Patent ID: 7405627

Claim:
A PLL frequency synthesizer which outputs a plurality of signals having a plurality of frequencies, comprising: a plurality of voltage-controlled oscillators which have oscillation frequencies and output said plurality of signals, where the oscillation frequencies are controlled according to control voltages; a first switch which selects one of said plurality of signals, and outputs the one of the plurality of signals; a frequency divider which divides a frequency of said one of the plurality of signals by a changeable frequency-division ratio, and outputs a frequency-divided signal; a phase comparator which outputs a phase difference between said frequency-divided signal and a reference signal; a second switch which selects as an output path of said phase difference one of paths respectively connected to the plurality of voltage-controlled oscillators; a plurality of low-pass filters each of which is arranged in correspondence with one of said plurality of voltage-controlled oscillators, has a changeable time constant, and converts said phase difference outputted through said output path, into one of said control voltages; and a controller which cyclically controls operations of said first switch, said second switch, and said frequency divider so that said plurality of voltage-controlled oscillators continuously outputs said plurality of signals, and changes said changeable time constant of each of said plurality of low-pass filters after all of said plurality of signals become stable.