SSC展频

marsman_123

Clock modulation, also known as Spread Spectrum, began to appear in computer system designs as early as 1995. Most personal computers designed today use Spread Spectrum
technologgy to reduce EMI. Spread Spectrum provides a cost effective method to keep EMI low.
Clock modulation is a technique whereby an input reference clock at some frequency is
modulated such that the frequency of the output clock varies slightly. For example, a
40 MHz reference clock with Spread Spectrum applied can produce an output swing from
39.60 MHz to 40.40 MHz. This would represent a Spread Spectrum clock that has a 2%
bandwidth centered on the reference frequency of 40 MHz. The purpose of modulating
the frequency of a clock is to distribute the energy of a single or narrow band over amuch wider band of frequencies. This reduces the amount of peak energy at any one frequency in the spectrum. The amount of EMI reduction is affected by the modulation profile, the percentage of frequency variation (bandwidth), and the modulation rate.