Browsing by Author "Manteghi, Majid"
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- 6.78MHz Omnidirectional Wireless Power Transfer System for Portable Devices ApplicationFeng, Junjie (Virginia Tech, 2021-01-11)Wireless power transfer (WPT) with loosely coupled coils is a promising solution to deliver power to a battery in a variety of applications. Due to its convenience, wireless power transfer technology has become popular in consumer electronics. Thus far, the majority of the coupled coils in these systems are planar structure, and the magnetic field induced by the transmitter coil is in one direction, meaning that the energy power transfer capability degrades greatly when there is some angle misalignment between the coupled coils. To improve the charging flexibility, a three–dimensional (3D) coils structure is proposed to transfer energy in different directions. With appropriate modulation current flowing through each transmitter coil, the magnetic field rotates in different directions and covers all the directions in 3D space. With omnidirectional magnetic field, the charging platform can provide energy transfer in any direction; therefore, the angle alignment between the transmitter coil and receiver coil is no longer needed. Compensation networks are normally used to improve the power transfer capability of a WPT system with loosely coupled coils. The resonant circuits, formed by the loosely coupled coils and external compensation inductors or capacitors, are crucial in the converter design. In WPT system, the coupling coefficient between the transmitting coil and the receiving coil is subject to the receiver's positioning. The variable coupling condition is a big challenge to the resonant topology selection. The detailed requirements of the resonant converter in an omnidirectional WPT system are identified as follows: 1). coupling independent resonant frequency; 2). load independent output voltage; 3). load independent transmitter coil current; 4). maximum efficiency power transfer; 5). soft switching of active devices. A LCCL-LC resonant converter is derived to satisfy all of the five requirements. In consumer electronics applications, Megahertz (MHz) WPT systems are used to improve the charging spatial freedom. 6.78 MHz is selected as the system operation in AirFuel standard, a wireless charging standard for commercial electronics. The zero voltage switching (ZVS) operation of the switching devices is essential in reducing the switching loss and the switching related electromagnetic interference (EMI) issue in a MHz system; therefore, a comprehensive evaluation of ZVS condition in an omnidirectional WPT system is performed. And a design methodology of the LCCL-LC converter to achieve ZVS operation is proposed. The big hurdle of the WPT technology is the safety issue related to human exposure of electromagnetic fields (EMF). A double layer shield structure, including a magnetic layer and a conductive layer, is proposed in a three dimensional charging setup to reduce the stray magnetic field level. A parametric analysis of the double shield structure is conducted to improve the attenuation capability of the shielding structure. In an omnidirectional WPT system, the energy can be transferred in any direction; however the receiving devices has its preferred field direction based on its positioning and orientation. To focus power transfer towards targeted loads, a smart detection algorithm for identifying the positioning and orientation of receiver devices based on the input power information is presented. The system efficiency is further improved by a maximum efficiency point tracking function. A novel power flow control with a load combination strategy to charge multiple loads simultaneously is explained. The charging speed of the omnidirectional WPT system is greatly improved with proposed power flow control.
- Active Source Management to Maintain High Efficiency in Resonant Conversion over Wide Load RangeDanilovic, Milisav (Virginia Tech, 2015-09-18)High-frequency and large amplitude current is a driving requirement for applications such as induction heating, wireless power transfer, power amplifier for magnetic resonant imaging, electronic ballasts, and ozone generators. Voltage-fed resonant inverters are normally employed, however, current-fed (CF) resonant inverters are a competitive alternative when the quality factor of the load is significantly high. The input current of a CF resonant inverter is considerably smaller than the output current, which benefits efficiency. A simple, parallel resonant tank is sufficient to create a high-power sinusoidal signal at the output. Additionally, input current is limited at the no-load condition, providing safe operation of the system. Drawbacks of the CF resonant inverter are associated with the implementation of the equivalent current source. A large input inductor is required to create an equivalent dc current source, to reduce power density and the bandwidth of the system. For safety, a switching stage is implemented using bidirectional voltage-blocking switches, which consist of a series connection of a diode and a transistor. The series diode experiences significant conduction loss because of large on-state voltage. The control of the output current amplitude for constant-frequency inverters requires a pre-regulation stage, typically implemented as a cascaded hard-switched dc/dc buck converter. The pre-regulation also reduces the efficiency. In this dissertation, a variety of CF resonant inverters with two input inductors and two grounded switches are investigated for an inductive-load driver with loaded quality factor larger than ten, constant and high-frequency (~500 kHz) operation, high reactive output power (~14 kVA), high bandwidth (~100 kHz), and high efficiency (over 95 %). The implementation of such system required to question the fundamental operation of the CF resonant inverter. The input inductance is reduced by around an order of magnitude, ensuring sufficient bandwidth, and allowing rich harmonic content in the input current. Of particular importance are fundamental and second harmonic components since they influence synchronization of the zero-crossing of the output voltage and the turn-on of the switches. The synchronization occurs at a particular frequency, termed synchronous frequency, and it allows for zero switching loss in the switches, which greatly boosts efficiency. The synchronous conditions were not know prior this work, and the dependence among circuit parameters, input current harmonics, and synchronous frequency are derived for the first time. The series diode of the bidirectional switch can reduce the efficiency of