
ICS84314-02
700MHZ, CRYSTAL-TO-3.3V/2.5V LVPECL FREQUENCY SYNTHESIZER W/FANOUT BUFFER
IDT / ICS 3.3V/2.5V LVPECL FREQUENCY SYNTHESIZER
13
ICS84314AY-02 REV. A MARCH 24, 2009
Spread Spectrum
Spread-spectrum clocking is a frequency modulation technique for
EMI reduction. When spread-spectrum is enabled, a 30kHz
triangle waveform is used with 0.6% down-spread (+0.0% / -0.6%)
from the nominal 100MHz clock frequency. An example of a
triangle frequency modulation profile is shown in Figure 5A below.
The ramp profile can be expressed as:
The ICS84314-02 triangle modulation frequency deviation will not
exceed 0.8% down-spread from the nominal clock frequency
(+0.0% / -0.6%). 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 5B. The ratio of this width to the
fundamental frequency is typically 0.6%, and will not exceed 0.8%.
The resulting spectral reduction will be greater than 7dB, as shown
in Figure 5B. It is important to note the ICS84314-02 7dB minimum
spectral reduction is the component-specific EMI reduction, and
will not necessarily be the same as the system EMI reduction.
Fnom = Nominal Clock Frequency in Spread Off mode
(100MHz with 20MHz IN)
Fm = Nominal Modulation Frequency (30kHz)
δ = Modulation Factor (0.6% down spread)
,
Figure 5A. Triangle Frequency Modulation
Figure 5B. 100MHz Clock Output In Frequency Domain
(A) Spread-Spectrum OFF
(B) Spread-Spectrum ON
1
δ
–
()Fnom 2fm δ Fnom t
×
when0 t
1
2fm
----------
<<
+
1
δ
–
()Fnom 2
– fm
δ Fnom t
×
when
1
2fm
---------- t
1
fm
-------
<<
1/fm
0.5/fm
Fnom
(1 -
δ) Fnom
B
A
– 10 dBm
δ = 0.6%