Piezo electric oscillator

Principle :

In a crystal , if mechanical pressure is applied along the two opposite faces ,then in the other two opposite faces of crystal, electric voltages are induced. This is known as Piezo electric effect.

Conversely, if electric voltages are given along two opposite faces of the crystal , the along the other two opposite faces, mechanical vibrations are produced . This is known as Inverse Piezo electric effect.

Inverse Piezo electric effect is used as principle in producing ultrasonic waves in piezo electric generator.

Construction :

An  X cut or Y cut quartz crystal is placed between two metal electrodes A and B. The electrodes are connected to the coil L₃ which is inductively coupled to coils L₁ and L₂. 

The two ends of the rod are wound by coils L₁ and L₂. The coil L₁ is connected to the collector output of the transistor and the coil L₂ is connected to the base of the transistor as shown. The frequency of the  oscillatory circuit can be adjusted by the capacitor  C₁ connected across the coil L₁. A battery is connected in the circuit that acts as a source.

 

                          

Working :

The battery is switched on and oscillating current is produced by the collector of the transistor . This is current is passed through the coil L₁ and an e.m.f is induced in the neighbouring coil L₂. This induced e.m.f is mutually inducted to coil L₃ due to transformer action. The electrodes receive this alternating e.m.f and the crystal placed between electrodes experiences this oscillating force. The crystal is thrown into vibration due to inverse Piezo electric effect.

Resonance condition :

When the frequency of oscillatory circuit is adjusted with the help of capacitor to match the natural frequency of the vibrating quartz crystal , then resonant condition is said to have been achieved and ultrasonics waves are produced from the crystal.

                                   

Where ,

            E -Young’s Modulus of the rod.

            ρ- density of the material of rod

                                                            p = 1,2,3…….

Mertis :

1. It can produce frequencies above 500 MHz.
2. It can produce stable single frequency
3. It has wide range of frequencies

Demerits :

1. The Crystal is expensive
2. Cutting and shaping is very difficult.