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W159B Datasheet(PDF) 3 Page - SpectraLinear Inc |
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W159B Datasheet(HTML) 3 Page - SpectraLinear Inc |
3 / 10 page W159B Rev 1.0, November 21, 2006 Page 3 of 10 system timing margins. The use of spread spectrum modulation is controlled by an external signal input. The W159B also includes power management control inputs. By using these inputs, system logic can stop CPU and/or PCI clocks or power down the entire device to conserve system power. Spread Spectrum Generator The device generates a clock that is frequency modulated in order to increase the bandwidth that it occupies. By increasing the bandwidth of the fundamental and its harmonics, the ampli- tudes of the radiated electromagnetic emissions are reduced. This effect is depicted in Figure 1. As shown in Figure 1, a harmonic of a modulated clock has a much lower amplitude than that of an unmodulated signal. The reduction in amplitude is dependent on the harmonic number and the frequency deviation or spread. The equation for the reduction is dB = 6.5 + 9*log10(P) + 9*log10(F) Where P is the percentage of deviation and F is the frequency in MHz where the reduction is measured. The output clock is modulated with a waveform depicted in Figure 2. This waveform, as discussed in “Spread Spectrum Clock Generation for the Reduction of Radiated Emissions” by Bush, Fessler, and Hardin produces the maximum reduction in the amplitude of radiated electromagnetic emissions. The deviation selected for this chip is –0.5% downspread. Figure 2 details the Cypress spreading pattern. Cypress does offer options with more spread and greater EMI reduction. Contact your local Sales representative for details on these devices. Spread Spectrum clocking is activated or deactivated by selecting the appropriate values for the SPREAD# input pin. Figure 1. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation Spread Spectrum Enabled Highest Peak Spread Spectrum Non- 100% 60% 20% 80% 40% 0% –20% –40% –60% –80% –100% Time Figure 2. Modulation Waveform Profile |
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