DDS芯片/AD9851在频率合成信号发生器中的应用

本文介绍美国 AD(模拟器件 )公司采用先进的 DDS直接数字合成技术生产的高集成度产品— AD9851芯片 ,以及它在直接数字式频率合成信号发生器中的应用。     

DDS芯片／AD9851在频率合成信号发生器中的应用

本介绍美国AD（模拟器件）公司采用先进的DDS直接数字合成技术生产的高集成度产品-AD9851芯片，以及它在直接数字式频率合成信号发生器中的应用。     

基于DDS的高性能信号源的设计

直接数字频率合成(DDS)是频率合成领域中的一项新技术.本文简要介绍了DDS工作原理及其特性,详细阐述了应用DDS技术和单片机控制技术、选用AD9851芯片研制一台高分辨率、高稳定度和较高纯度信号源的工程实现方法,给出了系统主要硬件电路、频率控制字传送程序以及抑制杂散的方法.本设计结构简单,拓展性好;人机交互界面友好,数据输入简单、可靠;电磁兼容设计合理,保证了仪器性能稳定.     

基于单片机与AD9851的信号发生器

基于直接数字频率合成(DDS)原理,以单片机AT89S52为控制核心,结合现场可编程门阵列(FPGA),利用DDS集成芯片AD9851实现了在1 Hz～20 MHz的正弦信号发生器。AD9851输出的信号通过滤波、自动增益控制、功率放大后,在输出为50Ω负载的条件下,输出电压峰峰值可达10 V。此外,本系统还附有输出AM、FM、ASK、PSK、FSK等多种调制信号的功能。信号信噪比高,驱动能力强;系统复杂度低,易于在实验室中自制使用。     

基于AD9834构成的DDS波形发生器实现与仿真

DDS（直接数字合成）技术被视为第三代频率合成技术。该文对基于AD9834DDS信号发生器进行分析、设计、编程。仿真实现用DDS方法产生任意波形（AWG）。     

程控信号发生器的设计

本文介绍了利用直接数字频率合成(DDS)技术构成数字频率合成器的工作原理、电路组成、电路设计并对利用AD9850、单片机和微机来构成信号发生器作了介绍。     

DDS芯片AD9852在信号产生器中的应用

首先介绍了DDS（直接数字频率合成）的工作原理,描述了DDS芯片AD9852的性能和结构特点,给出了以AD9852芯片和单片机为核心的高精度信号发生器的硬件设计方案,最后通过上位机软件进行软件编程,以此来控制DDS信号发生器的工作模式和输出频率值。实验结果表明,此信号发生器使用方便、结构简单、性能稳定可靠。     

AD9850 DDS芯片信号源的研制

直接数字合成（Direct Digital Synthesize,DDS)是一种重要的频率合成技术,具有分辨率高,频率变换快等优点。阐述了性能价格比较高的AD9850直接数字频率合成器芯片的基本原理和性能特点,以及用其研制的0－30MHz信号源。     

DDS数字移相器

直接数字式频率合成技术(DDS)是一种先进的全数字频率合成技术,本文基于DDS的相位调制功能,给出了一种DDS数字移相器的设计方案.     

基于FPGA的基带信号发生器的设计

采用基于DDS模块的硬件实现方法设计基带信号发生器,在FPGA内部用DDS模块进行频率合成和叠加,利用EDA技术和FPGA实现直接数字频率综合器DDS的设计。可以完成快速的频率切换,并且在改变时能够保持相位的连续,很容易实现频率、相位和幅度的数控调制。实验结果表明该信号发生器达到了一个比较好的设计精度。     

Unpacking (in)formal learning in an academic development programme: a mixed-method social network perspective

How and with whom academics develop and maintain formal and informal networks for reflecting on their teaching practice has received limited attention even though academic development (AD) programmes have become an almost ubiquitous feature of higher education. The primary goal of this mixed-method study is to unpack how 114 academics in an AD programme developed internal (within their programme) and external (outside their programme) learning and teaching relations. A secondary goal is to highlight the affordances of social network analysis (SNA) methods in conjunction with qualitative approaches for academic developers to understand the (in)formal learning processes in their AD programme. The quantitative results indicate that participants maintained 4.84 relations within their AD programme and 3.17 external ties. The qualitative results indicate that most academics developed a range of emotional, academic, and professional support links, which were mostly outside the AD context. Participants needed an outlet to share their feelings, challenges, and frustrations about their teaching and their experiences on the AD programme. These feelings were shared with people they trusted, primarily close friends and colleagues. This study provides a social perspective on the formal and informal relations of AD, and argues that SNA techniques can help academic developers to make these relationships visible.     

