, and quantization noise. There are two system components creating harmonics: the sampler and the quantizer. In this thesis, I determine, analytically and experimentally, the degree of correlation between signals, noise, and harmonics in a parallel ADC array. To test the analysis experimentally, I developed a 16-channel test-bed using 16-bit, state-of-the-art ADCs and 16 direct-digital synthesizers as low-noise signal sources. The test bed provides excellent signal isolation between channels and minimal digital noise to enable the measurement of very low levels of correlation. I investigated the feasibility of measuring the very high levels of signal correlation in the presence of channel nonlinearities with different measurement signals. For a completely linear channel, the channel matching is limited by noise. With nonlinearities, the ability to measure the signal correlation depends on the measurement signal. I verified that the thermal noise is uncorrelated across 16 channels as expected. I also demonstrated that sampling noise is fully correlated from channel-to-channel when a common clock drives the ADCs. Efforts to reduce the correlation using two previously developed de-correlation techniques-phase randomization and frequency offsets-successfully reduced the correlated noise by a factor of two. I then demonstrated analytically and experimentally that harmonics from quantizers are largely uncorrelated; however, harmonics from the sampler are largely correlated confirming the need for decorrelation techniques. I demonstrated the impact of the previously developed decorrelation techniques to reduce harmonic correlation and developed two new decorrelation techniques: phase cancellation and clock offsets, which offer significant advantages over phase randomization and frequency offsets. Each technique offers different levels of dynamic range improvement and complexity, allowing for a range of techniques to target the optimal level of decorrelation.Item Sensory Integration During De-adaptation to Visuomotor Distortions(2006-08-31) King, Bradley Ross; Clark, Jane E.; Kinesiology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Previous research has demonstrated that adults can adapt to novel sensorimotor perturbations, a process thought to be achieved by the gradual update of an adaptive internal representation. However, few research studies have investigated the persistence of a newly acquired representation, as assessed by the reduction of performance errors after the perturbation has been removed (i.e., de-adaptation). The primary objective of this thesis was to determine if the central nervous system (CNS) could flexibly utilize visual and proprioceptive afference to de-adapt to novel sensorimotor perturbations. It has been previously demonstrated that the CNS relies more heavily on visual information for hand localization in the azimuthal direction whereas proprioception is more heavily weighted for hand localization in the radial direction. Seventy-two right-handed adults executed reaching movements during exposure to either an incremental visuomotor rotation or gain distortion. Visual feedback provided during post-exposure was manipulated. Results indicate that the CNS predominantly utilized visual afference to de-adapt to both perturbations, despite the fact that rotation adaptation resulted in movement errors in the azimuthal direction whereas gain adaptation resulted in movement extent errors. These data suggest that the CNS did not flexibly re-weight proprioceptive afference in the absence of visual feedback during a center-out drawing task.