User`s guide

52 Agilent E5505A User’s Guide
3 Phase Noise Basics
Caution must be exercised when is calculated from the spectral density of
the phase fluctuations because the calculation of is dependent on
the small angle criterion. Figure 17, the measured phase noise of a free
running VCO described in units of , illustrates the erroneous results that
can occur if the instantaneous phase modulation exceeds a small angle line.
Approaching the carrier obviously increases in error as it indicates a
relative level of +45 dBc/Hz at a 1 Hz offset (45 dB more noise power at a 1 Hz
offset in a 1 Hz bandwidth than in the total power of the signal); which is of
course invalid.
Figure 17 shows a 10 dB/decade line drawn over the plot, indicating a peak
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 insures that the calculation of remains
valid. Above the line the plot of becomes increasingly invalid, and
must be used to represent the phase noise of the signal.
Figure 17 Region of validity of L(f)
L f()
S
φ
f()
L f()
L f()
L f()
L f()
L f()
S
φ
f()