The present invention provides a frequency synthesizer capable of
switching an oscillation frequency band while maintaining a lock state
realized with a small-scale and low-current-consumption circuit
configuration, and an oscillation control method of the frequency
synthesizer. A frequency synthesizer having an oscillation control
circuit capable of switching an oscillation frequency band, includes: a
differential bias unit for generating, as a differential bias signal, a
difference in a bias signal to the oscillation control circuit caused by
variations in the oscillation frequency band; a band limit detector for
detecting transition of the oscillation frequency, that exceeds a limit
value of the oscillation frequency band; and a transient controller for
performing control of gradually shifting an output bias signal on the
differential bias unit in a transient period in which operation of the
differential bias unit is switched for switching the oscillation
frequency band in accordance with detection of the band limit detector.