Keysight Technologies N5511A, Руководство пользователя

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N5511A Phase Noise Test System User’s Guide 37
Phase Noise Basics
What is Phase Noise?
Caution must be exercised when L(f) is calculated from the spectral density of
the phase fluctuations
because the calculation of L(f) is dependent on
the small angle criterion. Figure 3-7 , the 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 L(f) 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 3-7 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 L(f) remains
valid. Above the line the plot of L(f) becomes increasingly invalid, and
must be used to represent the phase noise of the signal.
Figure 3-7 Region of validity of L(f)