The present invention relates to a self-oscillating modulator (2) comprising a PWM cycle constrainer (21). The present invention further relates to a method of stabilising the switch frequency of a self-oscillating modulator operating on a high level input signal, whereby at least one PWM cycle constraint representative signal (26, 32, 33) is applied to a pulse width modulated signal (18) within said self-oscillating modulator (2). The present invention further relates to a method of avoiding pulses narrower than a predetermined minimum pulse width or wider than a predetermined maximum pulse width in a self-oscillating modulator (2) operating on a high level input signal, whereby at least one PWM cycle constraint representative signal (26, 32, 33) is applied to a pulse width modulated signal (18) within said self-oscillating modulator. The present invention further relates to a method of providing at least one PWM cycle constraint representative signal (26, 32, 33) for use in a self-oscillating modulator, whereby said PWM cycle constraint representative signal is established by the steps of: providing a periodic signal (17, 25, 31) by means of a reference signal generator (22), providing a square wave signal (31) on the basis of, and in synchrony with, said periodic signal (17, 25), and providing said PWM cycle constraint representative signal (26, 32, 33) on the basis of, and in synchrony with, said square wave signal (31).