Overview Of Clock Modulator - Fujitsu MB90390 Series Hardware Manual

CHAPTER 6 CLOCK MODULATOR
6.1

Overview of Clock Modulator

This section gives an overview of the Clock Modulator.
■ Overview
The clock modulator is intended for the reduction of electromagnetic interference - EMI, by spreading the
spectrum of the clock signal over a wide range of frequencies.
The modulator offers two modes: 1) phase modulation mode and 2) frequency modulation mode
In both cases the module is fed with an unmodulated reference clock with frequency F0, provided by the
PLL circuit. This reference clock is phase modulated by a triangular waveform or frequency modulated,
controlled by a random signal.
In general, the frequency modulation mode leads to better EMI behavior than phase modulation mode.
The mean frequency of the modulated clock is equal to the reference clock frequency F0.
Figure 6.1-1 Frequency Spectrum of the Modulated Clock (Fundamentals Only)
■ Modulation Degree and Frequency Resolution in Frequency Modulation Mode
Maximum and minimum frequencies (F
modulation degree parameter. Furthermore the resolution of the modulation range is selectable in 7 steps
from "L" (1) to "H" (7). Higher resolution implies a finer granularity of discrete frequencies in the spectrum
of the modulated clock but less possible modulation degrees.
In general the highest possible frequency resolution combined with the highest possible modulation degree
results in the highest EMI reduction. But for some cases lower modulation degrees may result in a better
EMI behavior. Please refer to the table of possible settings in Table 6.3-4.
106
modulation range
F F F
min 0 max
and F ) of the modulated clock are well defined by the
max min
frequency