Spread-Spectrum Clocking

Publish Date: Nov 02, 2009 | 748 Ratings | 4.73 out of 5 |  PDF

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).

Table of Contents

  1. What is Spread-Spectrum Clocking?
  2. Enabling/Disabling Spread-Spectrum Clocking in the System BIOS
  3. Use with National Instruments Products

1. 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.

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2. 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 Clocking

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3. Use 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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