977 research outputs found
Influence of the Feedback Filter on the Response of the Pulsed Digital Oscillator
This paper introduces a new feedback topology for the Pulsed Digital
Oscillator (PDO) and compares it to the classical topology. The `classic' or
single feedback topology, introduced in previous works, shows a strong behavior
dependence on the damping losses in the MEMS resonator. A new double feedback
topology is introduced here in order to help solving this problem. Comparative
discrete-time simulations and preliminary experimental measurements have been
carried out for both topologies, showing how the new double feedback topology
may increase PDO performance for some frequency ranges.Comment: Submitted on behalf of TIMA Editions
(http://irevues.inist.fr/tima-editions
Short-timescale Fluctuations in the Difference Light Curves of QSO 0957+561A,B: Microlensing or Noise?
From optical R band data of the double quasar QSO 0957+561A,B, we made two
new difference light curves (about 330 days of overlap between the time-shifted
light curve for the A image and the magnitude-shifted light curve for the B
image). We observed noisy behaviours around the zero line and no
short-timescale events (with a duration of months), where the term event refers
to a prominent feature that may be due to microlensing or another source of
variability. Only one event lasting two weeks and rising - 33 mmag was found.
Measured constraints on the possible microlensing variability can be used to
obtain information on the granularity of the dark matter in the main lensing
galaxy and the size of the source. In addition, one can also test the ability
of the observational noise to cause the rms averages and the local features of
the difference signals. We focused on this last issue. The combined
photometries were related to a process consisting of an intrinsic signal plus a
Gaussian observational noise. The intrinsic signal has been assumed to be
either a smooth function (polynomial) or a smooth function plus a stationary
noise process or a correlated stationary process. Using these three pictures
without microlensing, we derived some models totally consistent with the
observations. We finally discussed the sensitivity of our telescope (at Teide
Observatory) to several classes of microlensing variability.Comment: MNRAS, in press (LaTeX, 14 pages, 22 eps figures
RESOLUTION LIMITS FOR RESONANT MEMS SENSORS BASED ON DISCRETE RELAY FEEDBACK TECHNIQUES
Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/handle/2042/5920)International audienceThis paper is devoted to the analysis of resonant MEMS sensors based on discrete relay feedback techniques. One drawback of such techniques is that some synchronization usually occurs between the discrete part and the continuous part of the system: this results in sensor responses that are very similar to the curves known as devil's staircases, i.e. the frequency does not vary smoothly with the sensor's input. The main contribution of this paper is a theoretical calculation of the resolution of such systems. The resolutions of two existing resonant MEMS architectures are then calculated and these results are discussed
New VR magnification ratios of QSO 0957+561
We present VR magnification ratios of QSO 0957+561, which are inferred from
the GLITP light curves of Q0957+561A and new frames taken with the 2.56m Nordic
Optical Telescope about 14 months after the GLITP monitoring. From two
photometric approaches and a reasonable range for the time delay in the system
(415-430 days), we do not obtain achromatic optical continuum ratios, but
ratios depending on the wavelength. These new measurements are consistent with
differential extinction in the lens galaxy, the Lyman limit system, the damped
Ly-alpha system, or the host galaxy of the QSO. The possible values for the
differential extinction and the ratio of total to selective extinction in the V
band are reasonable. Moreover, crude probability arguments suggest that the ray
paths of the two components cross a similar dusty environment, including a
network of compact dust clouds and compact dust voids. As an alternative (in
fact, the usual interpretation of the old ratios), we also try to explain the
new ratios as caused by gravitational microlensing in the deflector. From
magnification maps for each of the gravitationally lensed images, using
different fractions of the surface mass density represented by the microlenses,
as well as different sizes and profiles of the V-band and R-band sources,
several synthetic distributions of V-band and R-band ratios are derived. In
some gravitational scenarios, there is an apparent disagreement between the
observed pair of ratios and the simulated distributions. However, several
microlensing pictures work well. To decide between either extinction, or
microlensing, or a mixed scenario (extinction + microlensing), new
observational and interpretation efforts are required.Comment: PS and PDF versions are created from the LaTeX file and 5 EPS
figures, two additional figues (Figs. 6 and 7) in JPEG format, scheduled for
the ApJ 20 January 2005 issu
Electronic structure of the molecule based magnet Cu PM(NO3)2 (H2O)2
We present density functional calculations on the molecule based S=1/2
antiferromagnetic chain compound Cu PM(NO3)2 (H2O)2; PM = pyrimidine. The
properties of the ferro- and antiferromagnetic state are investigated at the
level of the local density approximation and with the hybrid functional B3LYP.
Spin density maps illustrate the exchange path via the pyrimidine molecule
which mediates the magnetism in the one-dimensional chain. The computed
exchange coupling is antiferromagnetic and in reasonable agreement with the
experiment. It is suggested that the antiferromagnetic coupling is due to the
possibility of stronger delocalization of the charges on the nitrogen atoms,
compared to the ferromagnetic case. In addition, computed isotropic and
anisotropic hyperfine interaction parameters are compared with recent NMR
experiments
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