Hunt, William D.

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Now showing 1 - 10 of 48
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    A new method for wideband characterization of resonator-based sensing platforms
    (Georgia Institute of Technology, 2011-03) Munir, Farasat ; Wathen, Adam ; Hunt, William D.
    A new approach to the electronic instrumentation for extracting data from resonator-based sensing devices (e.g., microelectromechanical, piezoelectric, electrochemical, and acoustic) is suggested and demonstrated here. Traditionally, oscillator-based circuitry is employed to monitor shift in the resonance frequency of the resonator. These circuits give a single point measurement at the frequency where the oscillation criterion is met. However, the resonator response itself is broadband and contains much more information than a single point measurement. Here, we present a method for the broadband characterization of a resonator using white noise as an excitation signal. The resonator is used in a two-port filter configuration, and the resonator output is subjected to frequency spectrum analysis. The result is a wideband spectral map analogous to the magnitude of the S21 parameters of a conventional filter. Compared to other sources for broadband excitation (e.g., frequency chirp, multisine, or narrow time domain pulse), the white noise source requires no design of the input signal and is readily available for very wide bandwidths (1 MHz–3 GHz). Moreover, it offers simplicity in circuit design as it does not require precise impedance matching; whereas such requirements are very strict for oscillator-based circuit systems, and can be difficult to fulfill. This results in a measurement system that does not require calibration, which is a significant advantage over oscillator circuits. Simulation results are first presented for verification of the proposed system, followed by measurement results with a prototype implementation. A 434 MHz surface acoustic wave (SAW) resonator and a 5 MHz quartz crystal microbalance (QCM) are measured using the proposed method, and the results are compared to measurements taken by a conventional bench-top network analyzer. Maximum relative differences in the measured resonance frequencies of the SAW and QCM resonators are 0.0004% and 0.002%, respectively. The ability to track a changing sensor response is demonstrated by inducing temperature variations and measuring resonance frequency simultaneously using the proposed technique in parallel with a network analyzer. The relative difference between the two measurements is about 5.53 ppm, highlighting the impressive accuracy of the proposed system. Using commercially available digital signal processors (DSPs), we believe that this technique can be implemented as a system-on-a-chip solution resulting in a very low cost, easy to use, portable, and customizable sensing system. In addition, given the simplicity of the signal and circuit design, and its immunity to other common interface concerns (injection locking, oscillator interference, and drift, etc.), this method is better suited to accommodating array-based systems.
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    Trapped hybrid modes in solidly mounted resonators based on c-axis oriented hexagonal crystals
    (Georgia Institute of Technology, 2010-12) Wathen, Adam D. ; Munir, Farasat ; Hunt, William D.
    Assuming an idealized piezoelectric bulk acoustic wave resonator, one typically calculates the velocity of the fundamental bulk acoustic mode as the measured frequency times twice the thickness of the piezoelectric film. In c-axis 6mm hexagonal crystals of (e.g., ZnO or AlN), both the longitudinal and thickness shear modes are peizoelectrically active using thickness excitation and lateral-field excitation, respectively. Without a loss of generality, we concentrate our study on ZnO films. The theoretical velocity of the pure thickness shear mode in sputtered ZnO, based strictly on reported material properties, is calculated to be approximately 2580 m/s. However, a variety of acoustic velocities for the thickness shear mode in ZnO have been reported in the literature ranging from about 3100–3500 m/s. These reported values represent a 20%–36% increase in acoustic velocity relative to the theoretical values. In the literature, this deviation is typically attributed to ZnO film inconsistencies and other phenomena which can be difficult to quantify. We propose that the reported inconsistencies may be attributed to a hybrid acoustic mode comprised of a coupling of shear and longitudinal particle displacements. In this paper, we present a theoretical description of a hybrid mode in ZnO solidly mounted resonator (SMR) devices. We begin first with an experimental verification of a mode with a changing velocity in a ZnO SMR with the only variable being the ZnO thickness. Using the acoustic velocity through the thickness as an effective velocity with which to reference the mode, we find the effective acoustic velocity to range from 3100–3900 m/s, with increasing ZnO thickness. We then start from the first principles of piezoelectric acoustic wave propagation and derive three coupled partial differential equations describing a hybrid mode comprised of the coupling between longitudinal and shear particle displacement and the corresponding piezoelectrically generated potential in the ZnO film. The qualitative findings described by the derived equations are then further investigated with finite element simulation (COMSOL MULTIPHYSICS®). We simulate the performance of our experimental devices using the COMSOL platform, examine the eigenfrequencies of the structure, and find a hybrid mode which is trapped both vertically and laterally in the ZnO film. Calculating the effective velocity of the simulated modes, we find the simulated effective velocities to be within 1.5% of our measured results. Finally, we compare simulation results to experimentally measured results of a previously observed hybrid mode and achieve a 0.2% agreement.
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    Music, Science, and Technology
    (Georgia Institute of Technology, 2009-03-03) Hunt, William D. ; Valk, Henry
    Music and its performance have been part of our inheritance since primitive times. But what is music? How do we produce and hear it? How are popular instruments that we use to perform it, such as the guitar and piano, evolving? These and related questions will be discussed from the standpoint of current science and technology.
