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User’s Manual
197
15.2 Using the Clock Spectrum Spreader
The spectrum spreader is very powerful for reducing EMI because it will reduce all sources
of EMI above 100 MHz that are related to the clock by about 15 dB. This is a very large
reduction since it is common to struggle to reduce EMI by 5 dB in order to pass government
tests.
Figure 15-3. Peak Spectral Amplitude Reduction from Spectrum Spreader
The spectrum spreader modulates the clock so as to spread out the spectrum of the clock
and its harmonics. Since the government tests use a 120 kHz bandwidth to measure EMI,
spreading the energy of a given harmonic over a wider bandwidth will decrease the
amount of EMI measured for a given harmonic. The spectrum spreader not only reduces
the EMI measured in government tests, but it will also often reduce the interference cre-
ated for radio and television reception.
The spectrum spreader has three settings under software control (see Table 15-1 and
Table 15-2): off, standard spreading and strong spreading.
Two registers control the clock spectrum spreader. These registers must be loaded in a spe-
cific manner with proper time delays. GCM0R is only read by the spectrum spreader at the
moment when the spectrum spreader is enabled by storing 080h in GCM1R. If GCM1R is
cleared (when disabling the spectrum spreader), there is up to a 500-clock delay before the
spectrum spreader is actually disabled. The proper procedure is to clear GCM1R, wait for
500 clocks, set GCM0R, and then enable the spreader by storing 080h in GCM1R.
15dB
10
5
100
50
200
150
250
350
300
Normal Spreading
Strong Spreading
MHz
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