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4
Phase Noise Basics
What is Phase Noise?
4-6
E5500 Phase Noise Measurement System Version A.02.00
Caution must be exercised when
density of the phase fluctuations
L f ( )
is dependent on the small angle criterion.
measured phase noise of a free running VCO described in units of
L f ( )
, illustrates the erroneous results that can occur if the
instantaneous phase modulation exceeds a small angle line.
Approaching the carrier
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 power in the first order sideband which insures that
L f ( )
the calculation of
L f ( )
becomes increasingly invalid, and
represent the phase noise of the signal.
Figure 4-5 Region of Validity of L(f)
L f ( )
is calculated from the spectral
S φ f ( )
because the calculation of
L f ( )
obviously increases in error as it
remains valid. Above the line the plot of
S φ f ( )
Document Part No. E5500-90024 Ed. 1.0
Figure
4-5, the
must be used to
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