H
Harrkev
Guest
INTRODUCTION
You have this shiny new motherboard, and the BIOS lists an option for "Spread Spectrum." You have heard that it is best left turned off for overclocking, but you have no idea what it is or why it is there. That is what this article is about. Hopefully all of your questions will be answered after reading this.
From now on, spread spectrum will be abbreviated as SS.
DO I NEED SPREAD SPECTRUM?
No. As an end user, you ALMOST NEVER need SS. This feature was NOT designed to benefit the end user. It was designed to allow pre-built computers to pass FCC testing. If you do not know what FCC testing is, then you have absolutely no need for SS. But, if your computer interferes with your radio reception, it is possible that enabling SS may help a little.
The good news is that, unless you overclock, SS does absolutely no harm.
SO WHAT GOOD IS SPREAD SPECTRUM?
Spread spectrum is a trick used to allow products to pass FCC testing. Inside almost every electronic device is an oscillator of some sort. On a typical motherboard, the front-side-bus may operate at 200MHz. This means that there is a steady 200MHz clock which is running all of the time. This clock runs over long wires. And a long wire carrying a signal is also sometimes known as an antenna.
So, a typical motherboard will be radiating a 200MHz signal into the air as radio waves. Also, there is likely to be a strong 33Mhz and 66Mhz signal from the PCI and AGP ports. Plus, each clock will have overtones which will be odd multiples of the fundamental frequency. So you will also have signals at 100MHz, 167MHz, 200MHz, 233Mhz, etc. Not to mention harmonics plus the noise generated by the signal and control lines.
The problem comes when a product needs FCC certification. On a spectrum analyzer, each of these clock frequencies shows up as a "spike" on the spectrum display. If the spike goes over a certain threshold, the product fails the test. There are several ways to deal with a failure of this type, and they all require making changes in the prduct. Perhaps the board can be re-designed (expensive), or perhaps the case can be improved (possibly also expensive). Or perhaps SS can be turned on (free, if built-in).
SO HOW DOES SPREAD SPECTRUM WORK?
Spread spectrum works by changing the clock frequency. Instead of the FSB running at 200MHZ, it will be AROUND 200MHZ, but constantly varying. One instant it will be 200MHz. The next instant, it will be 199.5MHz, and 198MHz the next, and maybe 202MHz the next. The actual frequency of the clock goes up and down, but is always centered around 200MHz.
Now, if you look at this on a spectrum analyzer, instead of getting a sharp "spike" at 200Mhz, you will get a round "blob" centered at 200MHz. If the top of the blob is below the legal threshold, the product now passes FCC testing. It should be noted that the exact same amount of radio energy is being generated, but the energy has been spread out over a narrow range of frequencies instad of being centered on one frequency. To use a simple analogy, instead of putting $10 on #12 on a roulette wheel, you will instead put $2 on #10, $2 on #11, $2 on #12, $2 on #13, and $2 on #14. This is why it is called "spread spectrum" -- you are taking the same amount of radio energy and spreading it out around the spectrum.
SO WHY IS THIS BAD FOR OVERCLOCKING?
Let's suppose that the weak link in your computer is the RAM. No matter what you do, your RAM will not run faster than 230MHz. So, if SS is disabled, you can safely run at 230MHz. Now, if SS in enabled, and the frequency variation is +/-5MHz, then if you run at 230MHz, one instant you will be running at 225MHz (OK) and the next instant you will be running at 235MHz (blue screen of death). With a +/-5MHz variation, you cannot run faster than 225MHz.
It should be noted that, in this case, if you are running at the stock 200MHZ, then you might as well leave SS turned on because it does not hurt you.
SO WHY ARE YOU MISTER SMARTY-PANTS?
I have a Master's degree in Electrical Engineering. I currently design and debug digital control and communication products (including board and FPGA design) for a non-consumer market.
Note: Moderators, feel free to make this sticky or to move it.
