FREQUENCY SYNTHESIZER, RADIO COMMUNICATION DEVICE, AND RADIO COMMUNICATION DEVICE CONTROL METHOD

摘要

Provided is a frequency synthesizer which can suppress degradation of the transmission performance when applied to a radio communication device which can switch the transmission power from one to another and execute transmission with an ultra weak power.  The frequency synthesizer (1) includes a VCO (10), a PLL circuit (18), and a first control circuit (an amplification control circuit (16) and a reference signal generation circuit (17)).  The VCO (10) can modify the oscillation frequency in accordance with the frequency control voltage and modify the output signal width in accordance with an amplitude control signal.  The PLL circuit (18) generates a frequency control voltage in accordance with a result of comparison between a signal obtained by dividing the output signal of the VCO (10) and a reference frequency signal.  Moreover, the first control circuit (circuits 16 and 17) supplies the amplitude control signal to the VCO (10) and controls the VCO (10) so that the output signal amplitude has a value corresponding to a mode switching signal (control signal CODE_REF) to which the output signal amplitude is supplied from outside.