Frequency Stability of Oscillators
Even if an oscillator is set at an initial frequency, it cannot be maintained throughout. They keep on changing either in a uniform way or sometimes erratically. the term “frequency stability” is used to define the ability of the oscillator to maintain a single fixed frequency as long as possible over a time interval. These deviations in frequency are caused due to variations in the values of circuit features (circuit components, transistor parameters, supply voltages, stray-capacitances, output load etc.) that determine the oscillator frequency.
Other factors responsible for drift in oscillator frequency are given below.
- Operating point of active device – 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).
- Inter-element capacitances – If the operating point of the active device in the circuit is in the non-linear portion of its characteristics, there may be variations in the transistor parameters which, in turn, affects the oscillator frequency stability. So, the operating point, Q is carefully selected to work in the linear portion of the characteristics of the active device.
- Mechanical vibrations – Although the mechanical vibrations is not such a high frequency stability changing factor, they can be easily avoided by isolating the oscillator circuit from the source of mechanical vibrations.
- When the circuit operates for a long time, the heat starts to build up. As a result, the values of the frequency determining components like resistors, inductors and capacitors change with temperature. Thus the transistor parameter values also tend to change. But, the change in the values of R,L, and C will be slow and thus the change in oscillator frequency will also be slow.
- 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.