Phase-locked loop having charge pump driven with preserved energy for instant channel switching

Claim 1

A frequency synthesizer for communication channels, comprising: a voltage controlled oscillator to produce a frequency signal; a variable frequency divider for dividing the frequency of said frequency signal according to the frequency of one of said communication channels to produce a divided-frequency signal; an oscillator for generating a reference frequency signal; a phase comparator for detecting a phase difference of first polarity and a phase difference of second polarity between the divided-frequency signal and said reference frequency signal and producing a charge-control signal across output terminals thereof with a duration variable as a function of the phase difference of first polarity and a discharge-control signal across said output terminals with a duration variable as a function of the phase difference of second polarity; first and second integrator connected respectively to the output terminals of said phase comparator for respectively smoothing waveforms of said charge-control signal and said discharge-control signal; a charge pump arranged to be charged in response to the smoothed waveform of the charge-control signal and arranged to discharge energy in response to the smoothed waveform of the discharge-control signal to produce a frequency control voltage, said charge pump applying the frequency control voltage to said voltage controlled oscillator; and a differentiator coupled across the output terminals of said phase comparator for absorbing part of energy of said charge-control signal applied to said first integrator and part of energy of said discharge-control signal applied to said second integrator.