An oscillator includes a resonator, such as a crystal (12) coupled to first and second capacitor banks (14). The first and second capacitor banks (14) each comprise a plurality of capacitors (16) coupled to the resonator (12) through respective switching devices (18) that may be selectively enabled. The switches (18) are selectively enabled to couple a desired set of said capacitors (16) to said resonator (12). At least one of the switches (18sd) is controlled with a clock signal having a programmable duty cycle from a sigma-delta modulator (20) to enable at least one of said capacitors (16sd) during a first phase of the clock signal and disable that capacitor (16sd) during a second phase of the clock signal.