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  • 1
    Electronic Resource
    Electronic Resource
    Computational complexity and the observation of physical signals (1993)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 74 (1993), S. 5314-5319 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The effects of computational complexity on the characteristics of a physical signal that is reconstructed from its representation of sampled data are analyzed. It is found that a more complex algorithm does not only require longer time to implement, but also yields an erroneous reconstruction. The reconstruction suffers from contrast degradation, phase shifts, and attenuation of details relative to the true signal. These unwanted effects are caused by the existence of spurious frequencies in the computed spectrum due to rounding-off errors. The amplitude distribution of the spurious frequencies across the spectral bandwidth strongly depends on the number of data points handled and on the complexity of the particular reconstruction algorithm employed. Since the floating point representation of numbers in a computer is always finite, an upper limit exists in the maximum number of additions or multiplications required to compute a quantity reliably without errors.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.354232
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  • 2
    Electronic Resource
    Electronic Resource
    Modeling spatially extended complex systems from experimental data: Sensitivity to sampling intervals (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 77 (1995), S. 1374-1377 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The reliability of a phenomenological model in describing the profile of a spatially extended complex system is analyzed. An assessment is essential because an observed image of a two-dimensional object is sensitive to increases in sampling intervals. The range of interval values wherein the fractal dimension of the image remains invariant is determined for various kinds of object boundaries. Data sets of the same object that are obtained using sampling intervals that belong within the range contain the same amount of fractal information, and therefore adhere to one phenomenological model. This reliability range is determined for both coherent and incoherent measurement processes. Numerical experiments show that the range is the same for all fractal objects considered, and depends only on the type of measurement process. However, the measurement errors introduced when sampling at intervals beyond the reliability range are highly dependent on the type of fractal structures. Boundaries described by larger fractal dimensions are less sensitive. The existence of a nonzero reliability range implies that object fractal information can be recovered at sampling rates lower than the minimum value imposed by the Nyquist sampling criterion. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.358948
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  • 3
    Electronic Resource
    Electronic Resource
    Bandwidth and detection limit of a crossing–sampling spectrum analyzer (1997)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 240-242 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We report the performance of a spectrum analyzer that determines the Fourier spectrum of an analog input sa(t) using only as data the time locations {ti} where sa(t) intersects with a reference sinusoid r(t)=A cos(2πWt). If A≤|sa(t)| within the sampling period T, then one crossing exists within each interval Δ=1/2W. The spectrum is computed directly from {ti}, which consists of 2M=T/Δ crossing locations. The theoretical bandwidth and frequency resolution of the analyzer is given by 2W and 1/T, respectively. However, electronic circuits have a finite temporal response and the crossings can only be located with finite accuracy. A trade off, therefore, is expected between the measurement bandwidth and the detection limit in a real crossing-based analyzer. In our design, the accuracy can be varied to increase the bandwidth, through the use of a versatile field programable gate array. The relation between the detection limit, dynamic range, and bandwidth of our device is experimentally studied and analyzed. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147816
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  • 4
    Electronic Resource
    Electronic Resource
    Optical thickness profiling using a semiconductor laser confocal microscope (1996)
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 67 (1996), S. 2072-2078 
    Details
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A semiconductor laser confocal microscope is developed for measuring the optical thickness of thin transparent samples at high spatial resolution. The optical sectioning capability of a confocal microscope, and the sensitivity of the laser output to optical feedback, are both utilized to detect the presence of a nonabsorbing sample in the path of the probe beam that is initially focused onto a plane mirror. The index mismatch at the sample interface defocuses the beam away from the mirror and reduces the amount of optical feedback. The optical thickness is computed from the amount of axial displacement that the mirror must be given to regain maximum feedback. The laser power output is monitored using the monitor photodiode in the laser package. When the geometrical thickness of the sample is known a priori, the technique can be used to measure its refractive index and vice versa. The smallest and largest measurable sample thickness are determined by the sharpness of the axial intensity response of the microscope and the working distance of the focusing objective, respectively. Distortions in the central spot distributions of the response degrade the precision of the measurement technique. We demonstrate the technique for both the 830 and 780 nm laser output wavelengths, by classifying cover glasses of differing optical thickness (geometrical thickness range: 70–536 μm, refractive index=1.523), as well as to image the sudden index changes found in the phase ridge formed by two closely spaced cover glasses (surface flatness=7 μm) of different geometrical thickness. Our experiments show that at least for objectives with small numerical apertures (≤0.25), spherical aberrations are negligible and our measurements are to within the accuracy set by the glass manufacturer. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1147017
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  • 5
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    Charge movement in a GaN-based hetero-structure field effect transistor structure with carbon doped buffer under applied substrate bias (2015)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2015-12-02
    Description: Charge trapping and transport in the carbon doped GaN buffer of a GaN-based hetero-structure field effect transistor (HFET) has been investigated under both positive and negative substrate bias. Clear evidence of redistribution of charges in the carbon doped region by thermally generated holes is seen, with electron injection and capture observed during positive bias. Excellent agreement is found with simulations. It is shown that these effects are intrinsic to the carbon doped GaN and need to be controlled to provide reliable and efficient GaN-based power HFETs.
    Print ISSN: 0021-8979
    Electronic ISSN: 1089-7550
    Topics: Physics
    Published by American Institute of Physics (AIP)
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  • 6
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    Buffer transport mechanisms in intentionally carbon doped GaN heterojunction field effect transistors (2014)
    American Institute of Physics (AIP)
    Details
    Publication Date: 2014-07-01
    Description: Temperature dependent pulsed and ramped substrate bias measurements are used to develop a detailed understanding of the vertical carrier transport in the buffer layers in a carbon doped GaN power heterojunction field effect transistor. Carbon doped GaN and multiple layers of AlGaN alloy are used in these devices to deliver an insulating and strain relieved buffer with high breakdown voltage capability. However, understanding of the detailed physical mechanism for its operation is still lacking. At the lowest electric fields (
    Print ISSN: 0003-6951
    Electronic ISSN: 1077-3118
    Topics: Physics
    Published by American Institute of Physics (AIP)
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