The Complete Guide to Spread Spectrum

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.
 
X

Xee

Guest
Thanks for that info very intresting, so basically its not going to stop background static noise being heard in the audio? Thats what I thought was the purpose, of Spread Spectrum, no I know why it does not work, LOL :undecided:
 
M

madmolio

Guest
Great article, had no clue what SS did before I read this :) a reboot & disable SS comin right up :biggrin:
 
D

Deeez

Guest
Originally posted by Harrkev
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 could of just said "Spread Spectrum is BADD" mmmmmkay!
 
R

Rogue_Wulff

Guest
I never understood what that was for, until I read this. Now I see it has no real benefit for the end-user. Thanks for the very clear and consise explanation :biggthumbsup:
 

ex_forum_user_3

New member
GENERAL OF THE ARMY
Joined
Jul 3, 2002
Messages
23,397
Well I did, as I'm an Electronical Engineer and a HAM Radio Amature, but I never knew how to explain it properly...
Good job, well done!! :biggthumbsup:
 
G

goodcooper

Guest
i already knew this, but this is an EXCELLENT way of putting it...

... i have a question tho... (for a smarty pants)

i like to listen to radio station 103.5 here in NC, and if i put my 'ol KT133 machine with my duron in it... and oced it to 103mhz, then put it next to my radio tuner, would i get a lot of interference?
 
H

Harrkev

Guest
In theory, yes. But keep in mind that these boards are designed NOT to radiate (as much as possible). And if you use an all-metal case (like I do), then that will help even more. So I would expect a certain amount of interference, but not an excessive amount. Plus, there are a LOT of signals (not just clocks) running around. I would expect broad-band noise.
 
G

goodcooper

Guest
Originally posted by Harrkev
In theory, yes.


BUUUUUUT... if i had spread spectrum enabled, i wouldn't have any problems, right ?


just another way of understanding the NEED to keep the noise down in the air... the reason your new 2.4ghz cordless phone and your 2.4ghz wireless B AP don't interfere with each other.... spread spectrum....
 
H

Harrkev

Guest
I can honestly say that I have not done any side-by-side tests. Perhaps I will in the future when I have time. I have a Kenwood TH-F6a that should be up to the task. I should like to point out that one of the biggest contributors to RF noise is a cheap power supply. That probably throws off more power than your mobo ever will.

Also, keep in mind that the way tha spread-spectrum works, your mobo is now an FM transmitter.

As always, YMMV. Try it for yourself. If you are not an extreme overclocker, feel free to leave SS turned on.
 

ex_forum_user_3

New member
GENERAL OF THE ARMY
Joined
Jul 3, 2002
Messages
23,397
Old systems where really noisy transmitters of rubbish, heck a PS/2 Model30 would disturb half the neighberhood :biggrin:
Today's systems aren't that bad, SS Could help a little, but I doubt you will notice much difference.
Videocard's typically produce far more rubbish then most mobo's do.
 
D

Dan2012

Guest
Great stuff Harrkev I should of disabled SS ages ago when I have O/C my system. I always wondered what it was actually for in greater detail

Thanks.
:biggthumbsup:
 
M

Mr Woo

Guest
I work for a company the does medical research and they ahve all sorts of kit running, ruins my radio listening sometimes!!
 
R

ratusnatus

Guest
Great one. Very interesting.

Can i translate this to portuguese to post over here?

Ill keep the credits!

Cheers
 
R

Roger_kelly

Guest
Thanks for sharing your information with us.

Good to know what SS means before we go and tell anybody to fiddle with it.

Thanks
Rajiv Kulkarni
 
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