DDS and converter form signal generatorMany applications require low-frequency signal generators that can deliver high-performance, high-resolution signals. This Design Idea presents a circuit that generates frequencies of 0 to 1 MHz. Sinusoidal, triangular, and square-wave outputs are available. You can achieve frequency resolution of better than Hz and phase resolution of better than °; thus, you can program exact coherent frequencies. This feature is useful in digital modulation and frequency-tuning applications. The circuit uses the ADµC831 and AD9834 to generate the required frequencies (Figure 1). You can program the microcontroller from either a PC or a Unix-based workstation. You then program the AD9834 using a three-wire serial interface via the microcontroller. The interface word is 16 bits can program the AD9834 to provide sinusoidal, triangular, and square-wave outputs using the DDS (direct-digital-synthesis) architecture. The chip operates as an NCO (numerically controlled oscillator) using an on-chip, 28-bit phase accumulator, sine-coefficient ROM, and a 10-bit D/A converter. You typically consider sine waves in terms of their magnitude form, A(t)=sin(ωt). The amplitude is nonlinear and is, therefore, difficult to generate. The angular information, on the other hand, is perfectly linear. That is, the phase angle rotates through a fixed angle for each unit in time. Knowing that the phase of a sine wave is linear, and, given a reference interval (clock period), you can determine the phase rotation for that period: ΔPhase=ω dt; ω=ΔPhase/dt=2πf, and f=(ΔPhase×fMCLK)/(2π), where dt=1/fMCLK, and fMCLK is the master this formula, you can generate output frequencies, knowing the phase and master-clock frequency. The phase accumulator provides the 28-bit linear phase. The amplitude coefficients of the output sine wave are stored in digital format in the sine-coefficient ROM. The DAC converts the sine wave to the analog domain. If you bypass the ROM, the AD9834 delivers triangular waveforms instead of sinusoidal waveforms. A square-wave output is also available on the part. Figure 2 shows the various waveforms available from the system. As shown in Figure 1, the sinusoidal/ triangular output waveforms are available on the IOUT pin (Pin 19); and the square wave output is available on the SIGN_BIT_OUT pin (Pin 16). You program the DDS by writing to the frequency registers. The analog output from the part is then: fOUT=fMCLK/228×(frequency-register word).The outputs of the DDS have 28-bit resolution, so effective frequency steps on the order of Hz are possible to a maximum of approximately 1 MHz. Figure 2 shows the typical waveform outputs. Two phase registers are available that allow 12-bit phase resolution. These registers phase-shift the signal by: Phase shift=2π/4096×(phase-register word).A 50-MHz crystal oscillator provides the reference clock for the DDS. The output stage of the DDS is a current-output DAC loaded by an external resistor. A 200Ω resistor generates the required peak-to-peak voltage range. The output is ac-coupled through capacitor C1. The MicroConverter contains two on-chip, 12-bit DACs. DAC1 varies the current through R5, adjusting the full-scale current of the DDS via the FSADJUST pin. The equation to control the full-scale current of the DDS DAC is: IOUT (full-scale)=18×I×, the internal reference of the MicroConverter, and op amp 2 allow for offset control of the output voltage of the DDS. You can program this dc offset to ±10V at 10-bit resolution. When R1=R2 and the gain of op amp 2=8, then the output of op amp 2 is: VOUT=(DAC output–(VREF/2))×8, yielding a ±10V R6 through R9 allow for control of gain through op amp 3. The gain of the op amp is a function of resistor switching, which you enable using the RDRIVE pin available on the MicroConverter. This operation allows for an effective programmable-output amplitude of approximately ±10V p-p. Thus, the circuit allows for programmable sinusoidal and triangular waves, including dc offsets, and the ability to set peak-to-peak amplitude of approximately ±10V. The square wave output on the SIGN_BIT_OUT pin has 0 to 5V amplitude. For low-frequency operation, a lowpass filter normally serves to filter reference-clock frequencies, spurs, and other images. For applications in which the output signal needs amplification, you should use a narrowband filter to filter out unwanted noise before the gain stage. A third-order filter would be good enough to remove most of the unwanted noise. Figure 3 shows a typical spectral plot of the output. Applications for this circuit range from signal-waveform generation to digital modulation. You can use the system in frequency-sweeping and -scanning applications and in resonance applications that use the frequency as an excitation signal to determine circuit resonance. Another useful application is as a reference oscillator for a PLL system.
