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  #1 (permalink)  
Old 22nd January, 2003, 10:48 PM
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Spread Spectrum and your FSB

Anybody know what he concequences of enabling this is?

anybody over 200mhz have it off/on?

everybody under 200mhz have it off?

just some more questions.
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Old 22nd January, 2003, 10:55 PM
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I tried it and thought I had found something. I could crank up my fsb and multiplier and it would post at whatever settings I had chose but it didn't seem any faster, then when I opened WCPUID
it showed it at default 133 X 15 2000 MHz 2400+ same in Sandra
Even though it would show 200 X 11.5 or 220 X 10 in BIOS and on the initial post, in reality it was only running at default. As soon as I disabled it every thing was as it should be. Try and see if you find the same thing.
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Last edited by RandyB; 22nd January, 2003 at 10:58 PM.
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Old 24th January, 2003, 11:44 AM
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I read at www.rojakpot.com that Spread Spectrum Modulation reduces spikes and EMI. Dunno if it helps or not

140 *12.5 = 1775 = XP2100+
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Old 24th January, 2003, 12:08 PM
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Sounds like a good plan Randy....... I have wondered about this settting in the bios, but always turn it off when overclocked.........

I will check it on and off and see it comes out the same as you found Randy.....

I will run some benchmarks @200FSB with it on and off too......

Might find no gains or stability, but worth a try.....
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Old 24th January, 2003, 04:16 PM
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I asked about this some time ago, and this was the answer I got from Áedán.

Quote:
Spread Spectrum is a method for reducing the potential for EMI. As you probably know, PCs use various frequencies internally. Whenever you send frequencies down a wire, it acts as a radio transmitter! Admittedly, it's only a weak one, but with enough wires, and enough different frequencies, there's a good chance for local interferance. Things like radios, cordless phones and other devices can be affected by this interferance.

If you have a FSB, running at 133MHz, then the main frequency that will end up as RF will be 133MHz. If you profile the RF spectrum of such a system, you'd see a strong peak at 133MHz. If it's strong enough, then the item breaks various regulations, and hence isn't sellable. (Europe is quite keen on keeping RF leakage down! I don't know about other areas)

The spread spectrum setting is almost like a cheat to work around this. Instead of keeping the FSB at 133MHz, for one instant, the FSB runs at 134MHz, the next it runs at 133MHz, and the following it runs at 132MHz.

The average is 133MHz, agreed?

However, because it's only spent a 1/3 of the time at 133MHz, the RF peak is only a 1/3 of what it would be if the FSB was always at 133MHz. Hence, the average power at that frequency has dropped by 2/3, a pretty big reduction! This is generally enough to ensure that said devices pass through testing with flying colours.

In real life, spread spectrum uses far more than just 3 frequencies. It may use hundreds! This helps to keep the overall profile of the RF noise generated down. There's generally a percentage. This is the percentage deviation from the average clock frequency. If the percentage is 0.5%, then a 133MHz bus would actually vary from 133.665MHz down to 132.335MHz

The downside?

Items like the FSB are no longer running at one set frequency. If your machine locks up when the FSB hits 170MHz, and the spread spectrum range happens to hit that magic 170MHz, then your PC locks up. This may be despite the fact that the average FSB speed was 165MHz. Hence, the clock generator was generating frequencies from 160MHz up to 170Mhz. (Yes, it's an artificial example!)

If you can run spread spectrum without instability, then do so, please! It helps devices that rely on radio frequencies operate with less interferance. PCs are well known for causing a broad spectrum of RF "hash" that can interfere with many things.

Just a note: Square waves used in PCs don't produce just one frequency of RF interferance. A square wave is made up of many sine waves. Each of those sine waves causes it's own level of interferance! The fundimental (or strongest) sine wave is at the same frequency as the square wave. The next sine wave is 1/3 of the power, but at three times the frequency. The following sine wave is at 1/5 the power of the fundimental, but at five times the frequency!

Hence, your FSB running at, say, 170Mhz is causing interferance at 170MHz, 510MHz, 850MHz, 1.19GHz, 1.53GHz and so on!

Áedán
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Old 24th January, 2003, 05:27 PM
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Quote:
Originally posted by RandyB
I tried it and thought I had found something. I could crank up my fsb and multiplier and it would post at whatever settings I had chose but it didn't seem any faster, then when I opened WCPUID
it showed it at default 133 X 15 2000 MHz 2400+ same in Sandra
Even though it would show 200 X 11.5 or 220 X 10 in BIOS and on the initial post, in reality it was only running at default. As soon as I disabled it every thing was as it should be. Try and see if you find the same thing.
I JUST had the exact thing happen...I scratched my @zzz...changed hd...os...memory timings...all voltages...everything except rma the board...as soon as I disabled that mess...everything was fine...someone should post a warning about that settin...it can sure ruin Ur day...:beer:
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