Here, V o is the nominal amplitude of the signal, and f is the nominal frequency of oscillation. The phase term may be random or discrete and could be seen on the spectrum analyzer. As shown in Figure 1 there are two types of fluctuating phase terms. The first, the discrete signals called spurious, appear as distinct components in the spectral density plot. The second term, random in nature, appears as random phase fluctuations and is commonly called phase noise.
Most oscillators operate in saturation.
The AM noise component is usually 20dB lower than the phase noise component. Many methods are used to characterize phase noise of an oscillator. Essentially, all methods measure the frequency or phase deviation of the source under test in either frequency or time domain. Since frequency and phase are related to each other, all these terms are also related. One of the most common fundamental descriptions of phase noise is the one sided spectral density of phase fluctuations per unit bandwidth.