the system to below 90 %, and has to be removed from the system. The detrimental current-spikes can occur if the inverter is not operated in synchronous condition, such as in transients, or during parametric variations of the load coil. The resistance of the load coil has a wide variance, five times or more, while the inductance changes as well by a few percent. To accommodate for non-synchronous conditions, a low-loss current snubber is proposed as a safety measure to replace lossy diodes. The center-piece of the dissertation is the proposal of a two-phase zero-voltage switching buck pre-regulator, as it enables fixed frequency and synchronous operation of the inverter under wide parametric variations of the load. The synchronous operation is controlled by phase-shifting the switching functions of the pre-regulator and inverter. The pre-regulator reduces the dc current in the input inductors, which is a main contributor to current stress and conduction losses in the inverter switches. Total loss of the inverter switches is minimized since no switching loss is present and minimal conduction losses are allowed. The dc current in the input inductors, once seen as a means to transfer power to load, is now contradictory perceived as parasitic, and the power is transferred to the load using a fundamental frequency harmonic! The input current to the resonant tank, previously designed to be a square-wave, now resembles a sine-wave with very rich harmonic content. Additionally, the efficiency of the pre-regulator at heavy-load condition is improved by ensuring ZVS for with an additional inductive tank. The dissertation includes five chapters. The first chapter is an introduction to current-fed resonant inverters, applications, and state-of-the-art means to ensure constant frequency operation under load's parametric variations. The second chapter is dedicated to the optimization of the CF resonant inverter topology with a dc input voltage, two input inductors, and two MOSFETs. The topology is termed as a boost amplifier. If the amplifier operates away from the synchronous frequency, detrimental current spikes will flow though the switches since the series diodes are eliminated. Current spikes reduce the efficiency up to few percent and can create false functioning of the system. Operation at the synchronous frequency is achieved with large, bulky, input inductors, typically around 1-2 mH or higher, when the synchronous frequency follows the resonant frequency of the tank at 500 kHz. The input inductance cannot be reduced arbitrarily to meet the system bandwidth requirement, since the synchronous frequency is increased based on the inductance value. The relationship between the two (input inductance and the synchronous frequency) was unknown prior this work. The synchronous frequency is determined to be a complicated mathematical function of harmonic currents through the input inductors, and it is found using the harmonic decomposition method. As a safety feature, a current snubber is implemented in series with the resonant tank. Snubber utilizes a series inductance of cable connection between the tank and the switching stage, and it is more efficient than the previously employed series diodes. Topology optimization and detailed design procedure are provided with respect to efficiency and system dynamics. The mathematics is verified by a prototype rated at 14 kVA and 1.25 kW. The input inductance is reduced by around an order of magnitude, with the synchronous frequency increase of 2 %. The efficiency of the power amplifier reached 98.5 % and might be improved further with additional optimization. Silicon carbide MOSFETs are employed for their capability to operate efficiently at high frequency, and high temperature. The third chapter is dedicated to the development of the boost amplifier's large signal model using the Generalized State-space Averaging (GSSA) method. The model accurately predicts amplifier's transient and steady-state operation for any type of input voltage source (dc, dc with sinusoidal ripple, pulse-width modulated), and for either synchronous or non-synchronous operating frequency. It overcomes the limitation of the low-frequency model, which works well only for dc voltage-source input and at synchronous frequency. As the measure of accuracy, the zero-crossing of the resonant voltage is predicted with an error less than 2° over a period of synchronous operation, and for a range of interest for input inductance (25 μH – 1000 μH) and loaded-quality factor (10 – 50). The model is validated both in simulation and hardware for start-up transient and steady-state operation. It is then used in the synthesis of modulated output waveforms, including Hann-function and trapezoidal-function envelopes of the output voltage/current. In the fourth chapter, the GSSA model is employed in development of the PWM compensation method that ensures synchronous operation at constant frequency for the wide variation of the load. The boost amplifier is extended with a cascaded pre-regulator whose main purpose is to control the output resonant voltage. The pre-regulator is implemented as two switching half-bridges with same duty-cycle and phase-shift of 180°. The behavior of the cascaded structure is the same as of the buck converter, so the half-bridges are named buck pre-regulators. ZVS operation is ensured by putting an inductive tank between the half-bridges. Each output of half-bridges is connected to each of input inductors of the boost to provide the PWM excitation. Using the GSSA model, the synchronous condition and control laws are derived for the amplifier. Properties of the current harmonics in the input inductors are well examined. It is discovered that the dc harmonic, once used to transfer power, is unwanted (parasitic) since it increases conduction loss in switches of the boost. A better idea is to use the fundamental harmonic for power transfer, since it does not create loss in the switches. Complete elimination of the dc current is not feasible for constant frequency operation of the amplifier since the dc current depends on the load coil's resistance. However, significant mitigation of around 55 % is easily achievable. The proposed method improves significantly the efficiency of both the buck pre-regulator and the boost. Synchronous operation is demonstrated in hardware for fixed switching frequency of 480 kHz, power level up to 750 W, input voltage change from 300 V to 600 V, load coil's resistance change of three times, and load coil's inductance change of 3.5 %. Measured efficiency is around 95 %, with a great room for improvements. Chapter five summarizes key contributions and concludes the dissertation.
