A reference oscillator for a transmitter and/or a receiver of electromagnetic signals. The reference oscillator is suitable for generating a modified reference signal alternating ON times and OFF times with a predefined duty cycle from a signal supplied by a reference resonator. The reference oscillator also includes an adjustment circuit suitable for adjusting the duty cycle of the modified reference signal according to at least one adjustment parameter dependent on a rank of at least one harmonic component of the modified reference signal so as to minimize at least one harmonic component of the modified reference signal. A frequency synthesizer and a radio frequency signal receiver can include such a reference oscillator.