Frequency Stability of Oscillator:
An oscillator having initially been set at a particular frequency does not maintain its initial frequency, but instead drift and wander about in frequency, sometimes uniformly in one direction, sometimes quite erratically. The frequency stability of an oscillator is a measure of its ability to maintain as nearly a fixed frequency as possible over as long a time interval as possible. These deviations in frequency are caused due to variations in the values of circuit features (circuit components, transistor parameters, supply voltage, stray capacitances, output load etc.) that determine the oscillator frequency.
As the circuit operates, it warms up. Consequently the values of resistors, inductors and capacitors, which are frequency determining factors in such circuits, are changed with temperature. Variations in temperature also cause variations in transistor parameters. However, the variation or drift in oscillator frequency is slow because of slow variations in R, L, C and transistor parameters with time.
The other major factor responsible for deviation in frequency is variations in power supply (operating voltage applied to the active device). However, this problem can be overcome by using regulated power supply.
Any variation in load coupled to the tank circuit may cause a change in effective resistance of the circuit by transformer action which, in turn, causes the drift in frequency.
Other factors responsible for drift in oscillator frequency are:
Inter-element capacitances, operating point of active device and mechanical vibrations. The effects of variations in inter-element capacitances can be neutralized by introducing a swamping capacitor across the offending elements (the introduced capacitor becomes the part of the tank circuit). The operating point Q of the active device is so chosen as to restrict the operation of the device on the linear portion of its characteristic because operation on nonlinear portion causes the variations in transistor parameters which, in turn, affect the Frequency Stability of Oscillator. The instability in oscillator frequency caused due to mechanical vibrations can be avoided by isolating the oscillator circuit from the source of mechanical vibrations.