Fractional N frequency synthesis with residual error correction and method thereof

A fractional-N frequency synthesizer accepting a first signal to select a controllable oscillator output signal frequency by dividing the output signal frequency with a variable divisor divider controlled by a divider sequence signal from an accumulator network creating a divided signal, the divider sequence signal containing a desired value and an error value, the fractional-N synthesizer comprising:

• an error correction signal generator for approximating the error contained in the divider sequence signal and generating an error correction signal;
• a phase detector coupled to the divided signal output from the variable divisor divider for detecting a difference in phase of a reference signal and the divided signal and generating a phase difference signal;
• a modulator coupled to the phase difference signal output from the phase detector for modulating the phase difference signal with the error correction signal and generating a modulated signal having reduced residual error;
• and
• a controllable oscillator for accepting the modulated signal and generating an output signal having a predetermined frequency.

A fractional-N frequency synthesizer comprising:

• a controllable oscillator outputting a first signal having a first frequency and accepting a modulated signal for controlling the first frequency of the first signal;
• an accumulator network creating a divider sequence signal, the divider sequence signal containing a desired value and an error value;
• a divider having a signal input and a control input and a signal output, the first signal coupled to the signal input of the divider, the divider sequence signal coupled to the control input of the divider for controlling a variable divisor divider, the variable divisor divider for dividing the first frequency of the first signal and generating a first divided signal;
• an error correction signal generator for approximating an error contained in the divider sequence signal at least due in part to the error value in the divider sequence signal and generating an error correction signal;
• a phase detector coupled to the divided signal output from the variable divisor divider for detecting a difference in phase of a reference signal and the divided signal and generating a phase difference signal;
• and
• a modulator coupled to the phase difference signal output from the phase detector for modulating the phase difference signal with the error correction signal and generating the modulated signal having reduced residual error.

A method of frequency synthesizing, the frequency synthesizer having a controllable oscillator for generating a first signal having a first frequency, the method comprising the steps of:

• creating a divider sequence signal, the divider sequence signal containing a desired value and an error value;
• dividing the frequency of the first signal, the step of dividing dependent upon the divider sequence signal and forming a first divided signal having a phase;
• detecting a difference in phase of a reference frequency and the divided signal and generating a phase difference signal;
• approximating an error contained in the first divided signal at least due in part to the error value in the divider sequence signal and forming a first error correction signal;
• and
• modulating the phase difference signal with the first error correction signal and generating a modulated signal having reduced residual error and for controlling the controllable oscillator.