ICS841S012DI datasheet(12/22 Pages) IDT

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ICS841S012DKI REVISION A JULY 20, 2009

ICS841S012DI Data Sheet

CRYSTAL-TO-0.7V DIFFERENTIAL HCSL/LVCMOS FREQUENCY SYNTHESIZER

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 0.6% down-spread (+0.0% / -

0.5%) from the nominal output frequency. An example of a tri-

angle frequency modulation profile is shown in

Figure 4A be-

low. The ramp profile can be expressed as:

• Fnom = Nominal Clock Frequency in Spread OFF mode

• Fm = Nominal Modulation Frequency (30kHz)

•

δ = Modulation Factor (0.6% down spread)

The ICS841S012DI triangle modulation frequency deviation

will not exceed 0.7% down-spread from the nominal clock fre-

quency (+0.0% / -0.5%). 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 4B. The ratio of this

width to the fundamental frequency is typically 0.4%, and will

not exceed 0.7%. The resulting spectral reduction will be greater

than 5dB, as shown in

Figure 4B. It is important to note the

ICS841S012DI 5dB minimum spectral reduction is the compo-

nent-specific EMI reduction, and will not necessarily be the

same as the system EMI reduction.

FIGURE 4B. 200MHZ CLOCK OUTPUT IN FREQUENCY DOMAIN

(A) SPREAD-SPECTRUM OFF (B) SPREAD-SPECTRUM ON

FIGURE 4A. TRIANGLE FREQUENCY MODULATION

(1 -

δ) fnom + 2 Fm x δ x Fnom x t when 0 < t <

(1 -

δ) fnom - 2 Fm x δ x Fnom x t when

< t <

2Fm

➤

1/fm

0.5/fm

Fnom

(1 -

δ) Fnom

Time

Δ − 10 dBm

δ = .6% ➤

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