PRELIMINARY
W150
Document #: 38-07177 Rev. *B
Page 4 of 15
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 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 specified in Table 6. 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 is activated or deactivated by
selecting the appropriate values for bits 1–0 in data byte 0 of
the SMBus data stream. Refer to Table 7 for more details.
Power-on
Reset
Timer
Output Three-state
Data
Latch
Hold
QD
W150
VDD
Clock Load
R
10 k
Ω
Output
Buffer
Output
Low
Output Strapping Resistor
Series Termination Resistor
Jumper Options
Resistor Value R
Figure 2. Input Logic Selection Through Jumper Option
Figure 3. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
SSFTG
Typical Clock
Frequency Span (MHz)
–1.0
+1.0
0
–0.5%
–SS%
+SS%
5 dB/div
+0.5%
Russian ▼
English
Chinese
German
Japanese
Russian
Korean
Spanish
French
Italian
Portuguese
Polish
Vietnamese
Indian
Mexican
British
New Zealand
