PLL versus multiple crystals

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ccw27

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Suppose your chip set needs multiple clocks for different circuitry. Why use a PLL and one crystal to generate different clock frequencies instead of using multiple crystals to do so?

Thanks
 

Deriving all the frequencies from a single source, the exact ratio between frequencies and the phase coherence are preserved.

Z
 

Also multiple crystals would take up huge area on the PCB, and for some frequencies like
very high frequencies in the range of hundreds of MHz or GHz, crystal oscillators are not
available. But the most important reason is phase synchronisation between different parts of
the system.
 

It is for phase synchronization,or else you will have phase offset between different Crystals even if you could install them on board.
 

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