1. A device for calibrating the frequency of an oscillator, having a first and a second input and generating an output frequency as a response to a first voltage signal applied at said first input, and an output digital signal applied at said second input of the oscillator for calibrating the output frequency, said calibration device comprising: a counter having a first input frequency proportional to a reference frequency and a second input frequency proportional to the output frequency of the oscillator, said counter counting a time window number given by the ratio of the second input frequency to the first input frequency; comparing means for comparing said counted time window number with a reference time window number; and means for changing a value of said output digital signal if said counted time window number is different from said reference time window number and until said counted time window number is equal to said reference time window number.