Piezo - Electric Oscillator

Principle

The piezoelectric method of producing ultrasonic waves is based on the principle of Inverse piezo – electric effect.

Here, ultrasonic waves are produced when the frequency of the applied alternating voltage is equal to the vibrating frequency of the crystal.

Construction

A slice of quartz crystal is placed between the metal plates A and B so as to form a parallel plate capacitor with the crystal as a dielectric.
The two metal plates are connected with an induction coil L₃ of the transformer.
There are two more induction coil L₁ and L₂ .
The coil L₂ is connected a variable capacitor C₁ which they combined to form a tank circuit or oscillatory circuit.
The tank circuit and induction coil L₁ are connected with a NPN transistor.

Fig. 1.1 Piezo-electric oscillator

Working

When the battery is switched on, the electronic oscillator produces high frequency oscillations given by,

$\mathrm{f}=\frac{1}{2 \pi \sqrt{\mathrm{L}_1 \mathrm{C}_1}} \mathrm{~Hz}$

The current is passed through the primary coils L₁ and L₂ of the circuit.
This current is transferred to the secondary coil L₃ of the circuit. Now oscillatory e.m.f. is induced in the coil L₃ due to transformer action.
Due to the principle of inverse piezo electric effect the crystal starts vibrating along the mechanical axis of the crystal.
The frequency of the oscillator circuit is adjusted by the variable capacitor C₁ and when this frequency is equal to the frequency of the vibrating crystal resonance occurs.
At resonance the crystal vibrates large amplitude and ultrasonic waves are produced along both ends of the crystal.
The fundamental (natural) frequency of the quartz specimen is given by

\mathrm{n}=\frac{\mathrm{P}}{2 l} \sqrt{\frac{\mathrm{E}}{\rho}} \mathrm{Hz}

Where,

l- length of the material
E – Young’s modulus of the material
ρ- density of the material and
P – 1,2,3….etc., for fundamental, first overtone, second overtone etc.

Condition for resonance

Frequency of oscillatory circuit = Frequency of vibrating crystal

Advantages

i. Ultrasonic frequencies as high as 500MHz can be generated.
ii. The output power is high.
iii. The output power is independent of temperature and humidity.
iv. It can produce longitudinal as well as transverse ultrasonic waves.
v. It is more efficient than magnetostriction oscillator.
vi. The width of the resonance curve is so small and so we can get a stable and constant frequency of ultrasonic waves.