A voltage controlled oscillator is provided comprising resonant circuit means (301) and an active circuit means (303) having an active element T1, the input of the active circuit means being connected to the resonant circuit means (301). Variable capacitance ratio means (304) are connected to the active circuit means (303) in order to oscillate the active element T1 at a resonance frequency of the resonance circuit means (301). Variable control means are provided for changing the resonance frequency, so that the oscillator frequency of the active element T1 can be changed substantially linearly over a wide bandwidth, to thereby retain the amount of positive feed back at a predetermined level against the oscillator frequency of the active circuit means (303).