Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 5 Marks

Year: Dec 2013

1. Oscillators are based on amplifiers that have positive feedback through a frequency sensitive network. The quality of the frequency sensitive network determines the stability. Other factors may need to be controlled too, such as power supply voltage, gain, load isolation, temperature and the effects of parasitic components like stray capacitance and inductance.
2. Oscillators based on a resonator like a quartz crystal are generally easily stabilized because the mechanical resonance of the quartz element is highly stable, only influenced to a small degree by external factors.
3. It also has a high Q, which can be seen as the bandwidth is very narrow, so there is little chance for the oscillator to work at anything but the strong resonance of the crystal. A simple crystal oscillator can have $10^5$ accuracy easily.
4. With care like controlled gain, temperature controlled oven, and regulated supply, together with good load isolation they can achieve $10^{-9}$ stability.
5. Oscillators using electrical resonance based on a capacitor and inductor can have stability around $10^{-9}$ too with care. Issues include shielding of stray coupling, power regulation, gain control, load isolation, minimizing effects of changes in amplifier connected to the resonator, and choosing high quality L and C components.
6. It is also usual to use an LC resonator with a higher value of C (and thus lower L) to get better Q, and minimize the effects of stray capacitance changes.
7. The stability is less than for a quartz resonator because the resonator has less Q, so its bandwidth is more, and the actual resonant frequency is not as sharply defined as the quartz type resonator. Look up VFO and Clapp oscillator for examples.
8. With these the physical construction is just as important as the actual circuit.
9. Stability is mainly about attention to detail. However quartz oscillators tend to be better anyway.