An adjustable-frequency oscillator, is formed by two looped systems, functioning at the same frequency but the signals are phase shifted by 90°. Each looped system includes a phase shift device, an active element providing the gain and a resonator having a fixed phase-frequency characteristic. As the phase shift in each loop is imperatively a whole multiple of 2π, the phase shift added in each loop by the phase shift device entails that each resonator introduces a complementary phase shift to comply with the oscillation criterion. This complementary phase shift is produced at a frequency defined by the resonator, this then defining the frequency of oscillation. The frequency is adjusted by two phase shift stages, which carry out the analogue multiplication of the signals coming from the two looped systems by control voltages and the summing of these products.