Multiplying the frequency of a signal by a factor of N using an ideal frequency multiplier increases the phase noise of the multiplied signal by 20log(N) dB. Similarly dividing a signal frequency by N reduces the phase noise of the output signal by 20log(N) dB.
Real frequency multipliers increase the phase noise above the theoretical limit.
Real frequency dividers also have an output phase noise above the theoretical limit. One source of increased divider output noise, that is often overlooked, is aliasing of its input clock noise (see reference 1 for details). Regenerative dividers can work over a much wider frequency range than conventional digital dividers. They can also have much lower output phase noise than a conventional digital divider.
Isolation amplifiers are used in precision frequency systems to minimise the effect of load changes and injected signals on oscillators. It is important that an isolation amplifier has high reverse isolation and low phase noise, particularly flicker phase noise to avoid significantly increasing the phase noise of its output. Isolation amplifiers are used in frequency distribution systems to minimise the interaction between various loads. A low crosstalk between channels ensures that the phase changes of one output due to load changes at another output are small. Low input and output VSWR are useful in minimising phase changes due to load variations.
The phase noise of an oscillator is affected by several factors:
A crystal itself has intrinsic flicker noise of quantum mechanical origin. This noise is reduced by increasing the unloaded Q of the crystal and reducing the electrode area,
The sustaining amplifier also contributes flicker noise which is reduced by maximising the loaded Q of the oscillator and by using negative RF feedback in the sustaining amplifier.
The buffer amplifier in a well designed crystal oscillator is the principal contributor to the oscillator phase noise floor which is reduced by increasing the power extracted by the buffer amplifier.