You have this shiny new motherboard, and the BIOS lists an option for "Spread Spectrum." You have heard that it is best left turned off for overclocking, but you have no idea what it is or why it is there. That is what this article is about. Hopefully all of your questions will be answered after reading this.
From now on, spread spectrum will be abbreviated as SS.
DO I NEED SPREAD SPECTRUM?
No. As an end user, you ALMOST NEVER need SS. This feature was NOT designed to benefit the end user. It was designed to allow pre-built computers to pass FCC testing. If you do not know what FCC testing is, then you have absolutely no need for SS. But, if your computer interferes with your radio reception, it is possible that enabling SS may help a little.
The good news is that, unless you overclock, SS does absolutely no harm.
SO WHAT GOOD IS SPREAD SPECTRUM?
Spread spectrum is a trick used to allow products to pass FCC testing. Inside almost every electronic device is an oscillator of some sort. On a typical motherboard, the front-side-bus may operate at 200MHz. This means that there is a steady 200MHz clock which is running all of the time. This clock runs over long wires. And a long wire carrying a signal is also sometimes known as an antenna.
So, a typical motherboard will be radiating a 200MHz signal into the air as radio waves. Also, there is likely to be a strong 33Mhz and 66Mhz signal from the PCI and AGP ports. Plus, each clock will have overtones which will be odd multiples of the fundamental frequency. So you will also have signals at 100MHz, 167MHz, 200MHz, 233Mhz, etc. Not to mention harmonics plus the noise generated by the signal and control lines.
The problem comes when a product needs FCC certification. On a spectrum analyzer, each of these clock frequencies shows up as a "spike" on the spectrum display. If the spike goes over a certain threshold, the product fails the test. There are several ways to deal with a failure of this type, and they all require making changes in the prduct. Perhaps the board can be re-designed (expensive), or perhaps the case can be improved (possibly also expensive). Or perhaps SS can be turned on (free, if built-in).
SO HOW DOES SPREAD SPECTRUM WORK?
Spread spectrum works by changing the clock frequency. Instead of the FSB running at 200MHZ, it will be AROUND 200MHZ, but constantly varying. One instant it will be 200MHz. The next instant, it will be 199.5MHz, and 198MHz the next, and maybe 202MHz the next. The actual frequency of the clock goes up and down, but is always centered around 200MHz.
Now, if you look at this on a spectrum analyzer, instead of getting a sharp "spike" at 200Mhz, you will get a round "blob" centered at 200MHz. If the top of the blob is below the legal threshold, the product now passes FCC testing. It should be noted that the exact same amount of radio energy is being generated, but the energy has been spread out over a narrow range of frequencies instad of being centered on one frequency. To use a simple analogy, instead of putting $10 on #12 on a roulette wheel, you will instead put $2 on #10, $2 on #11, $2 on #12, $2 on #13, and $2 on #14. This is why it is called "spread spectrum" -- you are taking the same amount of radio energy and spreading it out around the spectrum.
SO WHY IS THIS BAD FOR OVERCLOCKING?
Let's suppose that the weak link in your computer is the RAM. No matter what you do, your RAM will not run faster than 230MHz. So, if SS is disabled, you can safely run at 230MHz. Now, if SS in enabled, and the frequency variation is +/-5MHz, then if you run at 230MHz, one instant you will be running at 225MHz (OK) and the next instant you will be running at 235MHz (blue screen of death). With a +/-5MHz variation, you cannot run faster than 225MHz.
It should be noted that, in this case, if you are running at the stock 200MHZ, then you might as well leave SS turned on because it does not hurt you.
SO WHY ARE YOU MISTER SMARTY-PANTS?
I have a Master's degree in Electrical Engineering. I currently design and debug digital control and communication products (including board and FPGA design) for a non-consumer market.
Note: Moderators, feel free to make this sticky or to move it.
a reboot & disable SS comin right up 