convertor和converter意思是一样的。英国人更喜欢用convertor。
2-3个月。因为converter论文从见刊到检索，需要一定的时间，具体多久与各converter期刊有关，一般是2-3个月左右。此时期刊方会把发表见刊的论文，送检converter数据库收录。
; Direct Digital SynthesisDirect Digital Synthesis (DDS) is an electronic method for digitally creating arbitrary waveforms and frequencies from a single, fixed source basic DDS circuit consists of an electronic controller, a random-access memory, a frequency reference (usually a crystal oscillator), a counter and a digital-to-analogue converter (DAC). Two operating steps are required to make the device work: we shall call these programming and the programming step, the electronic controller fills the memory with data. Each item of data is a binary word representing the amplitude of the signal at an instant of time. The array of data in the memory then forms a table of amplitudes, with time implied by the position in the table. If, for example, the first half of the table were filled with zeroes and the second half with values of 100%, then the data would represent a square wave. Any other wave shape can be created simply by altering the the running step, the counter (properly called the phase accumulator) is instructed to advance by a certain increment on each pulse from the frequency reference. The output of the phase accumulator (the phase) is used to select each item in the data table in turn. Finally, the DAC converts this sequence of data to an analogue generate a periodic waveform, the circuit is set up so that one pass through the table takes a time equal to the period of the waveform. For example, if the reference frequency is 1 MHz, and the table contains 1000 entries, then a complete pass through the table with a phase increment of 1 will take 1000 / 1 MHz = 1 ms, so the frequency of the output waveform will be 1/(1 ms) = 1 system can generate a higher output frequency simply by increasing the phase increment so that the counter runs through the table more quickly. In the example above, the phase increment is equal to 1, so the next possible frequency is obtained by setting the increment to 2, resulting in a doubling of output frequency. To obtain a finer control of frequency than this, the standard phase increment can be set to, say, 10. This then allows slightly higher or lower output frequencies. For example, increasing the increment to 11 would increase the output frequency by 10%, and reducing it to 9 would decrease the output frequency by the same proportion. The more precision required over the frequency, the more bits are needed in the detailsPractical implementations usually set the size of the lookup table to be a power of 2 and work with 32-bit phase accumulators and phase increments. Usually the upper 8 or 10 bits of the counter are used as lookup table index (lookup table size is 256 or 1024, respectively). The remaining lower bits can be used as a parameter or index to interpolate between the adjacent entries in the lookup table. Often linear interpolation suffices. The source frequency usually comes from a crystal of 1 MHz to 100 highest frequency that can be generated this way depends on the size of the lookup table and the