60,885 research outputs found
Stability implications of delay distribution for first-order and second-order systems
Kiss, G., & Krauskopf, B. (2009). Stability implications of delay distribution for first-order and second-order systems. Early version, also known as pre-print Link to publication record in Explore Bristol Research PDF-documen
Nonlinear Analysis of Irregular Variables
The Fourier spectral techniques that are common in Astronomy for analyzing
periodic or multi-periodic light-curves lose their usefulness when they are
applied to unsteady light-curves. We review some of the novel techniques that
have been developed for analyzing irregular stellar light or radial velocity
variations, and we describe what useful physical and astronomical information
can be gained from their use.Comment: 31 pages, to appear as a chapter in `Nonlinear Stellar Pulsation' in
the Astrophysics and Space Science Library (ASSL), Editors: M. Takeuti & D.
Sasselo
Invariant Measures for Dissipative Dynamical Systems: Abstract Results and Applications
In this work we study certain invariant measures that can be associated to
the time averaged observation of a broad class of dissipative semigroups via
the notion of a generalized Banach limit. Consider an arbitrary complete
separable metric space which is acted on by any continuous semigroup
. Suppose that possesses a global
attractor . We show that, for any generalized Banach limit
and any distribution of initial
conditions , that there exists an invariant probability measure
, whose support is contained in , such that for all
observables living in a suitable function space of continuous mappings
on .
This work is based on a functional analytic framework simplifying and
generalizing previous works in this direction. In particular our results rely
on the novel use of a general but elementary topological observation, valid in
any metric space, which concerns the growth of continuous functions in the
neighborhood of compact sets. In the case when does not
possess a compact absorbing set, this lemma allows us to sidestep the use of
weak compactness arguments which require the imposition of cumbersome weak
continuity conditions and limits the phase space to the case of a reflexive
Banach space. Two examples of concrete dynamical systems where the semigroup is
known to be non-compact are examined in detail.Comment: To appear in Communications in Mathematical Physic
Adaptive design of delta sigma modulators
In this thesis, a genetic algorithm based on differential evolution (DE) is used to generate delta sigma modulator (DSM) noise transfer functions (NTFs). These NTFs outperform those generated by an iterative approach described by Schreier and implemented in the delsig Matlab toolbox. Several lowpass and bandpass DSMs, as well as DSM\u27s designed specifically for and very low intermediate frequency (VLIF) receivers are designed using the algorithm developed in this thesis and compared to designs made using the delsig toolbox. The NTFs designed using the DE algorithm always have a higher dynamic range and signal to noise ratio than those designed using the delsig toolbox
POLOCALC: a Novel Method to Measure the Absolute Polarization Orientation of the Cosmic Microwave Background
We describe a novel method to measure the absolute orientation of the
polarization plane of the CMB with arcsecond accuracy, enabling unprecedented
measurements for cosmology and fundamental physics. Existing and planned CMB
polarization instruments looking for primordial B-mode signals need an
independent, experimental method for systematics control on the absolute
polarization orientation. The lack of such a method limits the accuracy of the
detection of inflationary gravitational waves, the constraining power on the
neutrino sector through measurements of gravitational lensing of the CMB, the
possibility of detecting Cosmic Birefringence, and the ability to measure
primordial magnetic fields. Sky signals used for calibration and direct
measurements of the detector orientation cannot provide an accuracy better than
1 deg. Self-calibration methods provide better accuracy, but may be affected by
foreground signals and rely heavily on model assumptions. The POLarization
Orientation CALibrator for Cosmology, POLOCALC, will dramatically improve
instrumental accuracy by means of an artificial calibration source flying on
balloons and aerial drones. A balloon-borne calibrator will provide far-field
source for larger telescopes, while a drone will be used for tests and smaller
polarimeters. POLOCALC will also allow a unique method to measure the
telescopes' polarized beam. It will use microwave emitters between 40 and 150
GHz coupled to precise polarizing filters. The orientation of the source
polarization plane will be registered to sky coordinates by star cameras and
gyroscopes with arcsecond accuracy. This project can become a rung in the
calibration ladder for the field: any existing or future CMB polarization
experiment observing our polarization calibrator will enable measurements of
the polarization angle for each detector with respect to absolute sky
coordinates.Comment: 15 pages, 5 figures, Accepted by Journal of Astronomical
Instrumentatio
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