844S012 Datasheet, PDF (17/24 Pages) Integrated Device Technology – Crystal-to-LVDS/LVCMOS Frequency Synthesizer

844S012 Data Sheet
CRYSTAL-TO-LVDS/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% / -0.6%) from the
nominal output 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
€ Fm = Nominal Modulation Frequency (30kHz)
€  = Modulation Factor (0.6% down spread)
1
–
Fnom
+
2Fm



Fnom

t
whe n 0 
t

-----1------
2Fm
,
The 844S012 triangle modulation frequency deviation will not exceed
0.7% 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 7B. 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 7B. It
is important to note the 844S012 5dB minimum spectral reduction is
the component-specific EMI reduction, and will not necessarily be the
same as the system EMI reduction.
1 – Fnom–2Fm    Fnom  t when -----1------ t  ---1----
2Fm Fm
Fnom
(1 - δ) Fnom
0.5/fm
1/fm
Time
Figure 7A. Triangle Frequency Modulation
' – 10 dBm
B
A
➤
G = 0.6% ➔
Figure 7B. 200MHz Clock Output In Frequency Domain
(A) Spread-Spectrum OFF
(B) Spread-Spectrum ON
844S012 REVISION B 08/25/15
17
©2015 Integrated Device Technology, Inc.