Phase locked loop based frequency modulator with accurate oscillator gain adjustment

A frequency modulator, comprising:

• a tuning circuit configured to
• determine a nominal gain characteristic of a digitally controlled oscillator (DCO) in an first mode, and
• determine an actual gain characteristic of the DCO in a second mode using the nominal gain characteristic;
• and
• a modulation circuit comprising the DCO coupled to the tuning circuit, configured to modulate a frequency of a DCO output signal from the DCO with a modulation signal, and to scale the modulation signal based on the actual gain characteristic in the second mode to provide gain compensation and frequency modulation of the DCO,
• wherein the first mode is an open loop mode and the second mode is a closed loop mode.

A frequency modulator, comprising:

• a digitally controlled oscillator (DCO), configured to supply a DCO output signal from the DCO based on an oscillator control word input;
• a select switch configured to couple one of an actual operating point word in a closed loop mode, and a controlled operating point word in an open loop mode to the oscillator control word input of the DCO based on a selection control signal;
• a frequency counter coupled to the DCO output signal, configured to measure a frequency of the DCO;
• a digital algorithm block coupled to the frequency counter and the select switch, configured to supply the selection control signal to the select switch for selection of the controlled operating point word to the oscillator control word input of the DCO in the open loop mode, and for selection of the actual operating point word to the oscillator control word input of the DCO in the closed loop mode, to read two or more of the measured frequencies from the frequency counter and determine a nominal gain for the DCO in the open loop mode based on the measured frequencies, and to determine an actual operating point gain for the DCO after a settling time to insure a locked state in the closed loop mode;
• and
• a modulation circuit coupled to the DCO output signal at the DCO, the digital algorithm block and the select switch, configured to modulate the DCO output signal with a modulation signal input scaled according to a reciprocal of the determined actual operating point gain in the closed loop mode, and to provide the actual operating point word to the select switch for closed loop control of the DCO frequency.

A frequency modulator, comprising:

• measuring means operable to measure and determine a nominal gain characteristic of a digitally controlled oscillator (DCO) in an open loop mode;
• determining means operable to determine an actual gain characteristic of the DCO in a closed loop mode using the nominal gain characteristic;
• modulating means operable to frequency modulate a DCO output signal of the DCO with a modulation signal input;
• and
• amplifying means operable to scale the modulated DCO output signal based on the actual gain characteristic in the closed loop mode to provide gain compensation and frequency modulation of the DCO.

A method for direct carrier frequency modulation by determining a nominal gain in one of an open loop mode and a closed loop mode and an actual gain in the closed loop mode to adjust the gain of the digitally controlled oscillator (DCO) of a frequency modulator, the method comprising:

• selecting one of the open and closed loop modes of the frequency modulator;
• tuning the DCO with a minimal tuning word value and measuring the minimal DCO frequency near a minimal end of a tuning range of the DCO;
• tuning the DCO with a maximal tuning word value and measuring the maximal DCO frequency near a maximal end of a tuning range of the DCO;
• determining the nominal gain of the DCO based solely on the minimal and maximal DCO frequency measurements;
• applying a modulation signal to the frequency modulator;
• applying a reciprocal of the nominal gain to the frequency modulator;
• selecting the closed loop mode of the frequency modulator;
• determining the actual gain of the DCO by reading the actual tuning word value at the DCO;
• determining a new gain value from the actual tuning word value in the closed loop mode;
• and
• applying a reciprocal of the new gain value to the frequency modulator.