> A software PLL is based on an NCO and an NCO unlike a VCO has a minimum
> step size so it can only achieve a number of discrete frequencies, i.e.
> the output frequency is quantized.
Why do you say this? An NCO is, of course based on some digital
number representation; but if you use IEEE doubles, the quantization
error of the phase is much smaller than the thermal noise of any
analog oscillator, maybe even smaller than the quantum limits
physics allows any analog VCO.
rhn A.T nicholson d.0.t C-o-M