TABLE OF CONTENTS The output spectrum of a frequency synthesizer is always corrupted by noise with various frequency dependences and by tones, called spurs, placed at specific offsets from the synthesized signal. Noise and spurs have different origins, but both are small signals affecting the phase of the output signal. They can, therefore, be studied in the framework of the linear model for phase transfer.
In the evaluation of the output phase noise, we assume that we know the noise spectra of the main building blocks (Figure 2.21(a)). The spectra 
, 
and 
are usually calculated as phase noise spectra, measured in rad 2/Hz, while 
and 
are current and voltage noise spectra, measured in A 2/Hz and V 2/Hz respectively.
In most cases, all the spectra in the figure are white within the frequency range of interest, with the important exception of . The oscillator phase noise , in fact, features at least two components, shaped as 1/ f 3 and 1/ f 2, the former clearly becoming dominant as f approaches d.c.
Some of the PLL building blocks are non-linear (above all the oscillator). These features must be taken into account in the evaluation of the noise spectra. After that, however, these spectra can be transferred...
