A quartz crystal exhibits the property that when mechanical force is applied, it generates electrical potential. This property of the crystal is known as piezo electric effect. Also if electric field is applied to a crystal it vibrates mechanically.

• If we apply mechanical vibrations to the crystal under proper conditions then we can obtained electrical oscillations from it.

• Crystal oscillator provides high frequency stability in spite of change in temperature, pressure and voltage .

The ac equivalent circuit diagram of crystal is as shown in the figure :

• In circuit R : Internal friction losses

                   L: mass of crystal and its inertia

                   C: Stiffness under the vibrating condition

                   Cm: crystal equivalent capacitance due to mounting arrangement

Resonant Frequency:

From the ac equivalent of the crystal we can say that it has two resonant frequencies, series and parallel resonant frequency.

Series Resonant frequency : Fs= 1/(2π√LC)

Parallel Resonant Frequency : Fp=1/(2π√LCeq)

Ceq: parallel capacitance between C and Cm is calculated as Ceq=(C*Cm)/((C+Cm))

The parallel resonant frequency can be varied by varying the value of Cm and is always greater than series resonant circuit.

The piezo electric crystal can operates in two modes

Fundamental Mode: In this mode crystal oscillator oscillates with fundamental crystal frequency.

Overtone mode: For higher frequency such as greater than 20 ghz overtone mode is used

Advantages:

• It provide very high frequency stability

• It gives very low frequency drift.

Disadvantages :

• Suitable only for high frequency oscillator

Application: • Used in computer for generating clock signal for CPU

• Used in frequency synthesizer circuit

• Radio and TV transmitter.