A comparison of the noise, and random frequency and amplitude fluctuations in different types of oscillator

No oscillator gives a pure sine-wave output. There is always some random noise present in the oscillator circuit, if only that due to thermal agitation. Because of this, noise will appear in the output even if steps are taken to stabilize the output amplitude. Also, the output power is spread over a finite frequency band, and not concentrated at a single frequency. The nature of the random fluctuation in the output is dependent on the form of feedback employed and the method by which the output amplitude is controlled. The frequency band over which the output power is distributed depends, however, on the relative magnitudes of signal and noise in the circuit rather than on the particular circuit employed. The conclusion is that an oscillator should be operated at as high a level as possible consistent with stability and constancy of the circuit components.