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Phase Noise Basics
What is Phase Noise?
L
Figure 4-4
(f) Described Logarithmically as a Function of Offset Frequency
L f ( )
Caution must be exercised when
is calculated from the spectral density
S φ f ( )
L f ( )
of the phase fluctuations
because the calculation of
is
dependent on the small angle criterion.
Figure
4-5, the measured phase noise
L f ( )
of a free running VCO described in units of
illustrates the erroneous
results that can occur if the instantaneous phase modulation exceeds a small
L f ( )
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 4-5
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
L f ( )
power in the first order sideband which insures that the calculation of
L f ( )
remains valid. Above the line the plot of
becomes increasingly
S φ f ( )
invalid, and
must be used to represent the phase noise of the signal.
Agilent Technologies E5500 Phase Noise Measurement System 4-5
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