A broadband frequency synthesizer including a VCO for supplying a high frequency output signal, a dual mode divider circuit, a means for selecting a division mode of the divider circuit, a phase detector and a low pass filter. The divider circuit divides the frequency of the output signal by a first division facto N1 in a first mode M1or by a second factor N2 different from the first factor N1 in a second mode M2 to provide the divided frequency signal (Fdiv). The selection means selects by determined time period the first or second division mode of the divider circuit as a function of the programmed frequency of the output signal in the frequency band defined by the first and second division factors N1 and N2. If this frequency is dose to the center of the frequency band, the dual mode divider divides the output signal frequency by a third factor N3 in a first mode M1 or by a fourth division factor N4. different from N3, in a second mode M2 according to the programming of the selection means.