W158

Rev 1.0, November 21, 2006

Page 3 of 12

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 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:

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% downspread. Figure

2 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.

Figure 1. Clock Harmonic with and without SSCG Modulation Frequency Domain Representation

100%

60%

20%

80%

40%

0%

–20%

–40%

–60%

–80%

–100%

Time

Figure 2. Modulation Waveform Profile