1,720,960 research outputs found
6/12-channel Synchronous Digital Phasemeter for Ultrastable Signal Characterization and Use
No full textNowadays, in primary time and frequency laboratories we can find high spectral purity signals in the 10 MHz - 10 GHz range generated from cryogenic oscillators or ultra-stable lasers together with frequency combs. Their short-term stability surpasses by one to two orders of magnitude the performances of active hydrogen masers (AHM), while in the long-term AHMs still have a better behavior. The new technology can be considered mature for what concern spectral purity, but we cannot say the same about complexity, power consumption and reliability. In this sense, it is important to measure ultra-stable sources with respect to AHMs. First, to test their spectral purity or, at least, to give it an upper bound; second to have a continuous monitoring; finally, to combine them in order to get the best of all in term of phase noise and frequency stability. All of these requirements can be satisfied by the system we are developing. It is a multi-channel synchronous and real-time phasemeter based on Tracking Direct Digital Synthesizer (TDDS) technique. The results related to the first prototype are presented
High-performing vapor-cell frequency standards
Full text linkMany nowadays scientific and technological applications need very precise time and frequency reference signals. Very often, only atomic clocks can guarantee the high level of accuracy and stability required by these signals. In the current scenario of atomic frequency standards, vapor-cell clocks are particularly suited to be employed in those activities that demand good frequency stability performances joined to compactness, reliability and low power consumption. Recently, due to better-performing laser sources and to innovative techniques to prepare and detect the atoms, several cell-based prototypes exhibiting unprecedented frequency stability have been developed. We review advances in the field of laser-pumped
vapor-cell clocks and we provide an overview of the techniques that allowed to
achieve frequency stabilities in the order of 1×10^−13 at 1 s (short term) and in the range of 10^−15 for the medium-long term. These stabilities are two orders of magnitude better than current commercial Rb clocks. We also prospect the possibility of further improving these results.Many nowadays scientific and technological applications need very precise time and frequency reference signals. Very often, only atomic clocks can guarantee the high level of accuracy and stability required by these signals. In the current scenario of atomic frequency standards, vapor-cell clocks are particularly suited to be employed in those activities that demand good frequency stability performances joined to compactness, reliability and low power consumption. Recently, due to better-performing laser sources and to innovative techniques to prepare and detect the atoms, several cell-based prototypes exhibiting unprecedented frequency stability have been developed. We review advances in the field of laser-pumped vapor-cell clocks and we provide an overview of the techniques that allowed to achieve frequency stabilities in the order of 1 x 10(-13) at is (short term) and in the range of 10(-15) for the medium-long term. These stabilities are two orders of magnitude better than current commercial Rb clocks. We also prospect the possibility of further improving these results
Simple-design ultra-low phase noise microwave frequency synthesizers for high-performing Cs and Rb vapor-cell atomic clocks
Full text linkWe report on the development and characterization of novel 4.596 GHz and 6.834 GHz microwave
frequency synthesizers devoted to be used as local oscillators in high-performance Cs and Rb vapor-cell atomic clocks. The key element of the synthesizers is a custom module that integrates a high
spectral purity 100 MHz oven controlled quartz crystal oscillator frequency-multiplied to 1.6 GHz
with minor excess noise. Frequency multiplication, division, and mixing stages are then implemented
to generate the exact output atomic resonance frequencies. Absolute phase noise performances of the
output 4.596 GHz signal are measured to be −109 and −141 dB rad2/Hz at 100 Hz and 10 kHz Fourier
frequencies, respectively. The phase noise of the 6.834 GHz signal is −105 and −138 dB rad2/Hz
at 100 Hz and 10 kHz offset frequencies, respectively. The performances of the synthesis chains
contribute to the atomic clock short term fractional frequency stability at a level of 3.1 ×10−14 for
the Cs cell clock and 2 ×10−14 for the Rb clock at 1 s averaging time. This value is comparable with
the clock shot noise limit. We describe the residual phase noise measurements of key components
and stages to identify the main limitations of the synthesis chains. The residual frequency stability
of synthesis chains is measured to be at the 10−15 level for 1 s integration time. Relevant advantages
of the synthesis design, using only commercially available components, are to combine excellent
phase noise performances, simple-architecture, low-cost, and to be easily customized for signal
output generation at 4.596 GHz or 6.834 GHz for applications to Cs or Rb vapor-cell frequency
standards
Frequency-doubled Laser System at 780 nm for Pulsed Vapor-cell Clocks
Full text linkWe present the development status of a low-noise pulsed laser source suitable for high-performing vapor-cell clocks. The laser is based on a 1560 nm source, frequency doubled to be resonant with the D-2 line of rubidium at 780 nm. The laser system is able to deliver laser pulses with programmable amplitude and length. The intensity noise of the laser during the pulses duration is also actively reduced by means of the same fast analog control loop generating the pulses. The pulses characteristics are shown to be compatible with the specifications of a high-performing Pulsed Optically Pumped (POP) clock
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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