The frequency stability of an oscillator can be defined as a measure of its ability to maintain a constant frequency over a long time interval.
However, it has been observed if an oscillator is set at some particular frequency it does not maintain it for a long per period, in other words, the frequency of an oscillator changes slowly (or drifts away) from the initially set value.
Sometimes, the change in frequency is uniform in one direction (i.e. either increasing or decreasing) but at some, it may be changing quite erratically.
The change in oscillation frequency can observe arise due certain factored
The operating point of the active device:
The operating point of the active device (i.e. visitor transistor or FET) is selected in such a manner so that its operation in the non-linear region changes the values of device parameter which in turn affects the frequency stability of the oscillator.
The value of circuit components (i.e. resistor, inductor and capacitors) increase with the variation in temperature. Because of such cause a drift in oscillator frequency. Supply voltage: the changes in d.c. supply voltage applied to the active changes shift the oscillator frequency this problem can be as avoided by using a highly regulated power supply .
a change in the output load can cause a change in the Q-factor of the tank circuit thereby causing a change in oscillator output frequency.
Inter-element capacitance and stray capacitances:
Any changes in the interelement capacitances of a transistor (particularly collector to emitter capacitance) cause changes in the oscillator output frequency stability of an oscillator. The effect of change interelement capacitance can be neutralized by putting an additional capacitor across the corresponding elements. However, it is difficult to completely avoid the effect of stray capacitances.