Keysight Technologies N5511A User Manual page 306

Evaluating Your Measurement Results
Problem Solving
Small angle line
Caution must be exercised where L(ƒ) is calculated from the spectral density of
the phase modulation Sφ(ƒ)/2 because of the small angle criterion. Refer to
Figure 14-13. Below the line, the plot of L(ƒ) is correct; above the line, L(ƒ) is
increasingly invalid and Sφ(ƒ) 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 (Sφ(ƒ)).
L(ƒ) 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(ƒ) is still valid.
Figure 14-13 L(ƒ) Is only valid for noise levels below the small angle line
306 N5511A Phase Noise Test System User's Guide