【正文】 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 EMCAnalyzerFCC 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 spectrumIn 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 methodDirectsequence 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 correlation of the transmitted PN sequence with the PN sequence that the receiver believes the transmitter is using.For despreading to work correctly, the transmit and receive sequences must be synchronized. This requires the receiver to synchronize its sequence with the transmitter39。s sequence via some sort of timing search process. However, this apparent drawback can 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。s position if the transmitters39。 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 different 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 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 spectrumFrequencyhopping spread spectrum (FHSS) is a method of t