Todays PLL Design ? Fully Digital or Analog ?

[amic]

I am working on a PLL project and presently trying to figure out which architecture to choose. Are todays PLLs fully digital except VCO or do they have major Analog parts as well ? If So, what is the advantage of using Analog ? is it precision ?

 

[sunking]

all kind use today.
and you can read some book about pll

 

[amic]

all kind use today.
and you can read some book about pll

If I am not getting it wrong....we do have fully Analog PLLs even today.
In that case can I get any possible referance where the same thing is quoted.

Thanks.

 

[goodboy_pl]

refer to "Phase Locked Loops" by Roland Best

in fact fully digital freq. synthesizers need a high and accurate reference freq. which is developed by an analog PLL as a freq. multiplier + a Xtal Oscillator.

in fact each method have its + and - wich makes it suitable for different applications!

BEST!

 

[khouly]

it depends on the application the PLL will be used for

in RF systems most of the PLL's are charge pump type PLL
which the analog parts in it are VCO and the charge pump anf the loop filter
the remaining parts are digital


khouly

 

[rakko]

To add to the last reply, Digital PLLs are only good for speeds not exceeding 250Mhz mostly because standard digital cells do not run faster than 500Mhz reliably and even at thos speed have excessive jitter. So if you need higher speeds design your vco and filter in anlog and your divider circuit needs to be designed using custom cell design techniques so you get the duty cycles close to 50/50. For lower speeds, use DCO instead of VCO and all other blocks can be digital. Consider analog filters since they are usually smaller than digital filters for lower speeds.