6.6 Programming for Spread Spectrum
Spread spectrum clocking (SSC) is available only on the multisynth outputs. Each multisynth can implement spread spectrum in either
the main divider or the backup divider (the FS option). Therefore, the user can program a maximum of four different spread spectrum
“profiles” from the same part, although only two profile are available on outputs at any given time. The amplitude of the SSC clock
frequency (as illustrated in
Figure 6.2 Illustration: Center and Down Spread SSC Clocks as Frequency vs Time Plots on page 21
) is
denoted by ssc%. The variable, Amod, in the equation below is a real number representation of the ssc%, which is a percentage value.
The modulation rate (also illustrated in
Figure 6.2 Illustration: Center and Down Spread SSC Clocks as Frequency vs Time Plots on
) is denoted by Fmod in the equations below.
A
mod = {
{
ssc % × 2}
100 for center spread
ssc %
100 for down spread
}
idxy
_
ss
_
step
_
num
=
{
vcoFreq
idxy
}
Fmod × 4
idxy
_
ss
_
step
_
res
=
{
Amod × idxy_den × idxy × 128}
2 ×
idxy_ss_step_num
time
frequency
F0
Fmax = F0 (1 + ssc%/200)
Fmin = F0 (1 - ssc%/200)
Fmod = one modulation cycle
Fmod = one modulation cycle
F0
frequency
time
Fmin = F0 (1 - ssc%/100)
Figure 6.2. Illustration: Center and Down Spread SSC Clocks as Frequency vs Time Plots
The table below shows the register fields (and terms)
idxy_ss_step_num
and
idxy_ss_step_res
.
idxy_ss_step_num
is the number of
frequency steps between the mean and the maximum/minimum frequencies in SSC clocking and
idxy_ss_step_res
is the frequency
resolution that is required in each step. The goal is to maximize the number of steps and minimize the resolution. However, the number
of steps is set by the modulation rate (typically 30–33 kHz). The step resolution can be minimized by setting the largest value possible
for
idxy_den
.
Idxy_den
is the denominator of the
id
divider and setting it as close as possible to 2
15
– 1 is desired.
Si5357 Reference Manual • Programming the Volatile Memory
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [email protected] • www.skyworksinc.com
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Rev. 0.2 • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice • July 26, 2021
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