A phase lock loop frequency synthesizer includes a phase rotator in the
feedback path of the PLL. The PLL includes a phase detector, a low pass
filter, a charge pump, a voltage controlled oscillator ("VCO"), and a
feed back path connecting output of the VCO to the phase detector. The
feedback path includes a phase rotator connected to the output of the VCO
and to an input of a frequency divider. Coarse frequency control is
implemented by adjusting the input reference frequency to the phase
detector or by adjusting the divider ratio of the frequency divider. Fine
frequency control is achieved by increasing or decreasing the rotation
speed of the phase rotator. The phase rotator constantly rotates phase of
the VCO output, thereby causing a frequency shift at the output of the
phase rotator. The rotation speed of the phase rotator is controlled by
an accumulator and a digital frequency control word. Any high frequency
noise generated by the phase rotator is rejected by the PLL by properly
setting the PLL bandwidth so that the noise falls outside the bandwidth
of the PLL. Therefore, a low noise synthesized output from the VCO is
generated.