34 N5511A Phase Noise Test System User’s GuidePhase Noise BasicsWhat is Phase Noise?Phase termsThere are two types of fluctuating phase terms:— spurious signals— phase noiseSpurious signalsThe first are discrete signals appearing as distinct components in the spectraldensity plot. These signals, commonly called spurious, can be related to knownphenomena in the signal source such as power line frequency, vibrationfrequencies, or mixer products.Phase noiseThe second type of phase instability is random in nature and is commonlycalled phase noise. The sources of random sideband noise in an oscillatorinclude thermal noise, shot noise, and flicker noise. Many terms exist toquantify the characteristic randomness of phase noise. Essentially, all methodsmeasure the frequency or phase deviation of the source under test in thefrequency or time domain. Since frequency and phase are related to eachother, all of these terms are also related.Spectral densityOne fundamental description of phase instability or phase noise is spectraldensity of phase fluctuations on a per-Hertz basis. The term spectral densitydescribes the energy distribution as a continuous function, expressed in unitsof variance per unit bandwidth. We can the convert rms phase fluctuations intoa spectral density by dividing by the bandwidth of the noise sideskirts:Where BW (bandwidth is negligible with respect to any changes in .Because phase modulation is a symmetric process (both sidebands areidentical), we need only consider one of the noise side skirts. We use theright-hand side noise side skirt and call that . is directly related to bya simple approximation which has generally negligible error if the modulationsidebands are such that the total phase deviation are much less than 1 radian(Δφpk << radian).
