FREQUENCY SYNTHESIZER

PURPOSE:To eliminate a cause to the increase in the circuit scale such as a variable frequency divider and to improve the degree of freedom of an output frequency able to be set by varying a reference phase increment step to vary the frequency of a reference signal, then controlling the output frequency of a voltage controlled oscillator. CONSTITUTION:A low pass filter 5 eliminates a harmonic component from an output of a D/A converter 4, extracts only a component of a fundamental frequency fR and it is supplied as a reference signal input to a phase comparator 6. The output frequency of the VCO 8, that is, the output frequency f0 of the frequency synthesizer is set optionally at an integral number DELTAphi of multiple of a setting frequency pitch through the PLL constitution comparing the phase comparator 6, a loop filter 7, the VCO 8 and a fixed frequency divider 9. Moreover, the frequency of the reference signal is set sufficiently larger even when the pitch of the setting frequency is small and a defect of an extended frequency switching time is not caused.