Astronomical time

Astronomical time serves as the basis for civil time and is required to describe the orientation of the Earth with respect to an inertial reference frame. The definitions of astronomical time scales are reviewed as well as observational methods. The causes of the variations in the Earth's rotation are explained and a brief overview of international efforts to maintain astronomical time scales is presented     

United colors and untied meanings: Benetton and the commodification of social issues

The Benetton United Colors campaign illustrates how modern advertising has been radicalized into an explicitly political forum. Although lifestyle companies often attempt to associate their products with progressive social movements, Benetton was the first company to eliminate pictures of its products from its advertisements. In 1989, ads depicting Benetton's sportswear were replaced with powerful and problematic visual images of AIDS, environmental disasters, terrorism, and racism. Social issues became the embodiment of Benetton's product and, through the transformation into commodities, lost their significance as problematic human conditions. The campaign illustrates how the decontextualization of placing issues within the framework of product promotion, creates a tone of discordant meaning not adequately explained by mass culture critiques of consumerism. This case study recommends that advertising should be studied as a complex and contested social discourse within 1990s consumer culture.     

Lateral-mode analysis of narrow-stripe semiconductor lasers with acylindrically concave end facet

A beam-propagation method is used to investigate numerically the lateral-mode characteristics of GaAlAs semiconductor lasers having a cylindrically concave facet at one end. Various degrees of index guiding have been considered. Simulations were made for both below- and above-threshold regimes. As expected, the numerical results show that the optical beam divergence parallel to the junction plane can be significantly reduced. In addition, this far-field narrowing is accompanied by lower threshold currents. However, for values of the radius of curvature of the concave facet lying in some specific ranges, the losses of the fundamental lateral mode are higher than that calculated for conventional lasers, and the corresponding eigenvalue shows nonmonotonic variations as the injection current is increased. This phenomenon is due to an enhancement of the index antiguiding over the gain-guiding mechanism     

AlGaN/GaN heterojunction bipolar transistor

We demonstrate the first GaN bipolar transistor. An AlGaN/GaN HBT structure was grown by MOCVD on c-plane sapphire substrate. The emitter was grown with an Al_0.1Ga_0.9N barrier to increase the emitter injection efficiency. Cl₂ RIE was used to pattern the emitter mesa, and selectively regrown base contact pads were implemented to reduce a contact barrier associated with RIE etch damage to the base surface. The current gain of the devices was measured to be as high as three with a base width of 200 nm. DC transistor characteristics were measured to 30 V V_CE in the common emitter configuration, with an offset voltage of 5 V. A gummel plot and base contact characteristics are also presented     

A 20-W stereo class-D audio output power stage in 0.6-/spl mu/m BCDMOS technology

This paper presents a highly power efficient 2/spl times/20-W class-D audio output power stage implemented in 0.6-/spl mu/m BCDMOS technology. The presented power stage is capable of driving 2/spl times/8-/spl Omega/ loads from a 20-V power supply at a power efficiency approaching 90%. Circuit details of thermal detection, over-current protection, and startup speaker click/pop are also presented. The performance of open-loop Class-D output stages are limited by the distortion mechanisms present within the power stage itself. A third-order PWM modulator was prototyped and used to dramatically improve the performance of the Class-D output stage by using feedback. The results of this work are also presented.     

Self‐Assembly of Atomically Thin Chiral Copper Heterostructures Templated by Black Phosphorus

The fabrication of 2D systems for electronic devices is not straightforward, with top‐down low‐yield methods often employed leading to irregular nanostructures and lower quality devices. Here, a simple and reproducible method to trigger self‐assembly of arrays of high aspect‐ratio chiral copper heterostructures templated by the structural anisotropy in black phosphorus (BP) nanosheets is presented. Using quantitative atomic resolution aberration‐corrected scanning transmission electron microscopy imaging, in situ heating transmission electron microscopy and electron energy‐loss spectroscopy arrays of heterostructures forming at speeds exceeding 100 nm s^?1 and displaying long‐range order over micrometers are observed. The controlled instigation of the self‐assembly of the Cu heterostructures embedded in BP is achieved using conventional electron beam lithography combined with site specific placement of Cu nanoparticles. Density functional theory calculations are used to investigate the atomic structure and suggest a metallic nature of the Cu heterostructures grown in BP. The findings of this new hybrid material with unique dimensionality, chirality, and metallic nature and its triggered self‐assembly open new and exciting opportunities for next generation, self‐assembling devices.     

温度测量系统对ADC的要求

为具体应用选择适当的温度传感器取决于待测温度范围和所需的精度.系统精度取决于温度传感器的精度和对传感器的输出数字化的模数转换器(ADC)的性能.     

时域在数字射频领域中的作用

许多新兴应用要求功能强大的射频分析工具,除传统的频率轴和幅度轴之外,同时捕获时间信息.当前许多射频信号在不同时间点之间变化.有些信号会跳频,有些信号则简单地达到尖峰,然后消失.许多信号采用复杂的调制,其可能会在瞬间发生动态变化.这些活动可能会产生自己的负面效应,如随机瞬变、干扰、开关异常事件等.这些现象都有一个共同点,那就是时间不再是一个可以忽略的轴.