W48S87-04 Datasheet, PDF (6/21 Pages) Cypress Semiconductor – Spread Spectrum 3 DIMM Desktop Clock

PRELIMINARY
W48S87-04
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 am-
plitudes of the radiated electromagnetic emissions are re-
duced. This effect is depicted in Figure 3.
As depicted 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 num-
ber 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.
5dB/div
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 ±0.5% of the center frequen-
cy. Figure 4 details the Cypress spreading pattern. Cypress
does offer options with more spread and greater EMI reduc-
tion. Contact your local Sales representative for details on
these devices.
Spread Spectrum clocking is activated or deactivated by se-
lecting the appropriate values for bits 1–0 in data byte 0 of the
I²C data stream. Refer to Table 4 for more details.
SSFTG
Typical Clock
-SS% ^{Frequency Span (MHz)} +SS%
+1.0
Figure 3. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX (+.0.5%)
MIN. (–0.5%)
Figure 4. Typical Modulation Profile
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