Abstract: A software controlled crystal oscillator employing a method for digitally controlling a reference frequency of an oscillator, the method includes the steps of: locking a first signal produced by a first tunable software oscillator on to a first resonant frequency of a temperature sensing resonator; locking a second signal produced by a second tunable software oscillator on to a second resonant frequency of the temperature sensing resonator; estimating a temperature of the temperature sensing resonator using the first signal and the second signal; estimating the first resonant frequency and the second resonant frequency based upon the temperature; and adjusting the first signal to approximate the estimated first resonant frequency. An additional step includes controlling the reference frequency of the oscillator based upon the first signal such that the reference frequency is compensated for temperature.