The circuit (1) comprises a closed control loop (2) including a voltage controlled oscillator (3), a frequency divider (4), a phase comparator (5), and a low-pass filter (6) cascade interconnected via respective inputs and outputs, with the filter (6) output connected to the oscillator (3) input to form the loop (2). The circuit further comprises a second comparator (10) connected in parallel to the phase comparator (5) between the frequency divider (4) output and the low-pass filter (6) input to compare a reference frequency (F1) to a frequency (F2) from the divider (4) when the values of such frequencies (F1,F2) lie far apart.