SI5335 Datasheet, PDF (33/47 Pages) Silicon Laboratories – WEB-CUSTOMIZABLE, ANY-FREQUENCY, ANY-OUTPUT QUAD CLOCK GENERATOR/BUFFER

Si5335
5. Spread Spectrum
To help reduce electromagnetic interference (EMI), the Si5335 supports spread spectrum modulation in clock
generator mode only. The output clock frequencies can be modulated to spread energy across a broader range of
frequencies, lowering system EMI. Spread spectrum modulation is generated digitally in the output MultiSynth
dividers, which means that the spread spectrum parameters are virtually independent of process, voltage, and
temperature variations.
If the SSENB function is assigned to a pin in ClockBuilder and asserted (driven low), PCIe-compliant spread
spectrum is applied to all 100 MHz output clocks with a default spreading rate of 31.5 kHz and 0.5% down spread.
If no 100 MHz output clocks are defined but the SSENB is assigned and asserted, none of the output clocks will
have spread spectrum clocking applied. Some custom spread-spectrum clocking profiles are available. If the
Si5335's default PCIe spread spectrum profile is not suitable for your application, submit your custom spread
spectrum requirements for review by visiting the Silicon Labs Technical Support web page at https://
www.silabs.com/support/pages/contacttechnicalsupport.aspx, or contact your local Silicon Labs sales
representative for more information.
Reduced
Amplitude and
EMI
Clock with
SSC Off
Clock with
SSC On
(downspread)
Carrier Frequency
f
Figure 23. Spread Spectrum Clocking Impact on Output Power Spectrum
Rev. 1.4
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