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WTF is the CPU Spread Spectrum



I have a Tyan S1854 Trinity 400 and I see in my bios there is an option to set the FSB with the Spread Spectrum on or off. In the mobo manual it says "disabling the Spread Spectrum will lower EMI levels. I'm just wondering wtf the spread spectrum is.


Beware the G-Man
Political User
Read this:

What would you say if I told you their was a way to squeeze a few more megahertz out of that CPU of yours, even thought your at the maximum front side bus supported by your motherboard. Or maybe you’re not even at the maximum front side bus; maybe you just can’t seem to make the next step that the clock generator supports. Even if you have a motherboard with stepless FSB selection, this tweak could work for you. I have known about this sweet little tweak for years, and yet have never read an article anywhere about it’s usefulness to overclockers. And get this; almost every motherboard on the market is capable of this sweet little tweak.

Enough with the BS just tell us what the hell it is already!

There is a feature built in to most motherboards, accessible within the BIOS, called spread spectrum. It’s a simple little feature that alters the intensity of source frequency being sent to the clock generator in order to prevent radio signal interference. Basically it allows very very small changes in the FSB.

Here is a pic of the setting under the AWARD BIOS on an Abit VA6.

Some motherboards allow you to decrease as well as increase the spectrum. A decrease might just be up your ally if you can’t quite hit that next FSB. Just set it to decrease as much as the board allows, reboot, and increase the FSB to the next setting, and again reboot. If it worked, great! If not, back the FSB down, and set the spectrum to the highest increase the board will allow.

If you’re already at the maximum FSB and feel you can squeeze a few more megahertz out of the chip, the just increase the spectrum.

This is from, Da OC Place



Spread Spectrum is not a tweak to gain performance. In fact if the spectrum is centered around the "correct" operating frequency your net gain will be zero. Spread Spectrum is used to reduce EMI emissions. A square wave (ie. clock signal) when viewed in the frequency domain will have the highest energy output at that frequency. You will also see energy output at the harmonics of that frequency although reduced. To pass EMI requirements you would have to put alot of effort/$ into shielding. The relatively inexpensive alternative to reducing EMI levels is Spread Spectrum. When enabled the operating frequency will drift above and below the centerpoint. What this does is it averages the energy output and reduces the overall peak EMI emmissions. The overall energy output is still the same but it averaged out rather than concentrated at one specific frequency. The only way to realize a performance boost would be to have the range shifted above the operating frequency rather than centered around it.


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