Why do we still use intiger N PLL?

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knack

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Hi,

As i understand, a fractional-N PLL gives higher resolution for output freq. steps,

Then, why do we still need to use integer N PLLs anymore??

Can't the first replace the latter?? Why not?

Cheers,

-- Knack
 

Fractional PLL does not provide enough filter frequency headroom between the nyquist frequency and the PLL loop bandwidth. Typical you try with a limited order lowpass filter to reduce the sigma-delta high frequency noise. This is increasing with frequency depening on sigma-delta order. But at the same time the lowpass impact the phase margin. So pratical limitation is between 1/50 and 1/20 of the reference frequency. If you need higher loop bandwidths integer does not suffer from sigma-delta shaping noise.
 

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    knack

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I am interested to learn more about Sigma-Delta modulator (if it's separated from PLL) or Fractional-N PLL (if they're cross-coupled)... where do you advice me to get what i want??

Although you're my mentor here, but excuse me, I don't want book names because this will take alot of time, app. notes, papers, and tutorials are what i seek..

Thnx again,
--Knack
 

The paper "Delta-Sigma Modulation in Fractional-N Frequency Synthesis" Tom A.D.Riley IEEE JSSC vol 28 No.5 May 1993 may help you.

A lecture of Mit at https://ocw.mit.edu Course:6.976
"Sigma-Delta Fractional-N Frequency Synthesis" Scoot Meninger Massachusetts Institute of Technology June 7, 2004" is a good tutorial.

hope above may help you.

Ryan
 

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    knack

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