【正文】 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 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。 es from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device39。s sequence via some sort of timing search process. However, this apparent drawback c an be a significant benefit: if the sequences of multiple transmitters are synchronized with each other, the relative synchronizations the receiver must make between them can be used to determine relative timing, which, in turn, can be used to calculate the receiver39。 positions are known. This is the basis for many satellite navigation systems. The resulting effect of enhancing signal to noise ratio on the channel is called process gain. This effect can be made larger by employing a longer PN sequence and more chips per bit, but physical devices used to generate the PN sequence impose practical limits on attainable processing gain. If an undesired transmitter transmits on the same channel but with a differe nt PN sequence (or no sequence at all), the despreading process results in no processing gain for that signal. This effect is the basis for the code division multiple access (CDMA) property of DSSS, which allows multiple transmitters to share the same channel within the limits of the crosscorrelation properties of their PN sequences. As this description suggests, a plot of the transmitted waveform has a roughly bellshaped envelope centered on the carrier frequency, just like a normal AM transmission, except that the added noise causes the distribution to be much wider than that of an AM transmission. In contrast, frequencyhopping spread spectrum pseudorandomly retunes the carrier, instead of adding pseudorandom noise to the data, which results in a uniform frequency distribution whose width is determined by the output range of the pseudorandom number generator. Benefits ? Resistance to intended or unintended jamming ? Sharing of a single channel among multiple users ? Reduced signal/backgroundnoise level hampers interception (stealth) ? Determination of relative timing between transmitter and receiver Uses ? The United States GPS and European Galileo satellite navigation systems ? DSCDMA (DirectSequence Code Division Multiple Access) is a multiple access scheme based on DSSS, by spreading the signals from/to different users with 17 different codes. It is the most widely used type of CDMA. ? Cordless phones operating in the 900 MHz, GHz and GHz bands ? IEEE GHz WiFi, and its predecessor . (Their successor uses OFDM instead) ? Automatic meter reading ? IEEE (used . as PHY and MAC layer for ZigBee) Frequencyhopping spread spectrum Frequencyhopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. It is utilized as a multiple access method in the frequencyhopping code division multiple access (FHCDMA) scheme. A spreadspectrum transmission offers three main advantages over a fixedfrequency transmission: 1. Spreadspectrum signals are highly resistant to narrowband interference. The process of recollecting a spread signal spreads out the interfering signal, causing it to recede into the background. 2. Spreadspectrum signals are difficult to intercept. An FHSS signal simply a