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Spread-Spectrum Clocking
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Overview
Spread-spectrum
clocking is a fundamental way that electronic devices can contain
oscillators but not produce more electromagnetic interference than
allotted by the Federal Communications Commission (FCC).
What is Spread-Spectrum Clocking?
The history of spread-spectrum clocking begins with the fundamentals
of electronic devices. Most electronic devices today are equipped with
a type of local oscillator that continuously generates a clock signal
for other synchronous components to use. An example of this type of
clock signal in the frequency domain is shown in Figure 1.
Figure 1. Simulated Clean Oscillator Signal From an Electronic
Device.
Most electrical components that use this type of oscillator can cause
this tone to be transmitted in the form of electromagnetic interference
(EMI) to other devices in the area. To prevent this interference, the
FCC in 1975 enacted FCC Part 15 that regulates the output power of
these clocks in electronic devices.
Engineers needed a method to keep their devices from overstepping this
limit, so they started using spread-spectrum clocking in their devices.
In theory, spread-spectrum clocking means that the tone is varied so
that the clock signal varies continuously around the desired frequency.
For example, for a 1 GHz processor, the frequency might be 999.5 Mhz at
one moment in time and 1.0005 GHz at another. Doing this constantly
causes the power of the tone to be "spread" out more over a broader
band of tight frequencies centered at the desired tone.
Figure 2. Simulated Spread-Spectrum Oscillator Signal.
As you can see, the clock signal falls below the FCC standard and can
be used in modern electronic systems with regulated EMI.
Enabling/Disabling Spread-Spectrum Clocking in the System BIOS
Some BIOS configurations allow you to enable or disable spread-spectrum
clocking. As seen in the following figure, enabling or disabling this
feature is as simple as setting the appropriate property to Enabled/Disabled.
Figure 3. BIOS Screen to Enable/Disable Spread-Spectrum ClockingUse with National Instruments Products
The user documentation for many National Instruments devices
explains how to enable or disable spread-spectrum clocking so that the
product meets or exceeds FCC regulations. Refer to your device
documentation for device-specific configuration settings.
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