Definitions
Historically there have been two conflicting yet widely used definitions for phase noise. The definition used by some authors defines phase noise to be the power spectral density (PSD) of a signal's phase, while the other definition is based on the PSD of the signal itself. Both definitions yield the same result at offset frequencies well removed from the carrier. At close-in offsets however, the two definitions differ.
The IEEE defines phase noise as where the "phase instability" Sφ(f) is the one-sided spectral density of a signal's phase deviation. Although Sφ(f) is a one-sided function, it represents "the double-sideband spectral density of phase fluctuation". The phase noise expression ℒ(f) is pronounced "script ell of f".
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