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Evaluating Your Measurement Results
Problem Solving
Small Angle Line
L(f)
Figure 15-4
Is Only Valid for Noise Levels Below the Small Angle Line
L(f)
Caution must be exercised where
is calculated from the spectral density
S
(f)
of the phase modulation
/2 because of the small angle criterion. Below
φ
L(f)
L(f)
the line, the plot of
is correct; above the line,
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
S
(f)
L(f)
Spectral Density of Phase Modulation (dB/Hz) graph type (
).
φ
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.
15-18 Agilent Technologies E5500 Phase Noise Measurement System
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