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Design >> RF Design >> Fractional Divider in Reference Path of PLL
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Message started by Daniel Nobonzo on Sep 10^th, 2004, 12:52am

Title: Fractional Divider in Reference Path of PLL
Post by Daniel Nobonzo on Sep 10^th, 2004, 12:52am

Hi,

I wonder if there is any drawback in placing a fractional divider (noise, spurs, ..) in the reference path, i.e. after the crystal, instead of after the VCO.

Thanks for your comments.

Dan

Title: Re: Fractional Divider in Reference Path of PLL
Post by emad on Sep 10^th, 2004, 8:01am

It is actually both!

A fractional divider based on DS modulation is pretty noisy. That is because the quantization noise shows as shaped noise with 20*(n-1) dB/dec, where n is the modulator order. This noise at the reference will be further magnified by 20*log(N), where N is the feedback divider ratio.

Same happens for spurs.

For the reference section you want a quiet configuration. You simply can't afford to mess up there.

Title: Re: Fractional Divider in Reference Path of PLL
Post by strabush on Jan 10^th, 2005, 5:58am

Matter of fact, A DDS as the reference is exactly that.