frequency. In order to generate a reasonable representation of the waveform, at least a minimum number of samples must be taken from it. If the phase increment becomes too large, then the counter would step through the lookup table too fast and the result may be a severe distortion of the output exist in both software and hardware. Due to the realtime nature of DDS, software implementations are usually limited to audio of DDS are: function generators, mixers, modulators, and sound 信号发生器 Signal Generator A signal generator, also known variously as a test signal generator, function generator, tone generator, arbitrary waveform generator, or frequency generator is an electronic device that generates repeating electronic signals (in either the analog or digital domains). They are generally used in designing, testing, troubleshooting, and repairing electronic or electroacoustic devices; though they often have artistic uses as are many different types of signal generators, with different purposes and applications (and at varying levels of expense); in general, no device is suitable for all possible , signal generators have been embedded hardware units, but since the age of multimedia-PCs, flexible, programmable software tone generators have also been purpose signal generatorsFunction generatorsA function generator is a device which produces simple repetitive waveforms. Such devices contain an electronic oscillator, a circuit that is capable of creating a repetitive waveform. (Modern devices may use digital signal processing to synthesize waveforms, followed by a digital to analog converter, or DAC, to produce an analog output). The most common waveform is a sine wave, but sawtooth, step (pulse), square, and triangular waveform oscillators are commonly available as are arbitrary waveform generators (AWGs). If the oscillator operates above the audio frequency range (>20 kHz), the generator will often include some sort of modulation function such as amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) as well as a second oscillator that provides an audio frequency modulation generators are typically used in simple electronics repair and design; where they are used to stimulate a circuit under test. A device such as an oscilloscope is then used to measure the circuit's output. Function generators vary in the number of outputs they feature, frequency range, frequency accuracy and stability, and several other parameters.[edit] Arbitrary waveform generatorsMain article: Arbitrary waveform generatorArbitrary waveform generators, or AWGs, are sophisticated signal generators which allow the user to generate arbitrary waveforms, within published limits of frequency range, accuracy, and output level. Unlike function generators, which are limited to a simple set of waveforms; an AWG allows the user to specify a source waveform in a variety of different ways. AWGs are generally more expensive than function generators, and are often more highly limited in available bandwidth; as a result, they are generally limited to higher-end design and test applications.