- Analysis of an Embedded Singularity Chipless RFID TagBlischak, Andrew Travis (Virginia Tech, 2011-06-30)The objective of this research is to explore the feasibility of a chipless RFID tag that stores a data signature in the form of complex frequency plane singularities. To this end an existing chipless RFID tag, the notched elliptical dipole tag, was analyzed first with simulations and then measurements. A pole signature was extracted from simulations, and individual poles were determined via experimentation to be attributable to specific controllable features of the tag. The poles were shown to be independent of both excitation and observation. A prototype tag was measured, and the pole signature was retrieved from the scattered fields. The tag was successfully read for different orientations showing that embedded singularities can be used as a means for encoding and retrieving data.
- Applications of Non-linearities in RF MEMS Switches and ResonatorsVummidi Murali, Krishna Prasad (Virginia Tech, 2015-04-06)The 21st century is emerging into an era of wireless ubiquity. To support this trend, the RF (Radio Frequency) front end must be capable of processing a range of wireless signals (cellular phone, data connectivity, broadcast TV, GPS positioning, etc.) spanning a total bandwidth of nearly 6 GHz. This warrants the need for multi-band/multi-mode radio architectures. For such architectures to satisfy the constraints on size, battery life, functionality and cost, the radio front-end must be made reconfigurable. RF-MEMS (RF Micro-Electro-Mechanical Systems) are seen as an enabling technology for such reconfigurable radios. RF-MEMS mainly include micromechanical switches (used in phase shifters, switched capacitor banks, impedance tuners etc.) and micromechanical resonators (used in tunable filters, oscillators, reference clocks etc.). MEMS technology also has the potential to be directly integrated into CMOS (Complementary metal-oxide semiconductor) ICs (Integrated Circuits) leading to further potential reductions of cost and size. However, RF-MEMS face challenges that must be addressed before they can gain widespread commercial acceptance. Relatively low switching speed, power handling, and high-voltage drive are some of the key issues in MEMS switches. Phase noise influenced by non-linearities, need for temperature compensation (especially Si based resonators), large start-up times, and aging are the key issues in Si MEMS Resonators. In this work potential solutions are proposed to address some of these key issues, specifically the reduction of high voltage drives in switches and the reduction of phase noise in MEMS resonators for timing applications. MEMS devices that are electrostatically actuated exhibit significant non-linearities. The origins of the non-linearities are both electrical (electrostatic actuation) and mechanical (dimensions and material properties). The influence of spring non-linearities (cubic and quadratic) on the performance of switches and resonators are studied. Gold electroplated fixed-fixed beams were fabricated to test the phenomenon of dynamic (or resonant) pull-in in shunt switches. The dynamic pull-in phenomenon was also tested on commercially fabricated lateral switches. It is shown that the resonant pull-in technique reduces the overall voltage required to actuate the switch. There is an additional reduction of total actuation voltage possible via applying an AC actuation signal at the correct non-linear resonant frequency. The demonstrated best case savings from operating at the non-linear resonanceis 50 % (for the lateral switch) and 60 % (for the vertical switch) as compared to 25 % and 40 % respectively using a fixed frequency approach. However, the timing response for resonant pull-in has been experimentally shown to be slower than the static actuation. To reduce the switching time, a shifted-frequency method is proposed where the excitation frequency is shifted up or down by a discrete amount 'Ω after a brief hold time. It was theoretically shown that the shifted-frequency method enables a minimum realizable switching time comparable to the static switching time for a given set of actuation frequencies. The influence of VDC on the effective non-linearities of a fixed-fixed beam is also studied. Based on the dimensions of the resonator and the type of resonance there is a certain VDC,Lin where the response is near linear (S ' 0). In the near-linear domain, the dynamic pull-in is the only upper bound to the amplitude of vibrations, and hence the amplitude of output current, thereby maximizing the power handling capacity of the resonator. Apart from maximizing the output current, it is essential to reduce the amplitude and phase variations of the displacement response which are due to noise mixing into frequency of interest, and are eventually manifested as output phase noise due to capacitive current nonlinearity. Two major aliasing schemes were analyzed and it was shown that the capacitive force non-linearity is the major source of mixing that causes the up-conversion of 1/f frequency into signal sidebands. The resonator's periodic response (displacement) is defined by a set of two first- order nonlinear ordinary differential equations that describe the modulation of amplitude and phase of the response. Frequency response curves of amplitude and frequency are determined from these modulation equations. The zero slope point on the amplitude resonance curve is the peak of the resonance curve where the phase ('dc) of the response is ±π/2. For a strongly non-linear system, the resonance curves are skewed based on the amount of total non-linearity S. For systems that are strongly non-linear, the best region to operate the resonator is the fixed point that correspond to infinite slope ('dc = ±2π/3) in the frequency response of the system. The best case phase noise response was analytically developed for such a fixed point. Theoretically at this fixed point, phase noise will have contributions only from 1/f noise and not from 1/f2 and 1/f3. The resonators phase can be set by controlling the rest of the phase in the loop such that the total phase around the loop is zero or 2π. In addition, this work has also developed an analytical model for a lateral MEMS switch fabricated in a commercial foundry that has the potential to be processed as MEMS on CMOS. This model accounts for trapezoidal cross sections of the electrodes and springs and also models electrostatic fringing as a function of the moving gap. The analytical model matches closely with the Finite Element (FEA) model.
