User`s guide

Reference Graphs and Tables 16
Agilent E5505A User’s Guide 363
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 shown in
Figure 289, the line 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
Figure 289 Region of validity
E5505a_user_phase_noise_levels
17 Mar 04 rev 3
8657A/B
8644B
ESG with Opt 1E5
8664A/65A/65B Opt 004