Digital synthesizer, communication unit and method therefor

A communication unit having a digital synthesizer comprising:

• a ramp generator configured to generate a signal of frequency control words, FCW, that describes a desired frequency modulated continuous wave;
• a digitally controlled oscillator, DCO configured to receive the FCW signal;
• a feedback loop;
• and
• a phase comparator coupled to the ramp generator and configured to compare a phase of the FCW output from the ramp generator and a signal fed back from the DCO via the feedback loop and output a N-bit oscillator control signal;
• wherein the digital synthesizer comprises a gain circuit coupled to a multiplier located between the ramp generator and the DCO and configured to:
• apply at least one gain from a plurality of selectable gains to the N-bit oscillator control signal that sets a selectable loop gain of the digital synthesizer and thereby set a selectable loop bandwidth;
• and
• calculate and apply a gain offset dependent upon the selected gain that is adapted when the selected gain is changed.

A method for maintaining an open loop gain of a digitally controlled oscillator, DCO, feedback loop as substantially constant, the method comprising:

• generating a signal of frequency control words, FCW, that describes a desired frequency modulated continuous wave;
• comparing a phase of the FCW signal with a signal fed back from an output of the DCO at a phase comparator and outputting a N-bit oscillator control signal from the phase comparator;
• selecting at least one gain from a plurality of selectable gains;
• applying the selected at least one gain from a plurality of selectable gains to the N-bit oscillator control signal to set a selectable loop gain of the digital synthesizer and thereby set a selectable loop bandwidth;
• and
• calculating and applying a gain offset dependent upon the selected gain that is adapted when the selected gain is changed.