- Broadband Parametric Impedance Matching for Small Antennas Using the Bode-Fano Limit: Improving on Chu's Limit for Loaded Small AntennasLoghmannia, Pedra; Manteghi, Majid (IEEE, 2021-07-09)In this work, a parametric up-converter amplifier is introduced as a wideband impedance-matching network. Chu’s limit restricts the minimum Q-factor of unloaded small antennas. However, the practical bandwidth (BW) of small antennas is defined by their loaded Q-factor. By connecting a small antenna to an amplifier with a real input impedance that is several times greater than the radiation resistance of the antenna, we propose increasing the return loss, which leads to a reduction in the loaded Q-factor and an increase in the BW. In addition, a parametric amplifier is used because, in comparison with transistor amplifiers, it offers low-noise characteristics. The gain of the low-noise parametric amplifier compensates for the loss due to the imposed mismatch. Our simulation result shows BW improvements up to 32 times can be accomplished by trading 2 dB of noise figure (NF), compared to the 15 dB suggested by Chu’s limit for a lossy antenna.
- Class-E Current Source Power ConversionLi, Bo (Virginia Tech, 2024-09-16)Current source is used in auxiliary power supplies, battery chargers, and LED drivers. The battery chargers are required to provide constant current within a wide output voltage range, similar to LED drivers. The load-independent (LI) Class-E inverter is a promising topology for such applications since it can realize zero-voltage switching (ZVS) within a wide load range. Class-E current source can be achieved by converting constant voltage (CV) Class-E inverter to current source with a trans-susceptance network or using parallel resonant topology. The design and analysis of LI Class-E inverters usually assume a high-Q resonant load tank so that the load current/voltage is sinusoidal. While this is the case in RF applications, it's not required in DC-DC power conversion. Besides, high-Q design leads to high inductance and increased voltage/current stress on the resonant components, increasing converter volume, loss, and cost. This work aims to provide a design guideline for the CC Class-E inverter when significant harmonics are present by reflecting the trade-off between load range and voltage stress, with the help of a modified frequency domain analysis method to eliminate the iteration existing in the time domain analysis. Output current variation and voltage stress can be automatically quantified when circuit parameters vary. Generalized load range contours are obtained to guide the circuit design. With the help of the analysis, a 10-W dual-output Class-E gate power supply is designed with optimized magnetics and reduced isolation capacitance. Compared with CC Class-E based on trans-susceptance network, the parallel resonant CC Class-E inverter has smaller part counts due to its low-order resonant network. However, the current topology suffers from limited maximum output power. In this work, a coupled-inductor based parallel resonant CC Class-E inverter is proposed with more than 2 times maximum power without increasing part counts.
- A Curvature-Corrected Rough Surface Scattering Theory Through The Single-Scatter Subtraction MethodDiomedi II, Kevin Paul (Virginia Tech, 2019-03-21)A new technique is presented to study radio propagation and rough surface scattering problems based on a reformulation of the Magnetic Field Integration Equation (MFIE) called the Single-Scatter Subtraction (S^3) method. This technique amounts to a physical preconditioning by separating the single- and multiple-scatter currents and removing the single-scattering contribution from the integral term that is present in the MFIE. This requires the calculation of a new quantity that is the kernel of the MFIE integral call the kernel integral or Gbar. In this work, 1-dimensional deterministically rough surfaces are simulated by surfaces consisting of single and multiple cosines. In order to truncate the problem domain, a beam illumination is used as the source term and it is shown that this also causes the kernel integral to have a finite support. Using the Single Scatter Subtraction method on these surfaces, closed-form expressions are found for the kernel integral and thus the single-scatter current for a well defined region of validity of surface parameters which may then be efficiently radiated into the far field numerically. Both the closed-form expressions, and the computed radiated fields are studied for their physical significance. This provides a clear physical intuition for the technique as an augmentation to existing ones as a bent-plane approximation as shown analytically and also validated by numeric results. Further analysis resolves a controversy on the nature of Bragg scatter which is found to be a multiple-scatter phenomenon. Error terms present in the kernel integral also raise new questions on the effect of truncation for any MFIE-based solution. Additionally, a dramatic enhancement of backscatter predicted by this new approach versus the Kirchhoff method is observed as the angle of incidence increases due to the error terms.
