Rabbit 3000, User Manual

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User’s Manual 79
7.3 Clock Spectrum Spreader
When enabled the spectrum spreader stretches and compresses the clocks in a complex
pattern that results in spreading the energy in the clock harmonics over a wide range of
frequencies. The spectrum spreader has a normal and a strong setting. With either setting
the peak spectral strength of the clock harmonics is reduced by approximately 15 dB for
frequencies above 100 MHz. For lower frequencies the strong spreading has a greater
effect in reducing the peak spectral strength as shown in the figure below.
Figure 7-3. Reduction in Peak Spectral Strength from Spectrum Spreader
In the normal spectrum spreading mode, the maximum shortening of the clock cycle is 3
nanoseconds at 3.3 V and 25°C. In the strong spreading mode the maximum shortening of
a clock cycle under the same conditions is 4.5 ns. The reduction in peak spectral strength
is roughly independent of the clock frequency. Special precautions must be followed in
setting the GCM0R and GCM1R registers (see Section 15.2, “Using the Clock Spectrum
Spreader”).
15dB
10
5
100 50 200 150 250
350
300
Normal Spreading
Strong Spreading
MHz