5,411 research outputs found
A new simulation technique for RF oscillators
The study of phase-noise in oscillators and the design of new circuit topologies necessitates an efficient technique for the simulation of oscillators. While numerous approaches have been developed over the years e.g. [1-3], each has its own merits and demerits. In this contribution, an asymptotic numeric method developed in e.g. [4-5] is applied to the simulation of RF oscillators. The method is closely related to the stroboscopic and high-order averaging method in [6] and the Heterogeneous Multiscale Methods in [7]. The method is advantageous in that the same methodology can be applied for the simulation of general circuit problems involving highly oscillatory ordinary differential equations, partial differential equations and delay differential equations. Furthermore and counter-intuitively, its efficacy improves with increasing frequency, a feature that is very favourable in modern communications systems where operating frequencies are ever rising. Results for a CMOS oscillator will confirm the validity and efficiency of the proposed method
Effective Hamiltonian for transition-metal compounds. Application to Na_xCoO_2
We describe a simple scheme to construct a low-energy effective Hamiltonian
H_eff for highly correlated systems containing non-metals like O, P or As (O in
what follows) and a transition-metal (M) as the active part in the electronic
structure, eliminating the O degrees of freedom from a starting Hamiltonian
that contains all M d orbitals and all non-metal p orbitals. We calculate all
interaction terms between d electrons originating from Coulomb repulsion, as a
function of three parameters (F_0, F_2 and F_4) and write them in a basis of
orbitals appropriate for cubic, tetragonal, tetrahedral or hexagonal symmetry
around M. The approach is based on solving exactly (numerically if necessary) a
MO_n cluster containing the transition-metal atom and its n nearest O atoms
(for example a CoO_6 cluster in the case of the cobaltates, or a CuO_n cluster
in the case of the cuprates, in which n depends on the number of apical O
atoms), and mapping them into many-body states of the same symmetry containing
d holes only. We illustrate the procedure for the case of Na_xCoO_2. The
resulting H_eff, including a trigonal distortion D, has been studied recently
and its electronic structure agrees well with angle-resolved photoemission
spectra [A. Bourgeois, A. A. Aligia, and M. J. Rozenberg, Phys. Rev. Lett. 102,
066402 (2009)]. Although H_eff contains only 3d t_2g holes, the highly
correlated states that they represent contain an important amount not only of O
2p holes but also of 3d e_g holes. When more holes are added, a significant
redistribution of charge takes place. As a consequence of these facts, the
resulting values of the effective interactions between t_2g states are smaller
than previously assumed, rendering more important the effect of D in obtaining
only one sheet around the center of the Brillouin zone for the Fermi surface
(without additional pockets).Comment: 11 pages, 1 figure, accepted for publication in Phys.Rev.
Resolving the Radio Source Background: Deeper Understanding Through Confusion
We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam
area at 3 GHz with 8 arcsec FWHM resolution and 1.0 microJy/beam rms noise near
the pointing center. The P(D) distribution from the central 10 arcmin of this
confusion-limited image constrains the count of discrete sources in the 1 <
S(microJy/beam) < 10 range. At this level the brightness-weighted differential
count S^2 n(S) is converging rapidly, as predicted by evolutionary models in
which the faintest radio sources are star-forming galaxies; and ~96$% of the
background originating in galaxies has been resolved into discrete sources.
About 63% of the radio background is produced by AGNs, and the remaining 37%
comes from star-forming galaxies that obey the far-infrared (FIR) / radio
correlation and account for most of the FIR background at lambda = 160 microns.
Our new data confirm that radio sources powered by AGNs and star formation
evolve at about the same rate, a result consistent with AGN feedback and the
rough correlation of black hole and bulge stellar masses. The confusion at
centimeter wavelengths is low enough that neither the planned SKA nor its
pathfinder ASKAP EMU survey should be confusion limited, and the ultimate
source detection limit imposed by "natural" confusion is < 0.01 microJy at 1.4
GHz. If discrete sources dominate the bright extragalactic background reported
by ARCADE2 at 3.3 GHz, they cannot be located in or near galaxies and most are
< 0.03 microJy at 1.4 GHz.Comment: 28 pages including 16 figures. ApJ accepted for publicatio
Automated optical identification of a large complete northern hemisphere sample of flat spectrum radio sources with S_6cm > 200 mJy
This paper describes the automated optical APM identification of radio
sources from the Jodrell Bank - VLA Astrometric Survey (JVAS), as used for the
search for distant radio-loud quasars. The sample has been used to investigate
possible relations between optical and radio properties of flat spectrum radio
sources. From the 915 sources in the sample, 756 have an optical APM
identification at a red (e) and/or blue (o) plate,resulting in an
identification fraction of 83% with a completeness and reliability of 98% and
99% respectively. About 20% are optically identified with extended APM objects
on the red plates, e.g. galaxies. However the distinction between galaxies and
quasars can not be done properly near the magnitude limit of the POSS-I plates.
The identification fraction appears to decrease from >90% for sources with a 5
GHz flux density of >1 Jy, to <80% for sources at 0.2 Jy. The identification
fraction, in particular that for unresolved quasars, is found to be lower for
sources with steeper radio spectra. In agreement with previous studies, we find
that the quasars at low radio flux density levels also tend to have fainter
optical magnitudes, although there is a large spread. In addition, objects with
a steep radio-to-optical spectral index are found to be mainly highly polarised
quasars, supporting the idea that in these objects the polarised synchrotron
component is more prominent. It is shown that the large spread in
radio-to-optical spectral index is possibly caused by source to source
variations in the Doppler boosting of the synchrotron component [Abridged].Comment: LaTex, 17 pages, 5 gif figures, 4 tables. Accepted for publication in
MNRAS. High resolution figures can be found at http://www.roe.ac.uk/~ignas
Discounting in Games across Time Scales
We introduce two-level discounted games played by two players on a
perfect-information stochastic game graph. The upper level game is a discounted
game and the lower level game is an undiscounted reachability game. Two-level
games model hierarchical and sequential decision making under uncertainty
across different time scales. We show the existence of pure memoryless optimal
strategies for both players and an ordered field property for such games. We
show that if there is only one player (Markov decision processes), then the
values can be computed in polynomial time. It follows that whether the value of
a player is equal to a given rational constant in two-level discounted games
can be decided in NP intersected coNP. We also give an alternate strategy
improvement algorithm to compute the value
Being Like A Girl in the Twenty-First Century: Branding and Identity Through Cultural Conversation
Branding exists at the intersection of culture and economics and is an increasingly important tool in distinguishing commodities. Over the past few years, more and more companies have started to use discourses of female empowerment, celebrating femininity, womanhood, and all things âgirlyâ to sell their products - spiking sales, racking up millions of online views, and starting important conversations in the process. Procter & Gamble\u27s âLike A Girlâ campaign for Always creates a productive tension and way of thinking about both brand and buyer. The âLike A Girlâ campaign is unique because it presents a feminist message to and about younger girls, a group that is often left out of broader discussions of feminism, and it creates a space for support, identification, and critical engagement with essentializations of the female body
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