Phase consistent numerically controlled oscillator

A numerically controlled oscillator system comprising:

• a first free-running counter;
• a coarse NCO circuit;
• and
• a fine NCO circuit, having an input coupled to an output of the coarse NCO circuit, and wherein when a data rate of the coarse NCO circuit is changed, a clock rate of the fine NCO circuit is changed to match a new data rate of data output by the coarse NCO for input by the fine NCO, and
• wherein, before starting the fine NCO, a representation of a current phase of a desired fine NCO frequency is determined using a counter value of the first free-running counter and sent to a phase accumulator of the fine NCO.

A method for maintaining a consistent phase reference for a digital periodic signal while switching digital data rates, the method comprising:

• starting a first free-running counter, wherein the first free-running counter is not reset during a time period in which the consistent phase reference is to be maintained;
• obtaining, from the first free-running counter, a representation of a current phase value of a desired NCO frequency that is consistent with a phase value at the time of starting the first free-running counter;
• and
• using the representation of the current phase value of the desired NCO frequency to load a starting value into a phase accumulator of an NCO.