应用工程师问答-4对数放大器(LOGARITHMIC AMPLIFIER)精解

问:我才刚阅读完ADI近期所发表的对数放大器相关技术手册,不过我对于对数放大器究竟是什么仍然感到有些困惑. 答:你并不是唯一感到困惑的人.许多年来,我已经处理过许多的询问,都是有关于对数放大器所执行功能的重点差异,以及在本质上完全不同的设计概念.就让我们这样子开始好了,你希望在对数放大器的输出中看到什么?     

Modeling Concrete Masonry Walls Subjected to Explosive Loads

Concrete masonry unit walls subjected to blast pressure were analyzed with the finite element method, with the goal of developing a computationally efficient and accurate model. Wall behavior can be grouped into three modes of failure, which correspond to three ranges of blast pressures. Computational results were compared to high-speed video images and debris velocities obtained from experimental data. A parametric analysis was conducted to determine the sensitivity of computed results to critical modeling values. It was found that the model has the ability to replicate experimental results with good agreement. However, it was also found that, without knowledge of actual material properties of the specific wall to be modeled, computational results are not reliable predictors of wall behavior.     

An analysis of success factors and benefits of partnering in construction

Partnering is increasingly being used on construction projects. Partnering involves the parties to a construction project working together in an environment of trust and openness to realise the project efficiently and without conflict. Using a UK-wide postal questionnaire survey, the opinions of different types of organisation — consultants, contractors, and clients were assessed in relation to the success factors and benefits of partnering. The study shows that UK contractors and clients are more positive about partnering than consultants. The research also indicates that certain requirements must be met if partnering is to succeed. In particular, trust, communication, commitment, a clear understanding of roles, consistency and a flexible attitude are necessary. It is recognised that nothing will change without considerable effort from all parties. Respondents believe that partnering can bring significant benefits, including fewer adversarial relationships and increased end-customer satisfaction, to the construction industry if all parties involved in a project strive for its success.     

Magnetic carbon nanotubes with particle-free surfaces and high drug loading capacity

Open-ended, multi-wall carbon nanotubes (CNTs) with magnetic nanoparticles encapsulated within their graphitic walls (magCNTs) were fabricated by a combined action of templated growth and a ferrofluid catalyst/carbon precursor, and tested as drug hosts. The hybrid nanotubes are stable under extreme pH conditions due to particle protection provided by the graphitic shell. The magCNTs are promising for high capacity drug loading given that the magnetic functionalization did not block any of the active sites available for drug attachment, either from the CNT internal void or on the internal and external surfaces. This is in contrast to typical approaches of loading CNTs with particles that proceed through surface attachment or capillary filling of the tube interior. Additionally, the CNTs exhibit enhanced hydrophilic character, as shown by water adsorption measurements, which make them suitable for biological applications. The morphological and structural characteristics of the hybrid CNTs are evaluated in conjunction to their magnetic properties and ability for drug loading (diaminophenothiazine). The fact that the magnetic functionality is provided from 'inside the walls' can allow for multimode functionalization of the graphitic surfaces and makes the magCNTs promising for targeted therapeutic applications.     

Reliability analysis of roof sheathing panels exposed to a hurricane wind

Light-frame wood roofs are frequently used in the US for residential and commercial construction. High wind events, such as hurricanes, may cause severe damage to these structures by breaking the roof envelope and allowing penetration of wind-driven rain. Most previous wood panel reliability studies have used static, uniform wind pressure load models and code-specified load distribution rules for analysis. This study re-estimates the reliability of roof sheathing panels exposed to a specific hurricane event using actual wind pressure data and a more refined structural analysis model. The objective is to examine the adequacy of the simplified wind load and structural analysis models used for roof panel reliability analysis. In the procedure here, panel failure behavior is modeled by individual fastener extraction from the panel as dynamic wind pressure is increased. For reliability analysis, the limit state is based on panel pull-off. The results show that the use of a refined model provides some significant differences in panel reliability found from simplified techniques.     

