ICS84314-02 Datasheet, PDF (14/17 Pages) Integrated Circuit Systems – 700MHZ, CRYSTAL-TO-3.3V/2.5V LVPECL FREQUENCY SYNTHESIZER W/FANOUT BUFFER

Integrated
Circuit
Systems, Inc.
PRELIMINARY
ICS84314-02
700MHZ, CRYSTAL-TO-3.3V/2.5V LVPECL
FREQUENCY SYNTHESIZER W/FANOUT BUFFER
SPREAD SPECTRUM
Spread-spectrum clocking is a frequency modulation tech-
nique for EMI reduction. When spread-spectrum is enabled,
a 30kHz triangle waveform is used with 0.5% down-spread
(+0.0% / -0.5%) from the nominal 200MHz clock frequency.
An example of a triangle frequency modulation profile is shown
in Figure 7A below. The ramp profile can be expressed as:
• Fnom = Nominal Clock Frequency in Spread OFF mode
(200MHz with 16MHz IN)
• Fm = Nominal Modulation Frequency (30kHz)
• δ = Modulation Factor (0.5% down spread)
(1
-
δ)
fnom
+
2
fm
x
δ
x
fnom
x
t
when
0
<
t
<
2
1
fm
,
(1
-
δ)
fnom
-
2
fm
x
δ
x
fnom
x
t
when2
1
fm
<
t
<
1
fm
The ICS84314-02 triangle modulation frequency deviation will
not exceed 0.6% 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 7B. The ratio of
this width to the fundamental frequency is typically 0.4%,
and will not exceed 0.6%. The resulting spectral reduction
will be greater than 7dB, as shown in Figure 7B. It is impor-
tant to note the ICS84314-02 7dB minimum spectral reduc-
tion is the component-specific EMI reduction, and will not
necessarily be the same as the system EMI reduction.
Fnom
Δ − 10 dBm
B
A
(1 - δ) Fnom
0.5/fm
1/fm
FIGURE 6A. TRIANGLE FREQUENCY MODULATION
δ = 0.4% ➤
➤
FIGURE 6B. 200MHZ CLOCK OUTPUT IN FREQUENCY DOMAIN
(A) SPREAD-SPECTRUM OFF
(B) SPREAD-SPECTRUM ON
84314AY-02
www.icst.com/products/hiperclocks.html
14
REV. B NOVEMBER 17, 2005