- Crystal oscillator is basically a tuned oscillator that works on the basis of piezoelectric effect i.e. when voltage is applied to crystal it vibrates at frequency of applied voltage or when mechanical pressure is applied it generates an ac voltage.
- Generally quartz is used for crystal oscillator which is more stiff compare to Roschell salt.Quartz is inexpensive and readily available in nature.
- Crystal can be represented by electrical model if it is suitably cut and mounted between two metal plates as shown below.
Figure 1: Equivalent circuit of crystal oscillator
- In above equivalent circuit.
L -> Mass of crystal, $C_S$ -> Stiffness of crystal
- When it vibrate it collide with plates due to collision there will be losses represented by R. Similarly crystal is mounted between two plates hence there is mounting capacitor $C_m$.
- From electrical model of crystal we can say it oscillates at two frequencies series frequency & parallel frequency.
$fseries = \frac{1}{2\pi\sqrt{LC_S}}$
- Similarly, $fparallel = \frac{1}{2\pi\sqrt{LC_T}}$
$C_T = \frac{C_S C_m}{C_S + C_m}$
- When crystal oscillator oscillates at series frequency it is known as pierce oscillator.
- In the above circuit $C_C$ block DC and pass AC $R_E$ make Q point stable Q pt. stable against temperature.
- When power supply is turned on noise in $R_2$ result small change in $i_b$ due to this change in $i_c$ is by large amount.
$\delta i_c = \beta \delta i_b $
such varying current pass through crystal which vibrates at its natural frequency vibrations appear at base which result large change in $i_b$ as compared to first cycle hence $i_c$ will change by large amount as compared to previous cycle. $i_c$ goes on increasing till transistor work in saturation for positive cycle and cutoff for negative cycle.
Advantages:
- Crystal oscillator provides high degree of frequency stability.
- Used in computer to generate clock signal for CPU.
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