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Design >> Mixed-Signal Design >> Divider jitter puzzle
https://designers-guide.org/forum/YaBB.pl?num=1163167916

Message started by Visjnoe on Nov 10^th, 2006, 6:11am

Title: Divider jitter puzzle
Post by Visjnoe on Nov 10^th, 2006, 6:11am

Hello,

I have a problem where both roads do not lead to Rome...maybe someone can explain my error in reasoning?

In 1 of his papers on PLL jitter, Ken states that the period_jitter_divider = sqrt(N)*period_jitter_vco. Given the explanation, this makes sense to me.
Then, a little mathematical derivation is made which leads to: Svco = N^2*Sdivider (relationship between phase noise spectra).

Now, the problem is that when I fill in this phase noise relationship in the classical formula to compute period jitter, I obtain that the period jitter of the VCO and of the divider output are the same!

I use following equation: Jper = 1/(2*pi*fosc)*sqrt(2*integral_of_phasenoise)
Filling in the stated phase relationship leads to the factor N disappearing from the equation.

However, the intuitive reasoning leads to a sqrt(N) relationship...

Where do I go wrong?

Kind Regards

Peter