why a the DCO need to be normalized in a ADPLL?

[fenggou55123]

why a DCO need to be normalized in a ADPLL?

Hi, I am a Master student studying ADPLL, I had read same articles and books about ADPLL ,including the STASZEWSKI 's classical book.I do not understand why the DCO need to be normalized, when we are designing a DCO, we definitely know its gain, just like VCO of analog PLL, so it seems like meaningless to make the DCO gain to be FREF/LSB just to calculate the Loop variables.

Thank you for your kind help!!

 

[vamshi990]

Hi fenngou,

Kdco although known, is not constant and varies quite a bit with process, voltage and temperature. By normalizing it, we make sure the variations do not impact the pll operation as long as we estimate its value correctly. Gain estimation is also straight forward because, unlike analog pll, OTW is digital and can be easily stored and used to compute Kdco. This is important when we use ADPLL as a direct frequency modulator.

Hope this helps.