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    High-Q micromechanical resonators in a two-dimensional phononic crystal slab
    (Georgia Institute of Technology, 2009-02) Mohammadi, Saeed ; Eftekhar, Ali Asghar ; Hunt, William D. ; Adibi, Ali
    By creating line defects in the structure of a phononic crystal (PC) made by etching a hexagonal array of holes in a 15 μm thick slab of silicon, high-Q PC resonators are fabricated using a complimentary-metal-oxide-semiconductor-compatible process. The complete phononic band gap of the PC structure supports resonant modes with quality factors of more than 6000 at frequencies as high as 126 MHz. The confinement of acoustic energy is achieved by using only a few PC layers confining the cavity region. The calculated frequencies of resonance of the structure using finite element method are in a very good agreement with the experimental data. The performance of these PC resonator structures makes them excellent candidates for wireless communication and sensing applications.
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    Functionalization of high frequency SAW RFID devices for ozone dosimetry
    (Georgia Institute of Technology, 2009) Westafer, Ryan S. ; Levitin, Galit ; Hess, Dennis W. ; Bergin, Michael H. ; Edmonsonx, Peter J. ; Hunt, William D.
    In this paper we report new work on the gravimetric detection of ozone at EPA and OSHA relevant concentrations (approximately 100 ppb) in filtered ambient air. We have extended our proof-of-concept work which used both quartz crystal microbalance (QCM) and surface acoustic wave (SAW) resonators. We now enable detection using our high frequency SAW RFID devices. Such surface wave devices are extremely sensitive to the viscosity, thickness, and uniformity of the reactive or sorbent coating. We report laboratory characterization of our polymer-coated SAW sensors operating between 200 and 600 MHz on lithium niobate substrates. Return loss measurements confirm adequate load bearing even at 550 MHz. We compare both the temperature and ozone sensitivity of the RFID devices to conventional resonators. In conclusion, we suggest the design improvements to yield a next generation of SAW RFID ozone sensors with even greater sensitivity.
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    Clues from Digital Radio Regarding Biomolecular Recognition
    (Georgia Institute of Technology, 2008-10-10) Hunt, William D.
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    In Situ Acoustic Temperature Measurement During Variable-Frequency Microwave Curing
    (Georgia Institute of Technology, 2008-10) Davis, Cleon E. ; Dickherber, Anthony ; Hunt, William D. ; May, Gary S.
    Variable-frequency microwave (VFM) curing can perform the same processing steps as conventional thermal processing in minutes, without compromising intrinsic material properties. With increasing demand for novel dielectrics, there is a corresponding demand for new processing techniques that lead to comparable or better properties than conventional methods. VFM processing can be a viable alternative to conventional thermal techniques. However, current limitations include a lack of reliable temperature measuring techniques. This research focuses on developing a reliable temperature measuring system using acoustic techniques to monitor low-k polymer dielectrics cured on silicon wafers in a VFM furnace. The acoustic sensor exhibits the capability to measure temperatures from 20 degrees C to 300 degrees C with an attainable accuracy of ± 2 degrees.
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    Evidence of large high frequency complete phononic band gaps in silicon phononic crystal plates
    (Georgia Institute of Technology, 2008-06-02) Mohammadi, Saeed ; Eftekhar, Ali Asghar ; Khelif, Abdelkrim ; Hunt, William D. ; Adibi, Ali
    We show the evidence of the existence of large complete phononic band gaps (CPBGs) in two-dimensional phononic crystals (PCs) formed by embedding cylindrical air holes in a solid plate (slab). The PC structure is made by etching a hexagonal array of air holes through a freestanding plate of silicon. A fabrication process compatible with metal-oxide-semiconductor technology is used on silicon-on-insulator substrate to realize the PC devices. Measuring the transmission of elastic waves through eight layers of the hexagonal lattice PC in the ΓK direction, more than 30 dB attenuation is observed at a high frequency; i.e., 134 MHz, with a band gap to midgap ratio of 23%. We show that this frequency region matches very well with the expected CPBG found through theoretical calculations.
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    Analogies Between Digital Radio and Chemical Orthogonality as a Method for Enhanced Analysis of Molecular Recognition Events
    (Georgia Institute of Technology, 2008-02) Edmonson, Peter J. ; Hunt, William D. ; Stubbs, Desmond D. ; Lee, Sang-Hun
    Acoustic wave biosensors are a real-time, label-free biosensor technology, which have been exploited for the detection of proteins and cells. One of the conventional biosensor approaches involves the immobilization of a monolayer of antibodies onto the surface of the acoustic wave device for the detection of a specific analyte. The method described within includes at least two immobilizations of two different antibodies onto the surfaces of two separate acoustic wave devices for the detection of several analogous analytes. The chemical specificity of the molecular recognition event is achieved by virtue of the extremely high (nM to pM) binding affinity between the antibody and its antigen. In a standard ELISA (Enzyme-Linked ImmunoSorbent Assay) test, there are multiple steps and the end result is a measure of what is bound so tightly that it does not wash away easily. The fact that this "gold standard" is very much not real time, masks the dance that is the molecular recognition event. X-Ray Crystallographer, Ian Wilson, demonstrated more than a decade ago that antibodies undergo conformational change during a binding event[1, 2]. Further, it is known in the arena of immunochemistry that some antibodies exhibit significant cross-reactivity and this is widely termed antibody promiscuity. A third piece of the puzzle that we will exploit in our system of acoustic wave biosensors is the notion of chemical orthogonality. These three biochemical constructs, the dance, antibody promiscuity and chemical orthogonality will be combined in this paper with the notions of in-phase (I) and quadrature (Q) signals from digital radio to manifest an approach to molecular recognition that allows a level of discrimination and analysis unobtainable without the aggregate. As an example we present experimental data on the detection of TNT, RDX, C4, ammonium nitrate and musk oil from a system of antibody-coated acoustic wave sensors.
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    Biological warfare agents
    (Georgia Institute of Technology, 2007-03-13) Hunt, William D.