- Design and Analysis of a Low-Power Low-Voltage Quadrature LO Generation Circuit for Wireless ApplicationsWang, Shen (Virginia Tech, 2012-08-31)The competitive market of wireless communication devices demands low power and low cost RF solutions. A quadrature local oscillator (LO) is an essential building block for most transceivers. As the CMOS technology scales deeper into the nanometer regime, design of a low-power low-voltage quadrature LO still poses a challenge for RF designers. This dissertation investigates a new quadrature LO topology featuring a transformer-based voltage controlled oscillator (VCO) stacked with a divide-by-two for low-power low-voltage wireless applications. The transformer-based VCO core adopts the Armstrong VCO configuration to mitigate the small voltage headroom and the noise coupling. The LO operating conditions, including the start-up condition, the oscillation frequency, the voltage swing and the current consumption are derived based upon a linearized small-signal model. Both linear time-invariant (LTI) and linear time-variant (LTV) models are utilized to analyze the phase noise of the proposed LO. The results indicate that the quality factor of the primary coil and the mutual inductance between the primary and the secondary coils play an important role in the trade-off between power and noise. The guidelines for determining the parameters of a transformer are developed. The proposed LO was fabricated in 65 nm CMOS technology and its die size is about 0.28 mm2. The measurement results show that the LO can work at 1 V supply voltage, and its operation is robust to process and temperature variations. In high linearity mode, the LO consumes about 2.6 mW of power typically, and the measured phase noise is -140.3 dBc/Hz at 10 MHz offset frequency. The LO frequency is tunable from 1.35 GHz to 1.75 GHz through a combination of a varactor and an 8-bit switched capacitor bank. The proposed LO compares favorably to the existing reported LOs in terms of the figure of merit (FoM). More importantly, high start-up gain, low power consumption and low voltage operation are achieved simultaneously in the proposed topology. However, it also leads to higher design complexity. The contributions of this work can be summarized as 1) proposal of a new quadrature LO topology that is suitable for low-power low-voltage wireless applications, 2) an in-depth circuit analysis as well as design method development, 3) implementation of a fully integrated LO in 65 nm CMOS technology for GPS applications, 4) demonstration of high performance for the design through measurement results. The possible future improvements include the transformer optimization and the method of circuit analysis.
- Design and Detection Process in Chipless RFID Systems Based on a Space-Time-Frequency TechniqueRezaiesarlak, Reza (Virginia Tech, 2015-06-04)Recently, Radio Frequency Identification (RFID) technology has become commonplace in many applications. It is based on storing and remotely retrieving the data embedded on the tags. The tag structure can be chipped or chipless. In chipped tags, an integrated IC attached to the antenna is biased by an onboard battery or interrogating signal. Compared to barcodes, the chipped tags are expensive because of the existence of the chip. That was why chipless RFID tags are demanded as a cheap candidate for chipped RFID tags and barcodes. As its name expresses, the geometry of the tag acts as both modulator and scatterer. As a modulator, it incorporates data into the received electric field launched from the reader antenna and reflects it back to the receiving antenna. The scattered signal from the tag is captured by the antenna and transferred to the reader for the detection process. By employing the singularity expansion method (SEM) and the characteristic mode theory (CMT), a systematic design process is introduced by which the resonant and radiation characteristics of the tag are monitored in the pole diagram versus structural parameters. The antenna is another component of the system. Taking advantage of ultra-wideband (UWB) technology, it is possible to study the time and frequency domain characteristics of the antenna used in chipless RFID system. A new omni-directional antenna element useful in wideband and UWB systems is presented. Then, a new time-frequency technique, called short-time matrix pencil method (STMPM), is introduced as an efficient approach for analyzing various scattering mechanisms in chipless RFID tags. By studying the performance of STMPM in early-time and late-time responses of the scatterers, the detection process is improved in cases of multiple tags located close to each other. A space-time-frequency algorithm is introduced based on STMPM to detect, identify, and localize multiple multi-bit chipless RFID tags in the reader area. The proposed technique has applications in electromagnetic and acoustic-based detection of targets.
- Design of a Wireless Power Transfer System using Electrically Coupled Loop AntennasChandrasekhar Nambiar, Shyam (Virginia Tech, 2015-07-01)Wireless Power Transfer (WPT) has become quite popular over the recent years. This thesis presents some design challenges while developing a WPT system and describes a system-level methodology for designing an end-to-end system. A critical analysis of contemporary research is performed in the form of a literature survey of both academic and commercial research to understand their benefits and demerits. Some theoretical notes are presented on coupled-mode theory and coupled filter theory and the problems concerning WPT analyzed using these models. The need for higher power transfer efficiency (PTE) and power delivered to load (PDL) is studied using these models. The case for using magnetic antennas over electric antennas when surrounded by lossy media (specifically for the case of human body tissues at various frequencies) is made using some theoretical models and simulation results. An Electrically Coupled Loop Antenna (ECLA) is introduced, studied and designed for two main WPT applications, viz. free space transmission and that of powering implanted devices. An equivalent circuit is proposed to better understand the coupling effects of the antennas on a circuit level and to study the effect of various environmental and structural factors on the coupling coefficient. Some prototypes were created and measured for the two use cases of free space and implanted applications. In order to complete the system design, a negative resistance-based oscillator is designed and fabricated, that incorporates the antennas as a load and oscillates at the required frequency. Some changes in load conditions and power handling are studied by the use of two circuits for free-space (high-power) and implanted (low-power) applications. Finally, the salient points of the thesis are re-iterated and some future work outlined in the concluding chapter.
- Dynamic Spectrum Access Network Simulation and Classification of Secondary User PropertiesRebholz, Matthew John (Virginia Tech, 2013-06-17)This thesis explores the use of the Naïve Bayesian classifier as a method of determining high-level information about secondary users in a Dynamic Spectrum Access (DSA) network using a low complexity channel sensing method. With a growing number of users generating an increased demand for broadband access, determining an efficient method for utilizing the limited available broadband is a developing current and future issue. One possible solution is DSA, which we simulate using the Universal DSA Network Simulator (UDNS), created by our team at Virginia Tech. However, DSA requires user devices to monitor large amounts of bandwidth, and the user devices are often limited in their acceptable size, weight, and power. This greatly limits the usable bandwidth when using complex channel sensing methods. Therefore, this thesis focuses on energy detection for channel sensing. Constraining computing requirements by operating with limited spectrum sensing equipment allows for efficient use of limited broadband by user devices. The research on using the Naïve Bayesian classifier coupled with energy detection and the UDNS serves as a strong starting point for supplementary work in the area of radio classification.
