Agilent Technologies E5500 Phase Noise Measurement System
18-3
Reference Graphs and Tables
Phase Noise Floor and Region of Validity
Phase Noise Floor and Region of Validity
Caution must be exercised when
L(f)
is calculated from the spectral density
of the phase fluctuations,
S
φ
(f)
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(f)
is still valid. As show in
the graph above, below the line, the plot of
L(f)
is correct; above the line,
L(f)
is increasingly invalid and
S
φ
(f)
must be used to represent the phase
noise of the signal. (
S
φ
(f)
is valid both above and below the line.) When
using the
L(f)
graph to compute
S
φ
(f)
, add 3 dB to the Level.
S
φ
(f) = 2 (L(f)) or S
φ
(f)
dB
= L(f)
dBc
+ 3 dB
