An electronic device controlling a frequency modulation index has a frequency modulation index control loop having an input adapted to be connected to a frequency output of a frequency controllable oscillator. The oscillator has a center frequency Fc and an output adapted to be connected to an input of a frequency-modulation unit, the modulation index control loop being adapted to determine the modulation index. Further provided is a method of frequency-modulating with a modulation frequency Fm a non-linear controllable oscillator having a center frequency Fc with a constant modulation index.