841S104EGILF Datasheet, PDF (17/22 Pages) Integrated Device Technology – Crystal-to-HCSL 100MHz PCI ExpressTM Clock Synthesizer

ICS841S104I Data Sheet
CRYSTAL-TO-HCSL 100MHZ PCI EXPRESS^TM CLOCK 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 0.5% down-spread from the nominal 100MHz
clock frequency. An example of a triangle frequency modulation
profile is shown in Figure 6A below.
The ICS841S104I triangle modulation frequency deviation is 0.5%
down-spread from the nominal clock frequency. 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 6B. The ratio of
this difference to the fundamental frequency is typically 0.5%. The
resulting spectral reduction will be greater than 7dB, as shown in
Figure 2B. It is important to note the ICS841S104I 7dB minimum
spectral reduction is the component-specific EMI reduction, and will
not necessarily be the same as the system EMI reduction.
Fnom
(1 - δ) Fnom
0.5/fm
1/fm
➤
Figure 6A. Triangle Frequency Modulation
∆ – 7dBm
B
A
➔
δ = 0.25%
Figure 6B. 100MHz Clock Output In Frequency Domain
(A) Spread-Spectrum OFF
(B) Spread-Spectrum ON
ICS841S104EGI REVISION A JUNE 18, 2010
17
©2010 Integrated Device Technology, Inc.