W149 datasheet(4/14 Pages) CYPRESS

4 / 14 page

W149

Spread Spectrum Clocking

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 am-

plitudes of the radiated electromagnetic emissions are re-

duced. This effect is depicted in Figure 3.

As shown in Figure 3, 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

! is the percentage of deviation and is the frequency

in MHz where the reduction is measured.

The output clock is modulated with a waveform depicted in

Figure 4. 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 either –0.5% or ±0.5% of the

selected frequency. Figure 4 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 cannot be deactivated on the

W149.

Figure 3. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation

SSFTG

Typical Clock

Frequency Span (MHz)

Center Spread

MAX (+0.5%)

MIN (–0.5%)

Figure 4. Typical Modulation Profile

W149 Datasheet(PDF) 4 Page - Cypress Semiconductor