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ICS841484I Data Sheet

FEMTOCLOCK® CRYSTAL-TO-0.7V DIFFERENTIAL HCSL CLOCK GENERATOR

ICS841484DKI REVISION A NOVEMBER 7, 2012

Spread Spectrum

Spread-spectrum clocking is a frequency modulation technique for

EMI reduction. When spread-spectrum is enabled, a 32kHz triangle

waveform is used with 0.5% down-spread (+0.0% / -0.5%) from the

nominal output frequency. An example of a triangle frequency

modulation profile is shown in Figure 1A below. The ramp profile can

be expressed as:

Fnom = Nominal Clock Frequency in Spread Off mode

(200MHz with 25MHz IN)

Fm = Nominal Modulation Frequency (32kHz)

 = Modulation Factor (0.5% down spread)

(1 –

) Fnom + 2 fm x x Fnom x t when 0 < t < 1

2fm

(1 –

) Fnom + 2 fm x x Fnom x t when 1 < t < 1

2fm

An example of the amount of down spread relative to the nominal

clock frequency can be seen in the frequency domain, as shown in

Figure 1B. The ratio of this width to the fundamental frequency is

typically 0.5%. The resulting spectral reduction is typically 10dB, as

shown in Figure 1B. It is important to note the ICS841484 10dB

typical spectral reduction is the component-specific EMI reduction,

and will not necessarily be the same as the system EMI reduction.

Figure 1A. Triangle Frequency Modulation

Figure 1B. 200MHz Clock Output In Frequency Domain

(A) Spread-Spectrum OFF

(B) Spread-Spectrum ON

➤

1/fm

0.5/fm

Fnom

(1 -

δ) Fnom

Time

 – 10 dBm

 = 0.5%

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