25,797 research outputs found
Theoretical study of the mechanisms of fatigue in photomultipliers, phase 2 Final report
Fatigue testing of photomultipliers with gallium phosphide dynode
Programmable rate modem utilizing digital signal processing techniques
The engineering development study to follow was written to address the need for a Programmable Rate Digital Satellite Modem capable of supporting both burst and continuous transmission modes with either binary phase shift keying (BPSK) or quadrature phase shift keying (QPSK) modulation. The preferred implementation technique is an all digital one which utilizes as much digital signal processing (DSP) as possible. Here design tradeoffs in each portion of the modulator and demodulator subsystem are outlined, and viable circuit approaches which are easily repeatable, have low implementation losses and have low production costs are identified. The research involved for this study was divided into nine technical papers, each addressing a significant region of concern in a variable rate modem design. Trivial portions and basic support logic designs surrounding the nine major modem blocks were omitted. In brief, the nine topic areas were: (1) Transmit Data Filtering; (2) Transmit Clock Generation; (3) Carrier Synthesizer; (4) Receive AGC; (5) Receive Data Filtering; (6) RF Oscillator Phase Noise; (7) Receive Carrier Selectivity; (8) Carrier Recovery; and (9) Timing Recovery
A symbol-based algorithm for decoding bar codes
We investigate the problem of decoding a bar code from a signal measured with
a hand-held laser-based scanner. Rather than formulating the inverse problem as
one of binary image reconstruction, we instead incorporate the symbology of the
bar code into the reconstruction algorithm directly, and search for a sparse
representation of the UPC bar code with respect to this known dictionary. Our
approach significantly reduces the degrees of freedom in the problem, allowing
for accurate reconstruction that is robust to noise and unknown parameters in
the scanning device. We propose a greedy reconstruction algorithm and provide
robust reconstruction guarantees. Numerical examples illustrate the
insensitivity of our symbology-based reconstruction to both imprecise model
parameters and noise on the scanned measurements.Comment: 24 pages, 12 figure
Perfect magnetohydrodynamics as a field theory
We propose the generally covariant action for the theory of a self-coupled
complex scalar field and electromagnetism which by virtue of constraints is
equivalent, in the regime of long wavelengths, to perfect magnetohydrodynamics
(MHD). We recover from it the Euler equation with Lorentz force, and the
thermodynamic relations for a prefect fluid. The equation of state of the
latter is related to the scalar field's self potential. We introduce 1+3
notation to elucidate the relation between MHD and field variables. In our
approach the requirement that the scalar field be single valued leads to the
quantization of a certain circulation in steps of ; this feature leads,
in the classical limit, to the conservation of that circulation. The
circulation is identical to that in Oron's generalization of Kelvin's
circulation theorem to perfect MHD; we here characterize the new conserved
helicity associated with it. We also demonstrate the existence for MHD of two
Bernoulli-like theorems for each spacetime symmetry of the flow and geometry;
one of these is pertinent to suitably defined potential flow. We exhibit the
conserved quantities explicitly in the case that two symmetries are
simultaneously present, and give examples. Also in this case we exhibit a new
conserved MHD circulation distinct from Oron's, and provide an example.Comment: RevTeX, 16 pages, no figures; clarifications added and typos
corrected; version to be published in Phys. Rev.
Metastable Features of Economic Networks and Responses to Exogenous Shocks
It has been proved that network structure plays an important role in
addressing a collective behaviour. In this paper we consider a network of firms
and corporations and study its metastable features in an Ising based model. In
our model, we observe that if in a recession the government imposes a demand
shock to stimulate the network, metastable features shape its response.
Actually we find that there is a minimum bound where demand shocks with a size
below it are unable to trigger the market out from recession. We then
investigate the impact of network characteristics on this minimum bound. We
surprisingly observe that in a Watts-Strogatz network though the minimum bound
depends on the average of the degrees, when translated into the economics
language, such a bound is independent of the average degrees. This bound is
about GDP, where GDP is the gap of GDP between recession
and expansion. We examine our suggestions for the cases of the United States
and the European Union in the recent recession, and compare them with the
imposed stimulations. While stimulation in the US has been above our threshold,
in the EU it has been far below our threshold. Beside providing a minimum bound
for a successful stimulation, our study on the metastable features suggests
that in the time of crisis there is a "golden time passage" in which the
minimum bound for successful stimulation can be much lower. So, our study
strongly suggests stimulations to be started within this time passage.Comment: 13 pages, 10 figures, accepted for publication in PloS On
Satellite communication system and method Patent
Earth satellite relay station for frequency multiplexed voice transmissio
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