RE: [SI-LIST] : Micro Noise

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From: sweir (weirsp@a.crl.com)
Date: Mon Nov 08 1999 - 16:14:33 PST


One note to add to this, is that for a typical PLL, the closed loop
frequency response to either a phase ramp or step is much, much faster than
the response to frequency steps.

Regards,

Steve.
At 04:31 PM 11/8/99 -0800, you wrote:
>When spread spectrum is enabled, basically PLLs are trying to catching up a
>slowly modulated host clock. How slow? The modulation frequency is typically
>around 30kHz. Comparing that to the 100MHz host bus frequency, you would
>have over 3000 bus cycles in one modulation period. The clocks are not
>modulated by much (~0.5%). So the peak-to-peak period change is small
>(roughly 0.5%*10ns = 50ps), which takes about 1500 cycles to complete. Thus,
>the consecutive edge variation is extremely small (50ps/1500 = 33fs).
>
>Most PLLs can track such slowly modulated clocks with small spread amount
>and don't require any knowledge of the modulation scheme (or so called
>"modulation profile").
>
>-Michael T. Zhang
>PMD/PPD, Intel Corp
>(503)264-2301
>
>
>-----Original Message-----
>From: rbmccammon@mmm.com [mailto:rbmccammon@mmm.com]
>Sent: Monday, November 08, 1999 10:57 AM
>To: si-list@silab.eng.sun.com
>Subject: RE: [SI-LIST] : Micro Noise
>
>
>
>
>Michael T. Zhang wrote:
>One such example is Intel's BX-based motherboards have the optional spread
>spectrum clocking feature, which modulates the clocks for processor,
>chipset, memory, and PCI, but keeps the USB clock fixed.
>
>
>Would you elaborate a little bit?
>
>I have this vision of PLL's in different chips chasing
>the phase of the clock, but all of them at a different phase
>because of different dynamics. Do they all know the modulating
>algorithm so they can predict the next phase, or is the rate
>of change from one edge to the next held low enough not to be a
>problem?
>
>
>
>
>
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