Rabbit Rabbit 4000, User Manual

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Chapter 2 Clocks 13
2.3.2 Spectrum Spreader
When enabled, the spectrum spreader stretches and compresses the main clock in a complex
pattern that spreads the energy of the clock harmonics over a wider range of frequencies.
Figure 2-1. Effects of Spectrum Spreader
There are three settings that correspond to normal and strong spreading in the 0–50 MHz
and >50 MHz main clock range. Each setting will affect the clock cycle differently; the
maximum cycle shortening (at 1.8 V and 25°C) is shown in Table 2-2 below.
Table 2-2. Spectrum Spreader Settings
0–50 MHz > 50 MHz
GCM0R
Value
Description
Max. Cycle
Shortening
— Normal 0x40
Normal spreading of frequencies over
50 MHz
2.3 ns
Normal Strong 0x00
Normal spreading of frequencies up to
50 MHz; strong spreading of
frequencies over 50 MHz
3 ns
Strong — 0x80
Strong spreading of frequencies up to
50 MHz; normal spreading of
frequencies over 50 MHz
4.5 ns
FREQUENCY (MHz)
AMPLITUDE (dB)
400
405
410 415 420 425 430 435 440 445
0
-10
-20
-30
-40
-50
Spectrum Spreader
Disabled
Spectrum Spreader
Enabled (normal setting)