To avoid the risk of producing an artifact by providing a deviation with a positive value during a first period and a negative value during a second period and making the cumulative sum of the first period and the cumulative sum of the second period be actually equal in the lapse of time.

NRZ signals are sent to the input of a frequency control voltage generator 30 and the generator 30 generates frequency control signals MOD for modulating an oscillator 40 at the output. The signals are added to error signals arriving from a comparator 60 in an element 80 and the oscillator 40 is frequency modulated. A module 30 generates three control voltages respectively called as low, middle and high, combines a middle value to a '1' bit and combines a sometimes high and sometimes low value to a '0' bit. The high value is generated in a first time interval, the low value is generated in a second time internal and the time interval is decided so as to make the total of the first and second time intervals be actually equal in the lapse of the time.