Spread Spectrum Clocking; Demonstration Of Spread Spectrum Clocking (Ssc) - Intel Quark SoC X1000 Design Manual

Electromagnetic Interference—Intel
means that the signal quality must be checked for low-speed, full-speed, and high-
speed USB operation.
• Further common mode choke information can be found on the high-speed USB
Platform Design Guides available at www.usb.org.
17.4.7

Spread Spectrum Clocking

Although clock noise is inherently narrow band in nature, there are cases where the
clocks are deliberately modulated or "spread". The most common reason for doing this
is to reduce spectral peaks that might otherwise violate FCC and CE requirements for
unintentional (EMI) radiation.
The spread spectrum clocking technique reduces radiated emissions by spreading the
emissions over a wider frequency band. This band can be broadened, with subsequent
reductions in the measured radiation levels, by slowly frequency modulating the clock
over a few hundred kHz. Thus, instead of maintaining a constant system frequency,
SSC modulates the clock frequency/period along a predetermined path (i.e.,
modulation profile) with a predetermined modulation frequency. The modulation
frequency is usually selected to be larger than 30 KHz (above the audio band) while
small enough not to upset the PC system's timings.
Figure 70.

Demonstration of Spread Spectrum Clocking (SSC)

Although the spread spectrum clocking (SSC) technique has been demonstrated to
reduce peak radiation by approximately 8 dB and are widely applied in clocking system,
clocks are still key radiation sources that cause EMI. Designers must not assume SSC
clocking will eliminate EMI problems especially if the technique is not implemented
correctly.
Radiated emissions are typically confined in a narrow band centered around clock
frequency harmonics. By uniformly distributing the radiation over a band of a few MHz,
regulatory measurement levels (in a 120 kHz bandwidth at frequencies below 1 GHz
and in a 1 MHz bandwidth at frequencies above 1 GHz) will be reduced.
To conserve the minimum period requirement for bus timing, the SSC clock is
modulated between f
constant-frequency clock;
percentage of f
June 2014
Order Number: 330258-002US
®
Quark™ SoC X1000
H igh est peak
SSC
and ( )f


1
nom
 specifies the total amount of spreading as a relative
.
nom

non-SSC
 of f
nom
f
nom
where f is the nominal frequency for a
nom nom
®
Intel Quark™ SoC X1000
PDG
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