PLL reference feedthrough in PD

[neoflash]

Hi, folks:

In Rolland Best's PLL book, chapter 3.4.3, he stated that if the PLL used EXOR as the phase detector, the reference feed through will modulate the VCO output when divider N=3.

The modulated frequency component will be N*Fref +/- 2Fref and N*Fref +/- 4Fref.

My question is that why there is compenent located at N*Fref +/- 4Fref?

 

[nani]

Because of feedthrough u will have a periodic wave with period 1/Fref as output of phase detector.since the wave is periodic in freq domain u will have harmonics with integer multiples of Fref these component appear at vco output with vco frequency as centre freq

 

[neoflash]

Yes, I also found that I ignored that the output from PD is a square wave.

 

[nani]

hi,
neoflash i am working on fractional n freq synthesis. i am struck at sigmadelta part of that

is digital or analog[switched cp] preferred for implementing it

how to do behaviour level simulation of sigma delta [digital implementation] with initial condition applied to the accumulator for example mash 1-1-1

thanks in advance