- Electrically Small Antennas' Design Criteria and Measurement ChallengesManteghi, Majid (2023-05-24)The contrast between the design criteria for electrically small transmit and receive antennas is studied in this work. On the transmit side, radiation efficiency (ohmic loss plus return loss) and data throughput are critical. However, a higher impedance mismatch on the receiving front end may reduce ohmic loss and expand the frequency bandwidth. So, a calculated mismatch can be added to improve the performance of the receiving ESA by lowering the overall noise figure and widening the frequency bandwidth. These contradictory design criteria suggest utilizing separate transmit and receive antennas to improve the transmitter and receiver performances.
- Electrically-Small Antenna Performance Enhancement for Near-Field Detuning EnvironmentsHearn, Christian Windsor (Virginia Tech, 2012-12-13)Bandwidth enhancement of low-profile omnidirectional, electrically-small antennas has evolved from the design and construction of AM transmitter towers eighty years ago to current market demand for battery-powered personal communication devices. Electrically-small antenna theory developed with well-known approximations for characterizing radiation properties of antenna structures that are fractions of the radiansphere. Current state-of-the-art wideband small antennas near kaH1 have achieved multiple-octave impedance bandwidths when utilizing volume-efficient designs. Significant advances in both the power and miniaturization of microelectronics have created a second possible approach to enhance bandwidth. Frequency agility, via switch tuning of reconfigurable structures, offers the possibility of the direct integration of high-speed electronics to the antenna structure. The potential result would provide a means to translate a narrow instantaneous bandwidth across a wider operating bandwidth. One objective of the research was to create a direct comparison of the passive- multi-resonant and active-reconfigurable approaches to enhance bandwidth. Typically, volume-efficient, wideband antennas are unattractive candidates for low-profile applications and conversely, active electronics integrated directly antenna elements continue to introduce problematic loss mechanisms at the proof-of-concept level The dissertation presents an analysis method for wide bandwidth self-resonant antennas that exist in the 0.5dkad1.0 range. The combined approach utilizes the quality factor extracted directly from impedance response data in addition to near-and-far field modal analyses. Examples from several classes of antennas investigated are presented with practical boundary conditions. The resultant radiation properties of these antenna-finite ground plane systems are characterized by an appreciable percentage of radiated power outside the lowest-order mode. Volume-efficient structures and non-omnidirectional radiation characteristics are generally not viable for portable devices. Several examples of passive structures, representing different antenna classes are investigated. A PIN diode, switch-tuned low-profile antenna prototype was also developed for the comparison which demonstrated excessive loss in the physical prototype. Lastly, a passive, low-profile multi-resonant antenna element with monopole radiation is introduced. The structure is an extension of the planar inverted-F antenna with the addition of a capacitance-coupled parasitic to enhance reliable operation in unknown environments.
- Electromagnetic Interference Attacks on Cyber-Physical Systems: Theory, Demonstration, and DefenseDayanikli, Gokcen Yilmaz (Virginia Tech, 2021-08-27)A cyber-physical system (CPS) is a complex integration of hardware and software components to perform well-defined tasks. Up to this point, many software-based attacks targeting the network and computation layers have been reported by the researchers. However, the physical layer attacks that utilize natural phenomena (e.g., electromagnetic waves) to manipulate safety-critic signals such as analog sensor outputs, digital data, and actuation signals have recently taken the attention. The purpose of this dissertation is to detect the weaknesses of cyber-physical systems against low-power Intentional Electromagnetic Interference (IEMI) attacks and provide hardware-level countermeasures. Actuators are irreplaceable components of electronic systems that control the physically moving sections, e.g., servo motors that control robot arms. In Chapter 2, the potential effects of IEMI attacks on actuation control are presented. Pulse Width Modulation (PWM) signal, which is the industry–standard for actuation control, is observed to be vulnerable to IEMI with specific frequency and modulated–waveforms. Additionally, an advanced attacker with limited information about the victim can prevent the actuation, e.g., stop the rotation of a DC or servo motor. For some specific actuator models, the attacker can even take the control of the actuators and consequently the motion of the CPS, e.g., the flight trajectory of a UAV. The attacks are demonstrated on a fixed-wing unmanned aerial vehicle (UAV) during varying flight scenarios, and it is observed that the attacker can block or take control of the flight surfaces (e.g., aileron) which results in a crash of the UAV or a controllable change in its trajectory, respectively. Serial communication protocols such as UART or SPI are widely employed in electronic systems to establish communication between peripherals (e.g., sensors) and controllers. It is observed that an adversary with the reported three-phase attack mechanism can replace the original victim data with the 'desired' false data. In the detection phase, the attacker listens to the EM leakage of the victim system. In the signal processing phase, the exact timing of the victim data is determined from the victim EM leakage, and in the transmission phase, the radiated attack waveform replaces the original data with the 'desired' false data. The attack waveform is a narrowband signal at the victim baud rate, and in a proof–of–concept demonstration, the attacks are observed to be over 98% effective at inducing a desired bit sequence into pseudorandom UART frames. Countermeasures such as twisted cables are discussed and experimentally validated in high-IEMI scenarios. In Chapter 4, a state-of-art electrical vehicle (EV) charger is assessed in IEMI attack scenarios, and it is observed that an attacker can use low–cost RF components to inject false current or voltage sensor readings into the system. The manipulated sensor data results in a drastic increase in the current supplied to the EV which can easily result in physical damage due to thermal runaway of the batteries. The current switches, which control the output current of the EV charger, can be controlled (i.e., turned on) by relatively high–power IEMI, which gives the attacker direct control of the current supplied to the EV. The attacks on UAVs, communication systems, and EV chargers show that additional hardware countermeasures should be added to the state-of-art system design to alleviate the effect of IEMI attacks. The fiber-optic transmission and low-frequency magnetic field shielding can be used to transmit 'significant signals' or PCB-level countermeasures can be utilized which are reported in Chapter 5.