[edit] Special purpose signal generatorsIn addition to the above general-purpose devices, there are several classes of signal generators designed for specific applications.[edit] Tone generators and audio generatorsA tone generator is a type of signal generator optimized for use in audio and acoustics applications. Tone generators typically include sine waves over the audio frequency range (20 Hz–20 kHz). Sophisticated tone generators will also include sweep generators (a function which varies the output frequency over a range, in order to make frequency-domain measurements), multitone generators (which output several tones simultaneously, and are used to check for intermodulation distortion and other non-linear effects), and tone bursts (used to measure response to transients). Tone generators are typically used in conjunction with sound level meters, when measuring the acoustics of a room or a sound reproduction system, and/or with oscilloscopes or specialized audio tone generators operate in the digital domain, producing output in various digital audio formats such as AES-3, or SPDIF. Such generators may include special signals to stimulate various digital effects and problems, such as clipping, jitter, bit errors; they also often provide ways to manipulate the metadata associated with digital audio term synthesizer is used for a device that generates audio signals for music, or that uses slightly more intricate methods.[edit] Video signal generatorsMain article: Video signal generatorA video signal generator is a device which outputs predetermined video and/or television waveforms, and other signals used to stimulate faults in, or aid in parametric measurements of, television and video systems. There are several different types of video signal generators in widespread use. Regardless of the specific type, the output of a video generator will generally contain synchronization signals appropriate for television, including horizontal and vertical sync pulses (in analog) or sync words (in digital). Generators of composite video signals (such as NTSC and PAL) will also include a colorburst signal as part of the output. Video signal generators are available for a wide variety of applications, and for a wide variety of digital formats; many of these also include audio generation capability (as the audio track is an important part of any video or television program or motion picture).
中国期刊网是正规网站，中国期刊网又称为知网。
是国家知识基础设施的概念，由世界银行于1998年提出。CNKI工程是以实现全社会知识资源传播共享与增值利用为目标的信息化建设项目。由清华大学、清华同方发起，始建于1999年6月。
2019年5月，“科研诚信与学术规范”在线学习平台在中国知网正式上线发布。
成立背景
知网的概念是国家知识基础设施，由世界银行于1998年提出。CNKI工程是以实现全社会知识资源传播共享与增值利用为目标的信息化建设项目，由清华大学、清华同方发起，始建于1999年6月。
在党和国家领导以及教育部、中宣部、科技部、新闻出版总署、国家版权局、国家发改委的大力支持下，在全国学术界、教育界、出版界、图书情报界等社会各界的密切配合和清华大学的直接领导下。
建成了世界上全文信息量规模最大的"CNKI数字图书馆"，并正式启动建设《中国知识资源总库》及CNKI网格资源共享平台，通过产业化运作，为全社会知识资源高效共享提供最丰富的知识信息资源和最有效的知识传播与数字化学习平台。
以上内容参考 百度百科-中国知网
学术网站，一般人注册不了，即使注册了也有权限设置，想下载文献和资源都不行。一般只有高校的电子图书馆资源提供给学生和老师一些免费的ID和权限。一般都是学校花钱注册的。里面的文章是受版权保护的。
中国期刊网：
中国期刊库 啊
中国学术期刊网络出版总库是由清华大学学术期刊电子杂志社和清华同方知网技术有限公司承担的《中国学术期刊网络出版总库》，是国家“十一五”重大网络出版工程《中国学术文献网络出版总库》的最重要组成部分，其“雏形”是清华大学于1995年创办的《中国学术期刊(光盘版)》和在1999年全文上网的《中国学术期刊全文数据库》。经过十余年的发展，今天的《中国学术期刊网络出版总库》已经脱胎换骨，截至2006年9月30日，已收录期刊总数7556种，其中学术类期刊6624种，核心期刊、重要评价性数据库来源期刊收全率为99%，文献收全率为，总文献量达2100多万篇。目前，其网络出版时间相对各刊物纸制出版时间滞后平均不超过两个月，日均新增文献逾万篇。
知网（英文名称为HowNet）是一个以汉语和英语的词语所代表的概念为描述对象，以揭示概念与概念之间以及概念所具有的属性之间的关系为基本内容的常识知识库。知网是一个知识系统，而不是一部语义词典。尽管被我们称为知识 词典的常识性知识库是知网的最基本的数据库。知网的全部的主要文件包括知识词典构成了一个有机结合的知识系统。
检索范围层次范围：在题录、题录摘要、专题全文三个层次中选择检索。时间范围：检索某时间范围发表的文章。内容范围：同时检索若干个专题数据库。 在专项检索与全文检索中，本浏览器均支持词、非词和逻辑表达式三种形式 的检索词。刊名检索：检索某期刊发表的文章。标题检索：检索在文章标题中出现检索词的文章。作者检索：检索某作者发表的文章。作者单位检索：输入单位名称，检索该单位的作者发表的文章。关键词检索：检索在文章关键词中出现检索词的文章。摘要检索：检索在文章摘要中出现检索词的文章。分类检索：检索属于某类文章。引文检索：检索在文章引文中出现检索词的文章。基金检索：检索在文章基金项目中出现检索词的文章。 定题服务包括：文献传送服务、查新服务、科研成果评价、项目背景分析等。
作为目前国内最大的中文学术期刊网，自开通以来 , 赢得了广大学者，学术工作者以及电子商务界的关注。各大媒体多次专题报道中国学术期刊网。中国学术期刊网（qikanwang）是中国学术期刊协会的官方网站，拥有CN刊号的正规期刊会员1200多家，是中文学术期刊联合征稿平台。权威，安全，快捷，是广大追求事业晋升或学业进步人士的发表园地。中国学术期刊网一直致力于电子信息资源的研究、开发和应用。公司的业务范围包括数据库出版发行、电子期刊出版发行、网络信息服务、网络广告推广、文献资料数字化加工等多种个性化服务。聚刊社力量，建服务平台，让中国学术期刊网通过互联网走入每一个学术研究人员的身边是我们不懈的追求目标。· 中国学术期刊网以丰富的网络内容吸引了成千上万的知识份子，且每日的流量正处于急剧上升的状态。期刊网所有的数字出版平台是国家“十一五”重点出版工程。它提供了学科专业数字图书馆和行业图书馆。个性化服务平台由个人数字图书馆、机构数字图书馆、数字化学习平台等组成。· 中国学术期刊网有强大的技术后台支持, 完全实现并超越了目前国内的网络技术，一个学术类期刊的全功能网站。中国学术期刊网具有刊类、标题、关键词检索及全文检索等功能，独家研发出了完善的刊社管理和客服管理的系统软件 , 已从单一技术发展到拥有标准体系的电子商务网站。· 中国学术期刊网的刊社加盟系统，可提高各期刊社在读者中的知名度，并以零成本占领国内外网络阅读市场，从而扩大影响，亦增加纸版杂志的订户人数，达到电子版、纸版网上订阅互动促销的效果。此外，期刊社除获得电子期刊销售收入外，还可获得网络读者投稿的新稿源和具有重要参考价值的读者统计数据，并成为国内中文学术期刊协会的会员。· 中国学术期刊网将在刊社每月定期提供的样刊和电子数据的基础上，为签约刊社提供刊物图文的数字化处理，以及网上空间和专业技术维护，提供刊物电子版网上结算和客户管理，并为刊社在海外进行市场推广营销，定期向签约刊社提供客户服务报告，转达读者意见，反馈市场信息。· 中国学术期刊网还将向刊社免费提供刊社独立域名和主页，免费专用期刊网信箱，便捷的信息发布平台，刊社可制作发布自身广告，提前发布下期要目。并向大家提供最新的期刊资讯和期刊网上销售报告等所有网络优势。
文渊期刊是国家级别的期刊。
《文渊》杂志是经国家新闻出版广电总局批准，由中国出版传媒股份有限公司主管、主办的国家级教育社科综合类专业学术理论期刊。
国内统一刊号：CN11-9276/G，国际标准刊号：ISSN2096-6288。
主要栏目：教育前沿、高教研究、职教论坛、基础教育、教学论坛、管理纵横、经济视角、文体艺术、图书档案、信息工程、科学论坛、理论广角等。
作者投稿《文渊》要求：
观点新颖，主题明确，层次清楚，数据准确，语言简练，体现科学性、可读性和创新性。字数以3000-10000字符(两版起)为宜。作者署名人数一般不超过3人。
第一作者须附简介(姓名、出生年月、性别、民族、籍贯、学历、职称、研究方向、地址、邮编、邮箱、电话等)，其他作者附单位、邮编。来稿用word格式排版，图表等务必准确、清晰、规范。
综述：是的。
《文渊》国家级期刊，起发字符：2500字符1版起发。
收稿范围：高教研究、职教论坛、教育时空、思政教育、英语天地、管理纵横、经济视角、文体艺术、图书档案、网络科技、信息工程、科学论坛、理论广角
简介：
《文渊》杂志是经国家广播电视总局批准，由中国出版传媒股份有限公司主管、主办的国家级文教社科综合类专业学术理论期刊。
国内统一刊号：CN 11-9276/G，国际标准刊号：ISSN 2096-6288。主要栏目设置：高教研究、职教论坛、教育时空、思政教育、英语天地、写作交流、管理纵横、经济视角、文体艺术、图书档案、网络科技、信息工程、科学论坛、理论广角等。
《文渊》好。《文渊》是国家级正规的连续电子期刊，《文渊》杂志是经国家新闻出版广电总局批准的，内容要求极高，因此《文渊》好。《文渊》是经国家新闻出版总局批准备案的文化教育社科综合类专业学术理论期刊。
还可以 我也在看。 不过我觉得比较适合高三的人看 里面讲的大多数是高三人的经验和名校介绍，还有一些作文 《求学》也分几个版本，看你买什么版本的
以下为个人推荐：文学版——（提高写作水平）《儿童文学》，不要小看了这名字，虽说名字有点那啥，但是内容非常不错，很多短篇小说都有评价，文笔有清新的，有可爱的，还有真人故事，相当不错。《读友》，这杂志就有点小说味道了，封面十分漂亮，分两版，清新版，炫动版。清新版的封面后面还有些相当唯美的图片和句子。内容也十分符合咱中学生的胃口，小说不到，言情出头。《读者》，此乃经典版，有些深奥，不过看起来是想到爽，都是小文章，大道理。还有些小笑话能放松放松，这是休息时的第一推荐。《少年文艺》，也是经典啊，比较童话，文笔挺不错，有点叶圣陶的《稻草人》的感觉。《读书与作文（东方少年）》，这个就比较小学生了，个人不是很喜欢。娱乐版——《中学生青春阅读》，强推！！！我也订了，十分惊喜啊。封面唯美，水彩风，内容也有点儿小说的感觉，而且每期还随机有小说，上次我就看到了《虹之绽》（明晓溪的），插图太爱了！好漂亮的呢！十分潮流时尚，有明星的，还有网络流行的。上次拿到的书里面就有张翰等明星的手机壳图案，超级有爱。《花火》，多为悲剧，文风相当的唯美，典型的日本风。不过是言情的。《我们爱科学》，表小看了这个名字，相当不错，有趣儿，考古的，高科技的，实验的，都有。今天才从我同学那里借了本，强推。《新蕾》，不多说，好书一本。《仙度瑞拉》，言情的，明晓溪主编。文风清新可爱，只是多为连载，不过都是好文啊。……没了，好的没了，真想不出来了，采纳吧，我可是憋了好久的，~(@^_^@)~
《求学》月刊主办单位 广西出版杂志社 编辑出版 求学编辑部 国际标准刊号 ISSN1009-864X 国内统一刊号 CN45-1273/Z 邮发代号 48-108 国外发行代号 联合征定代号 LD451273 创刊日期 2001 出刊日期 每月中旬 报刊版式 16开48页 发行方式 邮局发行
还好吧，可以先到官方网站上看看。