W254B Datasheet, PDF (8/16 Pages) SpectraLinear Inc – 133MHz Spread Spectrum FTG for Mobile Pentium® III Platforms

W254B
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 7.
As shown in Figure 7, 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 8. 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% or –0.5% of the
selected frequency. Figure 8 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 value for bit 3 in data byte 0 of the
SMBus data stream. Refer to page 10 for more details.
SSFTG
Typical Clock
EMI Reduction
Spread
Spectrum
Enabled
Non-
Spread
Spectrum
Frequency Span (MHz)
Center Spread
Frequency Span (MHz)
Figure 7. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX.
MIN.
Rev 1.0, November 20, 2006
Figure 8. Typical Modulation Profile
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