Frequency modulation using a digital frequency locked loop

A circuit for frequency modulation (FM) transmission of an input signal, the circuit comprising:

• a digitally controlled oscillator (DCO) that outputs a modulated signal whose frequency is controlled by a digital tuning word;
• a carrier signal control logic circuit that generates a carrier signal word having most significant bits (MSB) and least significant bits (LSB);
• an LSB adder circuit that sums the LSB and the input signal to generate an LSB-input signal;
• a sigma delta modulator circuit that dithers the LSB-input signal to generate a dithered input signal word;
• an adder circuit that sums the MSB and the dithered input signal word to generate the digital tuning word;
• a feedback divider circuit that receives a feedback of the modulated signal and outputs a divided modulated signal;
• a reference divider circuit that receives a reference clock signal and outputs a divided clock signal;
• a frequency counter circuit that receives the divided modulated signal and the divided clock signal, counts a number of clock cycles of the divided modulated signal during a fixed period, and outputs the counted number of clock cycles, wherein the fixed period is determined by the divided clock signal;
• and
• a comparison circuit that compares the counted number of clock cycles to a predetermined number of clock cycles, and outputs a carrier frequency error,
• wherein the carrier signal control logic circuit is constructed to receive the carrier frequency error, and to adjust the carrier signal word based on the received carrier frequency error such that a carrier frequency of the modulated signal is locked.

A method for frequency modulation (FM) transmission of an input signal, the method comprising:

• a signal generating step of outputting a modulated signal whose frequency is controlled by a digital tuning word;
• a frequency locking controlling step of generating a carrier signal word and adjusting the carrier signal word based on a feedback of the modulated signal such that a carrier frequency of the modulated signal is locked;
• a sigma delta modulating step of dithering the input signal to generate a dithered input signal word;
• and
• an adding step of summing the carrier signal word and the dithered input signal word to generate the digital tuning word.

A method for frequency modulation (FM) transmission of an input signal, the method comprising:

• a signal generating step of outputting a modulated signal whose frequency is controlled by a digital tuning word;
• a carrier signal controlling step of generating a carrier signal word having most significant bits (MSB) and least significant bits (LSB);
• an LSB adding step of summing the LSB and the input signal to generate an LSB-input signal;
• a sigma delta modulating step of dithering the LSB-input signal to generate a dithered input signal word;
• an adding step of summing the MSB and the dithered input signal word to generate the digital tuning word;
• a feedback dividing step of receiving a feedback of the modulated signal and outputting a divided modulated signal;
• a reference dividing step of receiving a reference clock signal and outputting a divided clock signal;
• a frequency counting step of receiving the divided modulated signal and the divided clock signal, counting a number of clock cycles of the divided modulated signal during a fixed period, and outputting the counted number of clock cycles, wherein the fixed period is determined by the divided clock signal;
• and
• a comparing step of comparing the counted number of clock cycles to a predetermined number of clock cycles, and outputting a carrier frequency error,
• wherein the carrier signal controlling step adjusts the carrier signal word based on the carrier frequency error such that a carrier frequency of the modulated signal is locked.