1. A frequency synthesizer circuit comprising: a phase-locked loop circuit; a voltage controlled oscillator; a low pass filter; an input terminal for providing serial data provided from the exterior to said phase-locked loop circuit; an output terminal for providing an oscillation signal provided from said voltage controlled oscillator; and a testing unit providing a testing voltage with a binary value to said voltage controlled oscillator, wherein, said phase-locked loop circuit, said voltage controlled oscillator, said low pass filter, and said testing unit are mounted on a single chip; and usability of said frequency synthesizer circuit is determined based on the oscillation signal provided from said voltage controlled oscillator via said output terminal according to the testing voltage with a binary value provided from said testing unit.