Crystal oscillator works on the principle of piezo electric effect. According to piezo electric effect, when AC voltage is applied across the crystal, it vibrates at the frequency of the applied voltage. If the mechanical force is applied to make the crystal vibrate, then an AC voltage is generated across it. Quartz, Rochelle salt and Tourmaline show this phenomena of piezo electric effect.
The resistors R1, and R2, and RE form voltage divider stabilised dc bias circuit. The radio frequency choke prevents oscillations produced from entering the dc power supply.
When the circuit is switched on, the noise signal generated increases the collector current which charges the capacitors C1, and C2. The discharging current induces an emf across plates of the crystal which oscillates and produces the resonant frequency. The crystal, C1, and C2 produce a phase shift of 180° and the transistor produces a phase shift of 180°. Thus the phase shift around the loop is 360°, the require condition for the sustained oscillations.
The frequency of oscillations produced is
\(f_s=\frac{1}{2\pi \sqrt{LC_s}}\)
where \(C_s=\frac{C_1C_2}{C_1+C_2}\)
The frequency stability of the crystal oscillator is very high because frequency of oscillation is unaffected by changes in supply voltage and transistor parameters.
