Phase noise is the spreading of noise energy about a CW or single frequency signal. It is caused by minor fluctuations in phase of the CW signal. These in turn are caused by factors including noise on the control line of a voltage controlled oscillator, for example.
See also: Wikipedia article
Phase Noise
Frequency Control Tutorials
Phase Noise Measurement Methods and Techniques (2012)
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Converting Oscillator Phase Noise to Time Jitter
Receiver Dynamic Range - Part 1
HP Phase Noise Seminar Notes
Getting Its Measure: Oscillator Phase Noise Measurement Techniques and Limitations
RF Measurements for Cellular Phones and Wireless Data Systems
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