Keysight Technologies N5511A, User Manual

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34 N5511A Phase Noise Test System User’s Guide
Phase Noise Basics
What is Phase Noise?
Phase terms
There are two types of fluctuating phase terms:
— spurious signals
— phase noise
Spurious signals
The first are discrete signals appearing as distinct components in the spectral
density plot. These signals, commonly called spurious, can be related to known
phenomena in the signal source such as power line frequency, vibration
frequencies, or mixer products.
Phase noise
The second type of phase instability is random in nature and is commonly
called phase noise. The sources of random sideband noise in an oscillator
include thermal noise, shot noise, and flicker noise. Many terms exist to
quantify the characteristic randomness of phase noise. Essentially, all methods
measure the frequency or phase deviation of the source under test in the
frequency or time domain. Since frequency and phase are related to each
other, all of these terms are also related.
Spectral density
One fundamental description of phase instability or phase noise is spectral
density of phase fluctuations on a per-Hertz basis. The term spectral density
describes the energy distribution as a continuous function, expressed in units
of variance per unit bandwidth. We can the convert rms phase fluctuations into
a 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 are
identical), we need only consider one of the noise side skirts. We use the
right-hand side noise side skirt and call that
. is directly related to by
a simple approximation which has generally negligible error if the modulation
sidebands are such that the total phase deviation are much less than 1 radian
( Δφ
pk
<< radian).