Exemplary embodiments of the invention provide a system, method and apparatus for frequency calibration of a free-running, harmonic oscillator. A reference oscillator provides a reference frequency. An exemplary system comprises the harmonic oscillator, a frequency divider, a comparator, and a reactance modulator. The oscillator comprises a plurality of switchable reactance modules and a coefficient register, and provides an oscillation signal having an oscillation frequency. The frequency divider provides an output frequency as a fraction of the oscillation frequency. The comparator compares the output and reference frequencies and provides a comparison signal when the output frequency is not substantially equal to the reference frequency. The reactance modulator determines and provides to the coefficient register a first plurality of coefficients to control switching of a first subset of the reactance modules to increase the reactance of the oscillator when the output frequency is greater than the reference frequency, and a second plurality of coefficients to control switching of a second subset of the reactance modules to decrease the reactance of the oscillator when the output frequency is less than the reference frequency.