US Patent 7741928 Frequency modulation using a digital frequency locked loop

Circuits and methods for frequency modulation (FM) using a digital frequency-locked loop (DFLL). A digitally controlled oscillator (DCO) generates and adjusts a frequency of a modulated signal based on a digital tuning word. A DFLL control logic circuit receives a feedback of the modulated signal and generates a carrier signal word. A sigma delta modulator circuit receives an input signal and applies dithering to produce a dithered input signal word. An adder circuit receives and sums the dithered input signal word and the carrier signal word to produce the digital tuning word. The DFLL control logic circuit adjusts the carrier signal word to lock a carrier frequency of the modulated signal.