Cascading PLLs
 

Fuding Ge

fudingge@yahoo.com




Cycle-to-cycle jitter

Cypress gave the following results:
Jp-p=37 ps from crystal oscillator
Jp-p=55 ps from the first PLL
Jp-p=59 ps from the second PLL
Jp-p=68 ps from the third PLL

Phase Noise / Long term jitter

The following figures show the spectrum after the cascading PLLs. After cascading 5 PLLs, the low frequency jitter shows a gain of about 30dB. So although for cascading PLLs, the cycle-to-cycle jitter has a small gain, the long term jitter amplification is substantial!

Conclusion



Spectrum of the output signal from Crystall Oscillator



Spectrum of the output signal from PLL 1



Spectrum of the output signal from PLL 2



Spectrum of the output signal from PLL 5

All the PLLs are CY2308 (cycle-to-cycle jitter is about 200 ps.


References

  1. Cypress Appication Note, "Perfect Timing", www.cypress.com
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