Anticipating infectious disease re-emergence and elimination: a test of early warning signals using empirically based models

Timely forecasts of the emergence, re-emergence and elimination of human infectious diseases allow for proactive, rather than reactive, decisions that save lives. Recent theory suggests that a generic feature of dynamical systems approaching a tipping point—early warning signals (EWS) due to critical slowing down (CSD)—can anticipate disease emergence and elimination. Empirical studies documenting CSD in observed disease dynamics are scarce, but such demonstration of concept is essential to the further development of model-independent outbreak detection systems. Here, we use fitted, mechanistic models of measles transmission in four cities in Niger to detect CSD through statistical EWS. We find that several EWS accurately anticipate measles re-emergence and elimination, suggesting that CSD should be detectable before disease transmission systems cross key tipping points. These findings support the idea that statistical signals based on CSD, coupled with decision-support algorithms and expert judgement, could provide the basis for early warning systems of disease outbreaks.     

Effect of Edge Stiffening and Diaphragms on the Reliability of Bridge Girders

Secondary elements such as barriers, sidewalks, and diaphragms may affect the distribution of live load to bridge girders. The objective of this study is to evaluate their effect on girder reliability if these elements are designed to be sufficiently attached to the bridge so as not to detach under traffic live loads. Simple-span, two-lane structures are considered, with composite steel girders supporting a reinforced concrete deck. Several representative structures are selected, with various configurations of barriers, sidewalks, and diaphragms. Bridge analysis is performed using a finite-element procedure. Load and resistance parameters are treated as random variables. Random variables considered are composite girder flexural strength, secondary element stiffness, load magnitude (dead load and truck traffic live load), and live load position. It was found that typical combinations of secondary elements have a varying influence on girder reliability, depending on secondary element stiffness and bridge geometry. Suggestions are presented that can account for secondary elements and that provide a uniform level of reliability to bridge girders.     

多载波直接变频发射机为进入鼎盛时期做好准备

引言直接变频技术长期以来一直被誉为电信产业的“圣杯”。很显然,任何承诺减少元器件数量并且降低成本的新体系结构必定很诱人。然而,事情从未这样简单。超外差体系结构能够在中频(IF)滤除宽带噪声、镜像和杂散分量,直接变频发射机却没有这么多功能。使用直接变频从单载波过渡到多载波使问题更加具有挑战性,因为需要增加单载波的回退以避免元器件过驱动。解决这个问题唯一的方法就是使用具有高输出压缩点和低噪声的元器件。本文将提出解决这些问题的方法和存在的其它挑战,并且将现有元器件的性能与WCDMA和CDMA2000蜂窝基站要求进行比…     

Marginalised Care: Migrant Workers Caring for Older People in Ireland

Older adult care in Ireland is a mix of public, private, voluntary and family provision. This model is characterised by deficient funding and support structures for both care recipients and carers, leading ultimately to fragmented service delivery, both in the community and in residential care. Against this backdrop, there has been a significant and rapid growth in the number of migrant registered nurses and care assistants providing care to Irish older people. With two potentially marginalised groups now at the centre of the caring relationship, questions arise regarding the sustainability of quality of care and quality of life for both providers and recipients of care. This research study draws on the perspectives of the older person, the migrant carer and the employer to develop an understanding of migrant worker care provision within the disadvantaged ageing sector in Ireland. The paper will frame migrant care workers’ experiences within the perspective of a marginalised sector, whose central consumers, older people, are not prioritised in policy or practice. Providing evidence of disadvantage for older adults and migrant carers, the research findings demonstrate that it is necessary to improve caring experiences and conditions for both groups if quality of care is to be enhanced.     

Persistence of low wind speed conditions and implications for wind power variability

As the penetration of wind generation increases on power systems throughout the world, the effects of wind variability on power systems are of increasing concern. This study focuses on sustained occurrences of low wind speeds over durations ranging from 1 h to 20 days. Such events have major implications for the variability of energy yields from wind farms. This, in turn, influences the accuracy of wind resource assessment. The frequency analysis techniques commonly used to study wind variability cannot represent the autocorrelation properties of wind speeds and thus provide no information on the probabilities of occurrence of such sustained, low wind events. We present two complementary methods for assessing wind variability, runs analysis and intensity–duration–frequency analysis, both with emphasis on characterising the occurrence of continuous, extended periods (up to several days) of low wind speeds. Multi‐annual time series of hourly wind speeds from meteorological stations in Ireland are analysed with both techniques. Sustained 20‐day periods corresponding to extremely low levels of wind generation are found to have return periods of around 10 years in coastal areas. Persistent, widespread low wind speed conditions across the entire country are found to occur only rarely. Copyright © 2012 John Wiley & Sons, Ltd.