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CYW181-03SXT Datasheet(PDF) 3 Page - Cypress Semiconductor |
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CYW181-03SXT Datasheet(HTML) 3 Page - Cypress Semiconductor |
3 / 9 page W181 Document #: 38-07152 Rev. *D Page 3 of 9 Because the modulating frequency is typically 1000 times slower than the fundamental clock, the spread spectrum process has little impact on system performance. Frequency Selection With SSFTG In Spread Spectrum Frequency Timing Generation, EMI reduction depends on the shape, modulation percentage, and frequency of the modulating waveform. While the shape and frequency of the modulating waveform are fixed for a given frequency, the modulation percentage may be varied. Using frequency select bits (FS1:2 pins), the frequency range can be set. Spreading percentage is set to be 1.25% or 3.75% (see Table 1). A larger spreading percentage improves EMI reduction. However, large spread percentages may either exceed system maximum frequency ratings or lower the average frequency to a point where performance is affected. For these reasons, spreading percentages between 0.5% and 2.5% are most common. Spread Spectrum Frequency Timing Generation 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 2. As shown in Figure 2, 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 3. 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. Figure 3 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. Freq. Phase Modulating VCO Post CLKOUT Detector Charge Pump Waveform Dividers Divider Feedback Divider PLL GND VDD Σ Q P Clock Input Reference Input (EMI suppressed) Figure 1. Functional Block Diagram |
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