Frequency stability and measurement
G. H. Bryant · Institution of Engineering and Technology eBooks · 1993
An ideal source provides a single spectral line with zero width and finite energy. This strange mathematical world does not exist in practice, because real oscillators produce lines of finite width and have random amplitude, frequency and phase fluctuations. The spectral line, sometimes called the carrier, may also be accompanied by harmonics, usually generated by the non-linear behaviour of active components in the oscillator circuit. These are deterministic and not the subject of this discussion. Random variations modulate the carrier to produce noise sidebands in a bandwidth determined by the frequency range of the fluctuations. These noise sidebands rise above the general thermal background noise and tend to peak as the frequency approaches the carrier. Amplitude fluctuations can be minimised in a high-quality oscillator, leaving frequency and phase fluctuations as the main causes of source noise. Low-frequency fluctuations cause sidebands with very small separation from the carrier.