Phase noise is the frequency domain representation of rapid, short-term, random fluctuations in the phase of a wave, caused by time domain instabilities (“jitter”). Generally speaking radio frequency engineers speak of the phase noise of an oscillator, whereas digital system engineers work with the jitter of a clock.
An ideal oscillator would generate a pure sine wave. In the frequency domain, this would be represented as a single pair of delta functions (positive and negative conjugates) at the oscillator’s frequency, i.e., all the signal’s power is at a single frequency. All real oscillators have phase modulated noise components. The phase noise components spread the power of a signal to adjacent frequencies, resulting in sidebands. Oscillator phase noise often includes low frequency flicker noise and may include white noise.
Characterizing Phase Noise
The term phase noise is widely used for describing short term random frequency fluctuations of a
signal. Frequency stability is a measure of the degree to which an oscillator maintains the same value
of frequency over a given time. This may be specified in a number of ways. Three commonly used
terms for describing frequency stability are used here. An ideal sine wave oscillator may be described
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. The term spectral density describes the energy distributions as a
continuous function, expressed in units of energy per unit bandwidth. The phase noise of an oscillator
is best described in the frequency domain where the spectral density is characterized by measuring the
noise sidebands on either side of the output signal center frequency. Single sideband phase noise is
specified in dBc/Hz at a given frequency offset from the carrier. The frequency domain information
about phase or frequency is contained in the power spectral density SDq(f) of the phase or in the
power spectral density S Df (f) of the frequency. Here, f refers to the modulation frequency or offset
frequency associated with the noise-like variations in q(t).