what is phase locked oscillator

A phase-locked oscillator is a very stable source with low phase noise and stable
output frequency. A high-frequency oscillator can be phase locked to a lowfrequency,
stable crystal oscillator or crystal-controlled oscillator to achieve good
phase noise and frequency stability. A simplified phase-locked loop (PLL) block
diagram is given in Fig. 6.16. It consists of a very stable low-frequency oscillator
that acts as the reference source, a phase detector, a low-pass filter, a VCO, and a
frequency divider. The phase detector produces a DC control voltage at the output of
the low-pass filter, with the magnitude and polarity determined by the phase
(frequency) difference between the crystal oscillator and VCO output. The control
voltage is used to vary the VCO frequency. The process will continue until the VCO
frequency (or phase) is aligned with the multiple of the crystal oscillator frequency.
The frequency divider is used to divide the output frequency of VCO by N to match
the frequency of the reference oscillator. Because of the tracking, the output of the
PLL has phase noise characteristics similar to that of the reference oscillator.