Understanding Asperger Disorder: A Primer for Early Childhood Educators

The psychiatric community has only recognized Asperger Disorder (AD) as a distinct syndrome since 1994. Therefore, for most early childhood educators, AD is still a new diagnosis. The majority of the children with AD are being educated in the general education classroom although it's not uncommon to see specialized placements. Unfortunately, due to the uniqueness of this condition, teachers often question their academic and classroom management skills with these youngsters. This article suggests basic strategies in each area of AD that are useful in helping the student demonstrate success in the general education classroom or in specialized services. In addition, the characteristics of AD in young children are provided with some early indicators that may be important in early identification and referrral for specialized services.     

冼夫人时代高凉的社会性质

从经济、政治、文化等方面列举大量事实论证冼夫人时代高凉的社会性质。文章认为，东汉永元年间（公元89-105年），岭南进入封建社会；南朝初期刘宋时代（公元420-479年），高凉进入封建社会。因此，冼夫人时代的高凉已经是封建社会，但还存在奴隶社会的残余。     

阿尔茨海默病发病机制和动物模型研究进展

阿尔茨海默病（Alzheimer’s disease,AD）是一种慢性中枢神经系统退行性疾病,病因不明、发病机制复杂,因此动物模型的建立对探索其发病机制及寻找防治疾病的药物具有重要意义。在动物模型中,转基因小鼠模型和转基因果蝇模型已成为研究热点。该文就阿尔茨海默病的发病机制和动物模型的研究现状进行综述。     

Diffusion tensor imaging of the brain in patients with Alzheimer＇s disease and cerebrovascular lesions

Background： Recent autopsy study showed a high incidence ofcerebrovascular lesions in Alzheimer＇s disease （AD）. To assess the impact of cerebrovascular pathology in AD, we used diffusion tensor imaging （DTI） to study AD patients with and without cerebrovascular lesions. Materials and Methods： Conventional and DTI scans were obtained from 10 patients with probable AD, 10 AD/V patients （probable AD with cerebrovascular lesions） and ten normal controls. Mean diffusivity （D） and fractional anisotropy （FA） values of some structures involved with AD pathology were measured. Results： D value was higher in AD patients than in controls in hippocampus and the cingulate gyrus. In AD/V patients, increased D value was found in the same structures and also in the thalamus and basal ganglia compared to controls. There was a significant difference of D value between AD and AD/V patients. FA value reduced in the white matter of left inferior temporal gyrus and in the bilateral middle cingulate gyrus in patients with AD/V compared with controls. The MMSE （mini-mental state examination） score significantly correlated with FA value in the right hippocampus （r=0.639, P〈0.019）, in the right anterior cingulate gyrus （r=0.587, P〈0.035） and in left parahippocampal gyrus （r=0.559, P〈0.047）. Conclusion： Cerebrovascular pathology had stronger impact on the D value than the AD pathology alone did. Elevated D value in thalamic and basal ganglia may contribute to cognitive decline in AD/V patients. Reduced FA values in AD/V patients may indicate that cerebrovascular pathology induced more severe white matter damage than the AD pathology alone did.     

Medical students’ help-seeking recommendations for a person with Alzheimer’s disease: relationships with knowledge and stigmatic beliefs

ABSTRACT

Primary care physicians play a central role in the process of providing a timely diagnosis of Alzheimer’s disease (AD). Recent reviews reported that their lack of knowledge and stigmatic beliefs about the disease are important barriers to providing an adequate diagnosis and even affect physicians’ referral recommendations for help-seeking (HS). This topic however, has not been examined among medical students. Thus, the aim of this study was to examine Israeli medical students’ HS recommendations for a person with AD, and their associations with knowledge about the disease and stigmatic beliefs, including agism and AD stigma. A convenience sample of 142 six-year medical students completed a structured questionnaire (60% female; mean age = 25.8). Overall, regarding nonprofessional sources, the vast majority of the participants stated they would recommend seeking help from close family members and friends. Regarding medical sources, almost all the participants recommended seeking help from family physicians and neurologists. Results showed that AD stigma mediated the relationship between knowledge about AD and HS recommendations. Our results stress the importance of investing efforts during the training and socialization phases of future clinicians in order to improve the timely diagnosis of AD, placing special attention on AD stigma.     

阿尔兹海默症患者的语义记忆损伤:表现、理论及展望

图片命名、范畴流畅性等神经心理学测验表明,阿尔兹海默症(AD)患者普遍存在语义记忆的损伤,并随着病程的发展表现出进行性与弥散性特点。AD患者语义记忆的损伤既可能由语义知识的存储障碍造成,也可能由语义知识的提取障碍引发,还可能由存储与提取的双重损伤造成。未来研究应开发出对早期AD检测更为敏感的语义记忆测验,并采用横向与纵向多种研究技术与手段,考虑不同语言文化背景下AD患者语义损伤的情况,以综合分析AD患者语义记忆损伤的实质及其发展趋势。