In the hardware section is a perl script which produces a vhdl description of an irrational frequency divider which uses the idea of a phase accumulator which is kind of sort of related to an NCO.
Before anyone says anything its called an irrational divider cos it can divide the input frequency by an irrational number not cos it doesnt make any sense.
Thinking about the limitations of the digital realisation the closed loop transfer function for a VCO is a third order polynomial and the digital version is only going to approximate it closely with very high sampling rates and/or long data words.