N5511A Phase Noise Test System User’s Guide 329
Reference Graphs and Tables
Phase Noise Floor and Region of Validity
Phase Noise Floor and Region of Validity
Caution must be exercised when
L
(ƒ) is calculated from the spectral density of
the phase fluctuations,
S
φ
(ƒ) because of the small angle criterion. The –
10 dB/decade line is drawn on the plot for an instantaneous phase deviation of
0.2 radians integrated over any one decade of offset frequency. At
approximately 0.2 radians, the power in the higher order sidebands of the
phase modulation is still insignificant compared to the power in the first order
sideband which ensures the calculation of
L
(ƒ) is still valid. As shown in
, the line plot of
L
(ƒ) is correct; above the line,
L
(ƒ) is increasingly invalid
and
S
φ
(ƒ) must be used to represent the phase noise of the signal. (
S
φ
(ƒ) is
valid both above and below the line.) When using the
L
(ƒ) graph to compute
S
φ
(ƒ), add 3 dB to the Level.
S
φ
(ƒ) = 2 (
L
(ƒ)) or
S
φ
(ƒ)
dB
=
L
(ƒ)
dBc
+ 3 dB
Figure 16-2
Region of validity
Summary of Contents for N5511A
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