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There are two types of fluctuating phase terms. The first, deterministic, 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.
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.
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. Thus
be considered as:
Where BW (bandwidth is negligible with respect to any changes in
versus the fourier frequency or offset frequency (f).
Another useful measure of noise energy is L(f), which is then directly related
S φ f ( )
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 (
Figure 4-2
CW Signal Sidebands viewed in the frequency domain
Agilent Technologies E5500 Phase Noise Measurement System 4-3
∆φ 2 rms f ( )
S φ f ( )
------------------------------------------------------------------------ -
=
BW used to measure ∆φ rms
∆φ
<< radian).
pk
L f ( )
=
Phase Noise Basics
What is Phase Noise?
S φ f ( )
(Figure 4-2 on page
rad 2
----------- -
=
Hz
1
-- - S ∆φ f ( )
2
4-3) may
S φ
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