- Experiments with compact antenna arrays for MIMO radio communicationsBrowne, David W.; Manteghi, Majid; Fitz, Michael P.; Rahmat-Samii, Yahya (IEEE, 2006-11-01)The problem addressed in this study is how to design and test compact antenna arrays for portable Multiple-Input Multiple-Output (MIMO) transceivers.
- High-Frequency Design Consideration and EMI Mitigation in SiC-based Multilevel ConvertersYu, Jianghui (Virginia Tech, 2022-05-23)Medium Voltage (MV) power conversion systems are essential in high power applications to address the increasing demand of energy and the increasing penetration of renewable energy sources. MV power electronics converters are the key elements for power conversion in MV systems and are the focus of this study. Multilevel converter topologies are promising topologies in MV applications because of their reduced voltage stress on devices, excellent output quality, reduced semiconductor losses, lower common mode voltage among other advantages. However, they may suffer from the large number of switching devices and capacitors, as well as the need to regulate capacitor voltages. SiC MOSFETs can achieve higher switching speeds, higher switching frequencies, higher voltage ratings, higher operation temperatures compared to traditional Si devices. They have shown promise to increase the efficiency and power density of the converters, but may suffer from higher voltage overshoots, increased Electromagnetic Interference (EMI) emission and so on. In SiC-based multilevel converters, the features of multilevel topologies, and the features of SiC MOSFETs are coupled together. The benefits, challenges, and solutions of using SiC MOSFETs in multilevel converters are studied explicitly in this work. With the high switching speeds and high switching frequencies of SiC MOSFETs, and the large number of switches and capacitors in multilevel topologies, SiC-based multilevel converters need to be studied while considering high-frequency voltage and current behaviors and the interactions among them at different locations. Firstly, the use of SiC-based multilevel converter in the high-speed motor drive application is explored. A three-phase inverter is designed and built employing five-level Stacked Multicell Converter topology and SiC MOSFETs. The benefits and challenges of using multilevel converter topology and using SiC MOSFETs for this application are explored. A fitting topology is selected, and a prototype is designed, both with attentions paid to deal with the high switching speeds of SiC MOSFETs. The inverter is verified through experiments to meet all specifications with a high efficiency. Then a unique type of converter, converters with Integrated Capacitor Blocked Transistor (ICBT) cells are studied. Unlike the traditional methods, there are no fast-developing voltage unbalances, or high cell capacitor voltage ripples in ICBT-based converters. The ideal operation principle is analyzed and verified by the simulation results. Then the impacts of non-idealities on the operation are analyzed, and a control method is proposed for this type of converter. The operation and control of ICBT-based converters are verified by experimental results to achieve low cell capacitor voltage ripples and excellent voltage balance in Medium Voltage high power applications. Lastly, the conducted EMI emission in SiC-based multilevel converters are studied. Four SiC-based multilevel converters are studied, with the focus on the power circuit in one converter and the auxiliary circuits in the other three converters. The complexity of noise generation and propagation in multilevel converters is presented. The conducted EMI disturbances are experimentally evaluated, analyzed, and effectively mitigated in all four cases.
- A Hybrid DSP and FPGA System for Software Defined Radio ApplicationsPodosinov, Volodymyr Sergiyovich (Virginia Tech, 2011-04-07)Modern devices provide a multitude of services that use radio frequencies in continual smaller packages. This size leads to an antenna used to transmit and receive information being usually very inefficient and a lot of power is wasted just to be able to transmit a signal. To mitigate this problem a new antenna was introduced by Dr. Manteghi that is capable of working efficiently across a large band. The antenna achieves this large band by doing quick frequency hopping across multiple channels. In order to test the performance of this antenna against more common antennas, a software radio was needed, such that tested antennas can be analyzed using multiple modulations. This paper presents a software defined radio system that was designed for the purpose of testing the bit-error rate of digital modulations schemes using described and other antennas. The designed system consists of a DSP, an FPGA, and commercially available modules. The combination allows the system to be flexible with high performance, while being affordable. Commercial modules are available for multiple frequency bands and capable of fast frequency switching required to test the antenna. The DSP board contains additional peripherals that allows for more complex projects in the future. The block structure of the system is also very educational as each stage of transmission and reception can be tested and observed. The full system has been constructed and tested using simulated and real signals. A code was developed for communication between commercial modules and the DSP, bit error rate testing, data transmission, signal generation, and signal reception. A graphical user interface (GUI) was developed to help user with information display and system control. This thesis describes the software-defined-radio design in detail and shows test results at the end.
