An apparatus and method fore harmonic characterization and ratio correction of device mismatch between coarse and fine varactor tuning devices within a segmented unified varactor bank of an (RF) digitally controlled oscillator (DCO). The DCO is divided into an MSB bank, LSB bank and sigma-delta (SD-LSB) bank. Any ratio mismatches between MSBs and LSBs are digitally calibrated out using a DCO step-size pre-distortion scheme wherein LSB steps are adjusted to account for ratio mismatch between the MSB/LSB step sizes. A harmonic characterization technique is used to estimate the mismatches in the minimal size CMOS tuning varactors of a digitally controlled RF oscillator (DCO), wherein nominal ratio mismatch between the MSB and LSB devices is estimated using hybrid stochastic gradient DCO gain estimation algorithms. The nominal ratio mismatch and the mismatches in MSB and LSB banks are used to determine average MSB/LSB mismatch which is then used to correct the LSB steps.