Browsing by Author "Dr. Leda Lunardi, Committee Member"

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    A 1 Mbps Underwater Communications System using LEDs and Photodiodes with Signal Processing Capability
    (2008-12-07) Simpson, Jim Anto; Dr. Brian Hughes, Committee Member; Dr. Leda Lunardi, Committee Member; Dr. John F. Muth, Committee Chair
    The inability of radio frequency electromagnetic waves to propagate without attenuation in seawater has traditionally limited underwater communications to acoustics or tethered systems. High bandwidth optical communication systems have been demonstrated for terrestrial and space applications. There is growing interest to see if short range high bandwidth optical wireless systems can be made for the underwater environment. In this thesis we demonstrate a 1 Mbps optical wireless system using LEDs and PIN photodiodes that also incorporates capabilities for signal processing of the received data to be performed. Lasers and Photomultiplier tubes offer high performance, and are generally used in most underwater optical communication systems. However, these components are relatively expensive and can have large form factors. As an alternative solution the much cheaper and more compact LEDs and photodiodes are used as transmitters and receiver components. However, compared to a laser and PMT based system, such a system would be strongly disadvantaged in photon limited environments. If one assumes that photons actually reach the receiver, using signal processing techniques, optimized modulation formats, and error-correction coding, one expects that the range of the system can be extended. The development of a prototype system for the experimentation and verification of this proposition is the main motivation of this thesis. Small, compact transmitters using High Power LEDs and receivers using Si Photodiodes where the data can be digitally sampled such that signal processing techniques can be applied were constructed and demonstrated using a 12 foot, 1200 gallon tank that was also constructed for the project. It was shown that the LED and photodiode based system works well for short ranges, and that advantages can be obtained using digital signal processing. The applicability of this strategy to use digital signal processing techniques can be easily extended to higher performance Laser/PMT based systems.
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    Characterization and Modeling of III-N MOS-HFETs for High Frequency Applications
    (2006-11-20) Dandu, Krishnanshu; Dr. Doug Barlage, Committee Chair; Dr. Mark Johnson, Committee Member; Dr. Leda Lunardi, Committee Member; Dr. Kevin Gard, Committee Member
    This research focuses on the characterization and modeling of AlGaN⁄InGaN MOS-HFETs. DC and small signal S parameter measurements were used to characterize these FETs and the associated AlGaN⁄InGaN MOS heterojunction varactors. An equivalent circuit model was developed for the AlGaN⁄InGaN MOS varactor. The model accurately represents measured S-parameters of the device from 45 MHz to 10 GHz over the entire operating range of the device (gate bias varying from -8V to 6V). The extracted gate capacitance indicated the presence of an accumulation layer in the AlGaN barrier layer in forward bias (Vg > 3V). A physics based large signal model has been developed for the varactor. The model utilizes the triangular well approximation to describe charge control at the heterointerface and takes polarization into account. Free carrier generation and neutralization of donor atoms in the AlGaN barrier layer. The model accurately fits the extracted gate capacitance in HFET mode of operation and exhibits the real space transfer of free charge into the barrier layer. The second part of this work focused on the small signal characterization and modeling of the FETs. A direct extraction technique has been developed to extract the small signal components of a FET in presence of bias dependent series resistances. This method was applied to the extraction of small signal equivalent circuits for the AlGaN/InGaN MOS-HFET which exhibited varying source and drain resistances with bias. In the third part, a large signal model has been developed for these devices using the linear charge control equation while taking polarization effects into account. The intrinsic device model uses a quasi two dimensional solution of the Poisson equation in the channel to take velocity saturation effects into account. The capacitances are modeled by developing analytical expressions for the channel charge partitioned between the source and drain. The model has been implemented in Agilent ADS using Verilog-A and is compared to measured DC IV and small signal parameters. The limitations of this model are discussed and methods to enhance it are proposed. Finally, work done on characterization of GaN n-i-n structures utilizing re-grown source drain contacts is presented and discussed. The devices were modeled using expressions developed for devices with a uniform trap distribution in the bandgap.
