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Intel Pentium 4 Processor CK00 Clock Synthesizer/Driver Design Guidelines

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CK00 Clock Synthesizer/Driver Design Guidelines
Page 45
∆
SSC
non-SSC
Highest peak
δ
of f
nom
f
nom
Figure 6 Spectral Fundamental Frequency Comparison
7. To achieved sufficient system-level EMI reduction, it is desired that SSC reduce the spectral
peaks in the non-SSC mode by the amount specified in Table 4. The peak reduction
∆
is
defined, as shown in Figure 6, as the difference between the spectral peaks in SSC and
non-SSC modes at the specified measurement frequency
Table 4 Desired Peak Amplitude Reduction by SSC.
CPU Clock Freq.
Peak Reduction
∆
Measurement Freq.
133 MHz 7 dB 666 MHz (5
th
harmonics)
100 MHz 7 dB 700 MHz (7
th
harmonics)
Notes:
a) The spectral peak reduction is not necessarily the same as the system EMI reduction. However, this
relative measurement gives the component-level indication of SSC’s EMI reduction capability at the
system level.
b) It is recommended that a spectrum analyzer be used for this measurement. The spectrum analyzer
should have measurement capability out to 1 GHz. The measured SSC clock needs to be fed into the
spectrum analyzer via a high-impedance probe compatible with the spectrum analyzer. The output
clock should be loaded with 20 pF capacitance. The resolution bandwidth of the spectrum analyzer
needs to be set at 120 KHz to comply with FCC EMI measurement requirements. The video band
needs to be set at higher than 300 KHz for appropriate display. 100 KHz may be used as the resolution
bandwidth in case of measurement equipment limitation. The display should be set with maximum hold.
The corresponding harmonic peak readings should be recorded in both the non-SSC and the SSC
modes, and be compared to determine the magnitude of the spectral peak reduction.
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 downstream PLL tracking
skew, i.e., the accumulative phase difference between the downstream PLL input and