| Popis: |
An oscillator is an inherently non-linear device. If it had an effective loop gain of exactly unity for all signal levels, an initial low level oscillation would never build up in amplitude. Some non-linearity must be present to reduce the effective loop gain to unity at the full output level. During each RF cycle, the instantaneous voltage will sweep over the full range and thus will normally experience this non-linearity. The effects of such non-linearity will include the generation of harmonics and intermodulation between any noise components which may be present and also between such noise components and the carrier. Due to flicker effects in the active device used, noise close to DC will tend to follow a 1/f power law with frequency. For example, a noise component in a 1 Hz bandwidth at a frequency of 5 Hz will be relatively large and due to the non-linearity will beat with the carrier to produce noise components at (f0 ± 5) Hz which will be of significant amplitude. The only other significant contribution to the noise output of a good oscillator would be expected to be the noise components existing initially at frequencies adjacent to f0. These would be identical in an ideal linear oscillator to those in a real somewhat non-linear oscillator. Thus, it is to be expected that, if a linear model were used to predict the carrier to noise ratio of an oscillator, the main error would be due to the neglect of the effects of 1/f noise transposed to small offset frequencies. Such a model should, therefore, be adequate to determine the performance of high quality oscillators except for very small offset frequencies. |
