W233 Cypress Semiconductor Corp., W233 Datasheet - Page 5

• Manufacturer Part Number: W233
• Description: Spread Spectrum FTG For Via Mobile K7 Chipset
• Manufacturer: Cypress Semiconductor Corp.

Spread Spectrum Frequency Timing 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 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:
Document #: 38-07250 Rev. **
dB = 6.5 + 9*log
Figure 3. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
MAX (0%)
MIN (–0.5%)
SSFTG
10
(P) + 9*log
Frequency Span (MHz)
Center Spread
10
(F)
PRELIMINARY
Typical Clock
Figure 4. Typical Modulation Profile
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 5. Figure 4
details the Cypress spreading pattern. Cypress does offer op-
tions 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 se-
lecting the appropriate data bytes of the SMBus data stream.
Refer to Table 5 for more details.
Spread
Spectrum
Enabled
Frequency Span (MHz)
EMI Reduction
Down Spread
Speactrum
Spread
Non-
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