- Implanted Antennas and Intra-Body Propagation Channel for Wireless Body Area NetworkIbraheem, Ali Ahmed Younis (Virginia Tech, 2014-11-25)Implanted Devices are important components of the Wireless Body Area Network (WBAN) as a promising technology in biotelemetry, e-health care and hyperthermia applications. The design of WBAN faces many challenges, such as frequency band selection, channel modeling, antenna design, physical layer (PHY) protocol design, medium access control (MAC) protocol design and power source. This research focuses on the design of implanted antennas, channel modeling between implanted devices and Wireless Power Transfer (WPT) for implanted devices. An implanted antenna needs to be small while it maintains Specific Absorption Rate (SAR) and is able to cope with the detuning effect due to the electrical properties of human body tissues. Most of the proposed antennas for implanted applications are electric field antennas, which have a high near-zone electric field and, therefore, a high SAR and are sensitive to the detuning effect. This work is devoted to designing a miniaturized magnetic field antenna to overcome the above limitations. The proposed Electrically Coupled Loop Antenna (ECLA) has a low electric field in the near-zone and, therefore, has a small SAR and is less sensitive to the detuning effect. The performance of ECLA, channel model between implanted devices using Path Loss (PL) and WPT for implanted devices are studied inside different human body models using simulation software and validated using experimental work. The study is done at different frequency bands: Medical Implanted Communication Services (MICS) band, Industrial Scientific and Medical (ISM) band and 3.5 GHz band using ECLA. It was found that the proposed ECLA has a better performance compared to the previous designs of implanted antennas. Based on our study, the MICS band has the best propagation channel inside the human body model among the allowed frequency bands. The maximum PL inside the human body between an implanted antenna and a base station on the surface is about 90 dB. WPT for implanted devices has been investigated as well, and it has been shown that for a device located at 2 cm inside the human body with an antenna radius of 1 cm an efficiency of 63% can be achieved using the proposed ECLA.
- Implementation of Application Layer Protocol for an Active RFID SystemAgrawal, Ambuj (Virginia Tech, 2011-08-02)The emerging technology of active RFID tags has strong potential in the areas of real time health monitoring, sorting of cargo, and large scale inventory management because of their longer communication range and larger data storage capacity. The market of active RFID is growing very rapidly and therefore there has been an increase in the number of companies engaging in this field. But very often it is found that the products available in the market are not always suited to the application at hand. To overcome this problem, off the shelf active RFID products which were reconfigurable and followed a standard PHY and MAC layer protocol were used for this work. By reprogramming the application layer protocol of the RFID hardware, these devices were made suitable for the desired application. This also allowed the RFID tags to extend their functionality by interfacing extra modules with themselves. The work presented in this thesis describes the way in which the microcontroller on board the active RFID tags and readers can be programmed so that the functionality of the RFID hardware can be changed as per requirements. It also shows that extra modules can be added to the tag by successfully interfacing an accelerometer module with the tag.
- A Linear RF Power Amplifier with High Efficiency for Wireless HandsetsRefai, Wael Yahia (Virginia Tech, 2014-03-13)This research presents design techniques for a linear power amplifier with high efficiency in wireless handsets. The power amplifier operates with high efficiency at the saturated output power, maintains high linearity with enhanced efficiency at back-off power levels, and covers a broadband frequency response. The amplifier is thus able to operate in multiple modes (2G/2.5G/3G/4G). The design techniques provide contributions to current research in handset power amplifiers, especially to the converged power amplifier architecture, to reduce the number of power amplifiers within the handset while covering all standards and frequency bands around the globe. Three main areas of interest in power amplifier design are investigated: high power efficiency; high linearity; and broadband frequency response. Multiple techniques for improving the efficiency are investigated with the focus on maintaining linear operation. The research applies a new technique to the handset industry, class-J, to improve the power efficiency while avoiding the practical issues that hinder the typical techniques (class-AB and class-F). Class-J has been implemented using GaN FET in high power applications. To our knowledge, this work provides the first implementation of class-J using GaAs HBT in a handset power amplifier. The research investigates the linearity, and the nature and causes of nonlinearities. Multiple concepts for improving the linearity are presented, such as avoiding odd-degree harmonics, and linearizing the relationship between the output current and the input voltage of the amplifier at the fundamental frequency. The concept of bias depression in HBT transistors is introduced with a bias circuit that reduces the bias-offset effect to improve linearity at high output power. A design methodology is presented for broadband matching networks, including the component loss. The methodology offers a quick and accurate estimation of component values, giving more degrees of freedom to meet the design specifications. It enables a trade-off among high out-of-band attenuation, number/size of components, and power loss within the network. Although the main focus is handset power amplifiers, most of the developed techniques can be applied to a wide range of power amplifiers.
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