15-18
Agilent Technologies E5500 Phase Noise Measurement System
Evaluating Your Measurement Results
Problem Solving
Small Angle Line
Figure 15-4
L(f)
Is Only Valid for Noise Levels Below the Small Angle Line
Caution must be exercised where
L(f)
is calculated from the spectral density
of the phase modulation
S
φ
(f)
/2 because of the small angle criterion. Below
the line, the plot of
L(f)
is correct; above the line,
L(f)
is increasingly
invalid and Sf(f) must be used to accurately represent the phase noise of the
signal. To accurately plot noise that exceeds the small angle line, select the
Spectral Density of Phase Modulation (dB/Hz) graph type (
S
φ
(f)
).
L(f)
raises the noise floor by 3 dB.
The –10 dB per 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 sideband of the
phase modulation is still insignificant compared to the power in the first
order sideband. This ensures that the calculation of cal L(f) is still valid.
