Messages in this thread | | | Date | 10 Jun 2015 14:38:54 -0400 | From | "George Spelvin" <> | Subject | Re: Discussion: quick_pit_calibrate is slow |
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George Spelvin wrote: > spread spectrum clock modulation rates are typically about 30 kHz
Actually I found a source: http://download.intel.com/design/Pentium4/guides/24920601.pdf CK00 Clock Synthesizer/Driver Design Guidelines
Page 45 says "8. The modulation frequency of SSC is required to be in the range of 30-33 KHz to avoid audio band demodulation and to minimize system timing skew."
> The faster the modulation rate, the more the EMI peak is flattened. > But too fast produces unaceptable cycle-to-cycle timing variations, > also known as clock jitter.
This is actually wrong. It's the modulation *depth* that determines the peak reduction. (The above Intel spec says 0.5% typ, 0.6% max.) The only requirement is that the rate is well below the clock rate, and well above the averaging time of your frequency analyzer used to measure the noise.
But basically, they want the modulation rate above the audio band, but as low as possible to make it easy for downstream PLLs to track.
To quantify the error, consider a triangular modulation spectrum with a total of 0.6% of range. Intel requires <= 0.6% = 6000 ppm of down-modulation relative to nominal, but I'll consider it to be +/-3000 ppm from an average.
The total phase error accumulated during a frequency excursion is the integral, which is the area under the peak.
(The Lexmark/Hershey's kiss modulation waveform, which provides a slightly flatter peak, has a slightly smaller area due to the curved edges, so triangular is a safe overestimate.)
At 30 kHz, during one peak (1/60 ms = 16.66 us), the oscillator phase advances by 16.66 us * 1500 ppm = 25 ns. (At the more typical 0.5% value, that's 21 ns.)
25 ns is 100 ppm of 0.25 ms, so it should be okay if I go use a measurement interval of 1 ms or more.
Some BIOSes have offered 1% spread: http://www.techarp.com/showFreeBOG.aspx?bogno=266
which doubles that figure, but I don't think there's anything more.
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