Which of the following is the fixed frequency oscillator? 

Crystal Oscillator:

• Crystal oscillators are fixed frequency oscillators with a high Q-factor.
• It operates on the principle of the inverse piezoelectric effect in which alternating voltage applied across the crystal surfaces causes it to vibrate at its natural frequency.
• It is these vibrations that eventually get converted into oscillations.
• These oscillators are made of Quartz crystal Rochelle salt and Tourmaline.
• Quartz is inexpensive, naturally available, and mechanically strong when compared to others.

In a crystal oscillator, the crystal is suitably cut and mounted between two metallic plates as shown in the figure.

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Hartley Oscillator:

In a Hartley oscillator, the positive output feed-back is inductively coupled by a tank circuit consisting of an inductor coil with a center tap.

The tank circuit of Hartley Oscillator is shown:

Colpitts oscillator:

• The Colpitts oscillator consists of two capacitors and one inductor
• A capacitor with a center tap is used in the feedback system of the Colpitts oscillator
• It is used for the generation of sinusoidal output signals with very high frequencies

Phase Shift Oscillators:

• Phase Shift and Wein-bridge oscillators are used to generate audio frequencies, i.e. frequencies in AF Range.
• Commonly used phase shift oscillator using three lag circuit is as shown: