【導讀】師的指導下進行的研究工作及取得的成果。盡我所知，除文中特別?；?qū)W歷而使用過的材料。對本研究提供過幫助和做出過貢獻的個人?；蚣w，均已在文中作了明確的說明并表示了謝意。部分或全部內(nèi)容。研究所取得的研究成果。本人完全意識到本聲明的法律后果由本人承擔。子版，允許論文被查閱和借閱。本人授權大學可以將。影印、縮印或掃描等復制手段保存和匯編本學位論文。涉密論文按學校規(guī)定處理。意傳播，從而導致更大帶寬的信號的方法。斯拉在1900年7月提出的。特斯拉想出了這個想法后，在1898年時展示了世界上第。號控制船是安全的需要。兩個功能通過改變載波頻率或其他專用特征的干擾免疫。第一次在為使控制電路發(fā)射。整，必須在作出回應。第二個技術使用由預定的方式更改傳輸?shù)念l率的一個編碼輪控。麥克勞希爾，1915年)，雖然Zenneck自己指出德律風根在早幾年已經(jīng)試過它。工程師，在1929年提出了這個想法。其他幾個專利被帶到了20

　　

【正文】 ly those containing microprocessors, to reduce the spectral density of the electromagic interference (EMI) that these systems generate. A synchronous digital system is one that is driven by a clock signal and because of its periodic nature, has an unavoidably narrow frequency spectrum. In fact, a perfect clock signal would have all its energy concentrated at a single frequency and its harmonics, and would therefore radiate energy with an infinite spectral density. Practical synchronous digital systems radiate electromagic energy on a number of narrow bands spread on the clock frequency and its harmonics, resulting in a frequency spectrum that, at certain frequencies, can exceed the regulatory limits for electromagic interference (. those of the FCC in the United States, JEITA in Japan and the IEC in Europe). To avoid this problem, which is of great mercial importance to manufacturers, 14 spreadspectrum clocking is used. This consists of using one of the methods described in the Spreadspectrum telemunications section in order to reduce the peak radiated energy. The technique therefore reshapes the system39。s electromagic emissions to ply with the electromagic patibility (EMC) regulations. It is a popular technique because it can be used to gain regulatory approval with only a simple modification to the equipment. Spreadspectrum clocking has bee more popular in portable electronics devices because of faster clock speeds and the increasing integration of highresolution LCD displays in smaller and smaller devices. Because these devices are designed to be lightweight and inexpensive, passive EMI reduction measures such as capacitors or metal shielding are not a viable option. Active EMI reduction techniques such as spreadspectrum clocking are necessary in these cases, but can also create challenges for designers. Principal among these is the risk that modifying the system clock runs the risk of the clock/data misalignment. It is important to note that this method does not reduce the total energy radiated by the system, and therefore does not necessarily make the system less likely to cause interference. Spreading the energy over a large frequency band effectively reduces the electrical and magic field strengths that are measured within a narrow window of frequencies. Spreadspectrum clocking works because the measuring receivers used by EMC testing laboratories divide the electromagic spectrum into frequency bands approximately 120 kHz the system under test were to radiate all of its energy at one frequency, it would register a large peak at the monitored frequency band. Spreadspectrum clocking distributes the energy so that it falls into a large number of the receiver39。s frequency bands, without putting enough energy into any one band to exceed the statutory limits. The usefulness of spreadspectrum clocking as a method of actually reducing interference is often debated, but it is probable that some electronic equipment with sensitivity to a narrow band of frequencies will experience less interference, while other equipment with broadband sensitivity will experience more interference. Spread spectrum of a modern switching power supply (heating up period) incl. waterfall diagram over a few minutes. Recorded with a NF5030 EMCAnalyzer 15 FCC certification testing is often pleted with the spreadspectrum function enabled in order to reduce the measured emissions to within acceptable legal limits. However, some BIOS writers include the ability to disable spreadspectrum clock generation as a user setting, thereby defeating the object of the EMI regulations. This may be considered a loophole, but is generally overlooked as long as the default BIOS setting provided by the manufacturer has the spreadspectrum feature enabled. An ability to disable spreadspectrum clocking for puter systems is considered useful as the spreadspectrum techniques used can affect the maximum clockspeed achievable by the ponents involved due to clock skew, affecting overclocking efforts. Main techniques: Directsequence spread spectrum In telemunications, directsequence spread spectrum (DSSS) is a modulation technique. As with other spread spectrum technologies, the transmitted signal takes up more bandwidth than the information signal that is being modulated. The name 39。spread spectrum39。 es from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device39。s transmitting frequency. Features phasemodulates a sine wave pseudorandomly with a continuous string of pseudonoise (PN) code symbols called chips, each of which has a much shorter duration than an information bit. That is, each information bit is modulated by a sequence of much faster chips. Therefore, the chip rate is much higher than the information signal bit rate. 2. It uses a signal structure in which the sequence of chips produced by the transmitter is known a priori by the receiver. The receiver can then use the same PN sequence to counteract the effect of the PN sequence on the received signal in order to reconstruct the information signal. Transmission method Directsequence spreadspectrum transmissions multiply the data being transmitted by a noise signal. This noise signal is a pseudorandom sequence of 1 and ?1 values, at a frequency much higher than that of the original signal, thereby spreading the energy of the original signal into a much wider band. The resulting signal resembles white noise, like an audio recording of static. However, this noiselike signal can be used to exactly reconstruct the original data at the receiving end, by multiplying it by the same pseudorandom sequence (because 1 1 = 1, and ?1 ?1 = 1). This process, known as despreading, mathematically constitutes a 16 correlation of the transmitted PN sequence w