W48C111-17
PRELIMINARY
3
Spread Spectrum Feature
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 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% of the center frequen-
cy. Figure 2 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
Enabled
EM I R eduction
S pread
S pectrum
Non-
Figure 1. Typical Clock and SSFTG Comparison
MAX (+0.5%)
MIN (–0.5%)
Figure 2. Typical Modulation Profile
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