Patent ID: 7995697

Claim:
An apparatus for adapting a modulation system using a phase-locked loop (PLL) to operate using a reference frequency signal having a flexible frequency, comprising: an internal reference signal source configured to provide an internal reference signal having an internal frequency that is independent of the reference frequency; a frequency signal source configured to provide a plurality of first samples of a frequency signal wherein the first samples are taken at a first sampling frequency according to the internal reference signal; a pre-distortion filter configured to adapt the plurality of first samples to compensate for a low-pass frequency response of an output system; a resampling device configured to receive the internal reference signal, a divided frequency signal having a frequency proportional to frequency of the output system, and the plurality of adapted first samples and resample the plurality of first samples to generate a plurality of second samples taken at a second sampling frequency according to the reference frequency; a loop gain compensation device configured to receive the reference frequency and apply an offset gain to inversely offset a change in PLL open loop gain responsive to a change in the reference frequency; and a sigma-delta modulator configured to receive the plurality of second samples and the divided frequency signal and adapt a frequency division ratio used by the output system.