Frequency Stability of Oscillator

Frequency Stability of Oscillator

• For an oscillator, the frequency of the oscillations must remain constant. The analysis of the dependence of the oscillating frequency on the various factors like stray capacitance, temperature etc. is called as the frequency stability analysis.

• The measure of ability of an oscillator to maintain the desired frequency as precisely as possible for as long time as possible is called frequency stability of an oscillator.

• In a transistorised Colpitts oscillator or Hartley oscillator, the base-collector junction is reverse biased and there exists an internal capacitance which is dominant at high frequencies. This capacitance affects the value of capacitance in the tank circuit and hence the oscillating frequency.

• Similarly the transistor parameters are temperature sensitive. Hence as temperature changes, the oscillating frequency also changes and no longer remains stable. Hence practically the circuits cannot provide stable frequency.

Factors affecting the frequency stability :

• In general following are the factors which affect the frequency stability of an oscillator :

1. Due to the changes in temperature, the values of the components of tank circuit get affected. So changes in the values of inductors and capacitors due to the changes in the temperature is the main cause due to which frequency does not remain stable.

2. Due to the changes in temperature, the parameters of the active device used like BJT, FET get affected which inturn affect the frequency.

3. The variation in the power supply is another factor affecting the frequency.

4. The changes in the atmospheric conditions, aging and unstable transistor parameters affect the frequency.

5. The changes in the load connected, affect the effective resistance of the tank circuit.

6. The capacitive effect in transistor and stray capacitances, affect the capacitance of the tank circuit and hence the frequency.

• The variation of frequency with temperature is given by the factor denoted as S.

Sω T = Δω / ω_r / ΔT / T_r parts per million per ^oC ... (10.12.1)

where          ω_r = Desired frequency

T_r = Operating temperature

Δω = Change in frequency

ΔT = Change in temperature

• The frequency stability is defined as,

Sω  = d θ / dω... (10.12.2)

where dθ = Phase shift introduced for a small frequency change in desired frequency f_r.

Key Point : Larger the value of d 0/dra, more stable is the oscillator.

• The frequency stability can be improved by the following modifications :

1. Enclosing the circuit in a constant temperature chamber.

2. Maintaining constant voltage by using the zener diodes.

3. The load effect is reduced by coupling the oscillator to the load loosely or with the help of a circuit having high input impedance and low output impedance.

Review Question

1. Write a note on frequency stability of oscillators.