【正文】 es from the fact that the carrier signals occur over the full bandwidth (spectrum) of a device39。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。 ...just using spreadspectrum radio itself is not sufficient for munications security ? Resistance to fading. The high bandwidth occupied by spreadspectrum signals offer some frequency diversity, . it is unlikely that the signal would encounter severe multipath fading over its whole bandwidth, and in other cases the signal can be detected using . a Rake receiver. ? Multiple access capability. Multiple users can transmit simultaneously on the same frequency (range) as long as they use different spreading codes. See CDMA. Spreadspectrum clock signal generation Spreadspectrum clock generation (SSCG) is used in some synchronous digital systems, especially 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 making sense of it. What39。s radio navigation system for navigation of aircraft for crop dusting and similar applications, and Qualm39。s book was the first prehensive unclassified review of the technology and set the stage for increasing research into mercial applications. Initial mercial use of spread spectrum began in the 1980s in the US with three systems: Equatorial Communications System39。s existence did not bee known until the 1980s. The most celebrated invention of frequency hopping was that of actress Hedy Lamarr and poser Gee Antheil, who in 1942 received . Patent 2,292,387 for their Secret 11 Communications System. Lamarr had learned about the problem at defense meetings she had attended with her former husband Friedrich Mandl, who was an Austrian arms manufacturer. The AntheilLamarr version of frequency hopping used a pianoroll to change among 88 frequencies, and was intended to make radioguided torpedoes harder for enemies to detect or to jam. The patent came to light during patent searches in the 1950s when ITT Corporation and other private firms began to develop Code Division Multiple Access (CDMA), a civilian form of spread spectrum, though the Lamarr patent had no direct impact on subsequent technology. It was in fact ongoing military research at MIT Lincoln Laboratory, Magnavox Government amp。s book Wireless Telegraphy (German, 1908, English translation McGraw Hill, 1915), although Zenneck himself states that Telefunken had already tried it several years earlier. Zenneck39。 10 Spread Spectrum Techniques By Wikipedia Abstract: Spreadspectrum techniques are methods by which a signal (. an electrical, electromagic, or acoustic signal ) generated in a particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a wider bandwidth. These techniques are used for a variety of reasons, including the establishment of secure munications, increasing resistance to natural interference and jamming, to prevent detection, and to limit power flux density (. in satellite downlinks). History Frequency hopping: The concept of frequency hopping was first alluded to in the 1903 . Patent 723,188 and . Patent 725,605 filed by Nikola