spread spectrum clocking_xilinx pre-production_Free running(9)

FIG。 5 illustrates the distribution of the radiated electromagnetic energy around the third harmonic (960 MHz) from the clock signal generated using the adaptive SSC profile 404 described above。 As may be seen in the figure, energy radiated in the target band (GSM900 receiving band of 925-960 MHz), as shown at 502, is less than that radiated outside of the target band, as shown at 504。 It will be appreciated that similar energy distribution patterns as that shown in FIG。 5 would be generated around the base clock frequency of 320 MHz as well as the other harmonics, such as 640 MHz, 1280 MHz, and the like。

In further embodiments, the adaptive SSC profile 404 may comprise three, four, or more modulation rates according to the target band(s) for which it was configured。 For example, the modulation rate may be 60 kHz for clock frequencies between 316。8 MHz and 318。4 MHz; 55 kHz for clock frequencies between 318。4 MHz and 320 MHz; 25 kHz for clock frequencies between 320 MHz and 321。6 MHz, and 20 kHz for clock frequencies between 321。6 MHz and 323。2 MHz。

The higher and lower modulation rates used in the adaptive SSC profile 404 may be based on the suitability of the SSC clock signal for the components utilizing the clock。 For example, for the DRAM clock described herein, the DDR2 specification of modulation rates for SSC is from 20 kHz to 60 kHz。 The higher and lower modulation rates may be based on other factors as well, such as the base frequency of the clock signal, the deviation range of the SSC to be used, the target band(s), and the like。 The optimal high and low modulation rates may be determined by formulaic calculation, simulation, and/or experimentation considering EMI peak levels and the level of interference present in the target band(s)。

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