Keysight Technologies N5511A, User Manual

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N5511A Phase Noise Test System User’s Guide 33
Phase Noise Basics
What is Phase Noise?
Ideal vs Real Word Signals
Mathematically, an ideal sine wave can be described by
Where A
o
=nominal amplitude, f
o
=nominal frequency. In the time domain, this
signal is a perfect sinusoidal waveform, and in the frequency domain, it is
represented by a single spectral line. See Figure 3-2 .
Figure 3-2 Single Spectral Line
In practice however, there are always small, unwanted amplitude and phase
fluctuations present on the signal. An actual signal is better modeled by
Where E(t) = Amplitude fluctuations, and = randomly fluctuating phase
term, or phase noise. This randomly fluctuating phase term could be observed
on an ideal RF analyzer (one which has no sideband noise of its own) as seen in
Figure 3-3 . The signal is now represented by a spread of spectral lines - both
above and below the nominal signal frequency in the form of modulation
sidebands due to the random amplitude and phase fluctuations.
Figure 3-3 Spread of Spectral Lines