911,613 research outputs found
Digital second-order phase-locked loop
A digital second-order phase-locked loop is disclosed in which a counter driven by a stable clock pulse source is used to generate a reference waveform of the same frequency as an incoming waveform, and to sample the incoming waveform at zero-crossover points. The samples are converted to digital form and accumulated over M cycles, reversing the sign of every second sample. After every M cycles, the accumulated value of samples is hard limited to a value SGN = + or - 1 and multiplied by a value delta sub 1 equal to a number of n sub 1 of fractions of a cycle. An error signal is used to advance or retard the counter according to the sign of the sum by an amount equal to the sum
White holes and eternal black holes
We investigate isolated white holes surrounded by vacuum, which correspond to
the time reversal of eternal black holes that do not evaporate. We show that
isolated white holes produce quasi- thermal Hawking radiation. The time
reversal of this radiation, incident on a black hole precursor, constitutes a
special preparation that will cause the black hole to become eternal.Comment: 5 pages, 2 figures, revtex; revised version to appear in Classical
and Quantum Gravit
The Role of Primordial Kicks on Black Hole Merger Rates
Primordial stars are likely to be very massive \geq30\Msun, form in
isolation, and will likely leave black holes as remnants in the centers of
their host dark matter halos in the mass range
10^{6}-10^{10}\Ms. Such early black holes, at redshifts z\gtsim10, could
be the seed black holes for the many supermassive black holes found in galaxies
in the local universe. If they exist, their mergers with nearby supermassive
black holes may be a prime signal for long wavelength gravitational wave
detectors. We simulate formation of black holes in the center of high redshift
dark matter halos and explore implications of initial natal kick velocities
conjectured by some formation models. The central concentration of early black
holes in present day galaxies is reduced if they are born even with moderate
kicks of tens of km/s. The modest kicks allow the black holes to leave their
parent halo, which consequently leads to dynamical friction being less
effective on the lower mass black holes as compared to those still embedded in
their parent halos. Therefore, merger rates may be reduced by more than an
order of magnitude. Using analytical and illustrative cosmological N--body
simulations we quantify the role of natal kicks of black holes formed from
massive metal free stars on their merger rates with supermassive black holes in
present day galaxies. Our results also apply to black holes ejected by the
gravitational slingshot mechanism.Comment: 12 pages, 9 figure
Optimizing the Density of Holes of EWT Solar Cells Taking Into Consideration Mechanical Aspects
EWT solar cells start from drilled wafers with approximately 100 holes/cm2. These holes act as stress concentrators leading to a reduction in the mechanical strength of this type of wafers. The viability of cells with higher density of holes has been studied. To this end, sets of wafers with different density of holes have been characterized. The ring on ring test has been employed and FE models have been developed to simulate the test. The statistical evaluation permits to draw conclusions about the reduction of the strength depending on the density of holes. Moreover, the stress concentration around the holes has been studied by means of the FE method employing the sub-modeling technique. The maximum principal stress of EWT wafers with twice the density of holes of commercial ones is almost the same. However, the mutual interaction between the stress concentration effects around neighboring holes is only observed for wafers with a density of 200 holes/cm
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