# Prescaler noise contribution to PLL output noise

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#### carlosgg123

##### Newbie level 1
Prescaler noise contribution to FracN PLL output noise

Hello, I am writing a program to calculate the noise contributions of various PLL components (VCO, Charge Pump, etc.) to the output. The PLL is a FracN and it uses a P/(P+1) prescaler and a delta-sigma modulator. I can simulate the noise output of the prescaler in the circuit simulator. My question: In order to *approximate* its noise transfer function, can I take the overall divide ratio

N = $\frac{Fout }{Fref }$

and then multiply the prescaler noise output by

$\frac{1 }{Ndig }$*$\frac{G(s) }{1+H*G(s) }$

where

Ndig = N/P

Is that a semi-decent approximation, or does it even make sense?? Thank you.

http://www.ti.com/tool/pll_book
Page 49

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