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    Finite-Difference Time-Domain Analysis of Currents from a Human Electro-Muscular Incapacitation Device
    (2010-04-27) Mayhew, Rebecca; Dr. Leda Lunardi, Committee Member; Dr. H Troy Nagle, Committee Member; Dr. Gianluca Lazzi, Committee Chair
    This research studies the currents generated within the human body when subjected to a Human Electro-Muscular Incapacitation (HEMI) device – commonly known as a Stun Gun. The currents are calculated using the grid based Finite-Difference Time-Domain (FDTD) method of computational electromagnetics. The FDTD technique consists of first defining the Computational Domain - in this case the model of a human torso and a HEMI device. Then the electromagnetic source is defined and the FDTD algorithm and boundary logic is applied to the computational domain to calculate the magnetic and electric fields within the model. In this research, changes were made to the model of the electromagnetic source and to the model of the human torso in order to determine the changes’ effects on the peak currents observed at key observation points within the body. The electromagnetic source was modeled as both a gap current source and a magnetic frill source. The input signal waveform and the length of wires connecting the HEMI device to the subject were changed. In addition, the probe penetration depth, probe separation width, and probe contact locations were all changed. Finally, changes to the human torso model were made, including modeling the skin in wet and dry conditions, as well as adding clothing of various thickness and electromagnetic properties. This research shows that modeling the source as a gap source or magnetic frill are equivalent and the length of the wires connecting the HEMI device to the subject has no impact on the currents within the body. It also shows that probe penetration and probe separation increase the current penetration into the body. Wet skin, or any slightly conductive layer, reduces current penetration to a greater degree than clothing with no conductive properties. However, the greatest impact on the currents at set observation points within the body was the location of the contact probes. This suggests that when a person is shot with a HEMI device, the location of impact of the probes has more of an effect on the currents that a person is subject to than the depth or separation of the probes, or whether the person was wet or wearing clothes.
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    RF pHEMT Switch Model for Multiband Cell Phone Circuits
    (2004-11-05) Jasper, David Brian; Dr. Leda Lunardi, Committee Member; Dr. Michael Steer, Committee Member; Dr. Douglas Barlage, Committee Chair
    Simulation of Radio Frequency Switches used in the cellular phone industry is the main focus of this study. The RF pHEMT's used in an antenna switch for multiband cell phone circuits requires the use of an accurate model during simulation of the RF system. The pHEMT model extracted in this study utilizes theoretical methods within the extraction software and an analysis of simulated data and measured data. This study describes the techniques of calibration, model extraction, and data analysis.
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    Short Range Underwater Optical Communication Links
    (2005-10-25) Chancey, Mark Alan; Dr. Leda Lunardi, Committee Member; Dr. Gianluca Lazzi, Committee Member; Dr. John F. Muth, Committee Chair
    The future tactical ocean environment will be increasingly complicated. In addition to traditional communication links there will be a proliferation of unmanned vehicles in space, in the air, on the surface, and underwater. To effectively utilize these systems improvements in underwater communication systems are needed. Since radio waves do not propagate in sea water, and acoustic communication systems are relatively low bandwidth the possibility of high speed underwater optical communication systems are considered. In traditional communication systems, constructing a link budget is often relatively straight forward. In the case of underwater optical systems the variations in the optical properties of ocean water lead to interesting problems when considering the feasibility and reliability of underwater optical links. The main focus of this thesis is to understand how to construct an underwater link budget which includes the effects of scattering and absorption of realistic ocean water. The secondary focus of the thesis was to construct LED based optical communication systems. This required understanding the behavior of Gallium Nitride LEDs operated under intense electrical pulsing conditions. An optical FM wireless system was constructed for transmitting speech. An LED based Ethernet compatible digital communications system that was capable of operating at 10 Mbps was also constructed and packaged for underwater operation.