Abstract: A frequency synthesizer includes a multi-signal comparing phase frequency detector/converter, a loop filter, a controllable oscillator, and a frequency divider. The multi-signal comparing phase frequency detector/converter simultaneously receives N input reference frequency signals and N feedback reference frequency signals. Frequencies of the input reference frequency signals are equivalent to one another while phases thereof are different from one another. Frequencies of the feedback reference frequency signals are equivalent to one another while phases thereof are different from one another. The multi-signal comparing phase frequency detector/converter compares the input reference frequency signals and corresponding feedback reference frequency signals, and then outputs a comparison control signal according to the comparison result. The frequency synthesizer of the present invention is adapted to depress a reference spur, thus achieving an ideal output frequency signal.