Thanks RC, but can you elaborate on digital PWM chop? I don't exactly know how that works and googling wasn't of much help either!
In my question I meant some kind of a time to digital converter that quantizes the input time differences in terms of the number of fixed time delays, lets say an inverter delay. When I was referring to some papers on All digital PLLs I find that the spurs are much worse than in a standard charge pump PLL. So I am trying to understand what makes the digital version that bad? Is it just because of the quantization noise that is inevitable when one goes from A/D (here time to digital) and D/A (Digitally controlled oscillator) domains? or is there some other reason?