A description of spread spectrum clocking for WD SATA drives

Description of Spread Spectrum Clocking:

Computer electronics utilize “clocks” that alternate electricity to synchronize bits of data as they move from one circuit in a device to another. Similar to radio waves, these “clocks” have a certain frequency of energy radiated that can interfere with another device in the vicinity. This radiation is referred to as electromagnetic interference, or EMI.

The Federal Communications Commission (FCC) regulates broadcast emissions, such as radio and TV, as well as EMI emissions that may interfere with them. Computer and other electronic devices must meet certain requirements to limit the level of EMI they radiate. The design of a device must be changed to meet these requirements or be in violation of FCC regulations. The concept of spread-spectrum clocking is gaining popularity as a simple way to lower EMI, which is becoming more severe as processors and their “clocks” increase in speed. This method is an alternative to major design changes, which may be costly and require limiting the performance of a device. Spread-spectrum clocking speeds up and slows down the clock within a few percent of its target frequency, thus flattening out the EMI peak by spreading it across a range of frequencies.

Fluctuating the clocking speed must be tolerated by other circuits and devices involved, otherwise the devices lose their synchronization and may not communicate properly. As long as the devices are synchronized, they will communicate properly without it being necessary to lower the clocking frequency, which in turn lowers performance.

The effect of Spread Spectrum Clocking on drive/computer performance:

Spread-spectrum clocking does nothing more than help a device lower its emissions to meet FCC regulations. A computer may not need spread-spectrum clocking enabled to pass FCC regulations. Having spread spectrum clocking enabled does not affect performance. However, it requires that other devices involved tolerate fluctuating the clocking. If your Serial ATA controller or controller card do not support SSC, you should leave the jumper shunt on your WD Serial ATA drive in the SSC disabled position.

For more information on how to enable and disable spread spectrum clocking on your WD Serial ATA hard drive, please see Answer ID 1271: How to enable and disable SSC (spread spectrum clocking) on WD Serial ATA hard drives

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