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 ampli-
tudes of the radiated electromagnetic emissions are reduced.
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
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 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.
SSFTG
Typical Clock
Amplitude (dB)
Frequency Span (MHz)
Center Spread
Figure 3. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX (+0.5%)
FREQUENCY
10%
20%
30%
40%
50%
60%
70%
80%
90%
10%
20%
30%
40%
50%
60%
70%
80%
100%
90%
MIN (–0.5%)
Figure 4. Typical Modulation Profile
Rev 1.0, November 21, 2006
100%
Page 4 of 15
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