W137
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
amplitudes 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*log
10
(P) + 9*log
10
(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 –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 is activated or deactivated through
I/O pin #26.
Figure 3. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation
Figure 4. Typical Modulation Profile
Rev 1.0, November 24, 2006
Page 4 of 8
SPECTRALINEAR [ SPECTRALINEAR INC ]
