Method and arrangement for frequency synthesis

A fractional-N frequency synthesizer comprising:

• an accumulator that is incremented by a value representing a fractional part of a desired frequency;
• a variable divide-by-N circuit that is caused to divide by N.sub.1 a defined proportion of the time and by N.sub.2 a remaining portion of the time;
• a phase comparator for comparing an output pulse train from said variable divider with a reference frequency pulse train to produce phase error pulses proportional in width to a timing difference between said pulse trains;
• a first charge pump driven by said phase error pulses and producing first current pulses proportional in width to said phase error pulses and proportional in magnitude to a first programming current, said first current pulses being one of pull-up and pull-down current pulses;
• a second charge pump driven by said phase error pulses and producing second current pulses proportional in width to said phase error pulses and proportional in magnitude to a second programming current, said second current pulses being one of pull-up and pull-down current pulses;
• a first fractional-N ripple compensation current pulse generator producing pull-up current pulses proportional in width to contents of said accumulator and proportional in magnitude to a third programming current, an output of said first fractional-N ripple compensation current pulse generator being connected to a same circuit node as an output of said first charge pump such that said pull-up current pulses are added to said first current pulses to obtain a compensated proportional control circuit;
• a second fractional-N ripple compensation current pulse generator producing pull-up current pulses proportional in width to contents of said accumulator and proportional in magnitude to a fourth programming current, an output of said second fractional-N ripple compensation current pulse generator being connected to a same circuit node as an output of said second charge pump such that said pull-up current pulses are added to said second current pulses to obtain a compensated integral control circuit;
• and
• a loop filter having different input points to which respective ones of said compensated proportional control current and said compensated integral control current are applied to obtain an output voltage for controlling the frequency of a voltage controlled oscillator.