spread spectrum clocking_xilinx pre-production_Free running(2)

Many laptop computers, notebooks, tablets, PDAs, and other portable computing systems or devices include wireless communication components. The DRAM clock of an HDD suitable for inclusion in these portable systems may be configured so that the clock frequency and its harmonics do not fall into the communication bands of the wireless components. However, using SSC may cause the distributed frequency bands of electromagnetic energy to overlap with the wireless communication frequency bands and generate an unacceptable level of interference. For example, an HDD or other electronic device may use a 320 MHz DRAM clock and radiate electromagnetic energy on each harmonic of this frequency, such as 960 MHz as shown at 102 in FIG. 1.

A laptop computer or other portable system that implements the HDD may utilize wireless wide area networking (“WWAN”) for mobile Internet access。 Commercial WWAN receiving bands are composed of several different frequency ranges, such as GSM850 (869-894 MHz), GSM900 (925-960 MHz), DCS1800 (1805-1880 MHz), PCS1900 (1930-1990 MHz), W2100 (2110-2170 MHz), and the like。 Harmonics of the DRAM clock frequency (320 MHz) may not fall into any of the WWAN receiving bands, and thus the DRAM clock may not cause any WWAN interference。

However, if the HDD or electronic device uses SSC for the DRAM clock with ±10,000 ppm deviation range and a 32 kHz modulation rate, the occupied frequency range of radiated electromagnetic field may be broadened, as shown at 104。 This broadened range of frequencies for radiated electromagnetic energy may overlap with the WWAN receiving bands。 For example, the broadened range of frequencies around the third harmonic of the 320 MHz clock frequency (960 MHz) may overlap the GSM900 band, as shown in FIG。

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