controlled to deliver an output signal at a predefined output frequency a variable frequency divider to convert the output signal into a divided frequency signal a phase comparator to produce a signal measuring a phase difference between the divided frequency signal and a reference signal at a reference frequency and a loop filter to control the oscillator on the basis of the measurement signal to increase the speed of convergence of the synthesizer if the set point is changed the loop filter of the pll is a fractional i e non integer order low pass filter application_number US 23578705 A description with reference to fig1 a pll used in a frequency synthesizer according to the prior art comprises a vco 14 delivering a radiofrequency signal sout the frequency fout of which is for example of the order of a few hundred mhz this signal is addressed to a frequency divider 15 applying a variable division ratio n by modifying the set point associated with n the frequency of the output signal sout is changed in the example considered the pll is a charge pump pll cp pll which is one of the most widely used pll structures the divided frequency signal sdiv obtained from the frequency divider 15 is.