How does the spread spectrum crystal clock oscillator modulation scheme work to reduce EMI?
Figure 4 below shows a plot of output amplitude versus frequency for a modulated and un-modulated center-spread spectrum clock oscillator. As you can see from the figure, there is a large difference between the frequency span and the amplitude for a given modulated and un-modulated spread spectrum clock oscillator. By modulating the output frequency over a frequency spectrum, a reduction in output amplitude can be achieved. This reduction in output amplitude correlates with a reduction in radiated energy (EMI).