Spread-spectrum clock generation is use to reduce the spectral density of the electromagnetic interference (EMI) that 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. Many personal computers have a BIOS setting to turn spread-spectrum clocking on or off. It is important to note that this method does not reduce the peak electrical or magnetic field strength emitted by the system, nor the total energy, and therefore does not make the system any less likely to interfere with sensitive equipment such as TV and radio receivers. It works because the EMI receivers used by EMC testing laboratories divide the electromagnetic spectrum into frequency bands approximately 120 kHz wide. If the system under test were to radiate all of its energy at one frequency, then this energy would fall into a single frequency band of the receiver, which would register a large peak at that frequency. Spread-spectrum clocking distributes the energy so that it falls into a large number of the receiver's frequency bands, without putting enough energy into any one band to exceed the statutory limits. Even more problematic, the existence of spread spectrum clock generation in PCs may substantially increase the radiated EMI. For this reason, the FCC certification testing is done with the spread spectrum function enabled, but the preferred BIOS settings for actual use typically turn off the spread spectrum feature. Thus, the PC may emit up to 20 dB higher EMI than permitted by the FCC Part 15 rules. This loophole exists because the BIOS writers include the ability to disable spread spectrum clock generation as a user setting, thereby defeating the object of the EMI regulations.
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