PLO fundamentals
All microwave oscillators exhibit noise spectra and the mechanisms producing these are well understood. In general, the dominant effect impacting on applications is FM noise (or phase noise, to which it is physically related) which may limit sensitivity, bit rate and channel spacing (amongst other things) in a typical communication system.
There are external sources of FM noise such as mechanical vibration and radiation from nearby digital circuits and power supply lines. Elimination of all of these spurious sources still leaves the fundamental noise mechanisms in the oscillator such as flicker noise and thermal effects which exhibit a generally continuous spectrum whose amplitude is affected by the Q of the oscillator circuit.
The purpose of phase-locking the oscillator is to improve the close to carrier noise of an oscillator which has a good noise floor by superimposing a reference oscillator which has good close to carrier performance. In a typical PLO a crystal having excellent short-term stability is used as a reference for a DRO which has relatively poor short-term stability but an excellent far from carrier noise floor.