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Explain with neat lebelled diagram the construction and working of a piezoelectric oscillator.
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Principle:

  • Resonance is obtained between a natural frequency of suitable cut piezoelectric crystal and with a suitable frequency superimposed on it that is generated by an oscillator.

Construction:

  • In the diagram we have a thin plate of piezoelectric crystal cut on such axis so that we get inverse piezoelectric effect that is on application of high frequency electric field, it can vibrate in resonance.
  • Natural frequency of crystal slab cut is given by:η=k2tYρ
    where, t = Thickness of crystal slab
    Y = Young's modulus
    ρ=Density
    k= 1, 2, 3, ... which represent order or harmonic. We take k = 1 that is fundamental frequency.
  • It is important to know that natural frequencyη is inversely proportional to thickness t.
  • Hence for higher frequency, t has to be reduced. But very small thickness of slab without fracturing it is very difficult to cut.
  • Hence for frequencies above 20 MHz, we make use of higher overtones.

Working:

  • Function of the oscillator is very much similar to magnetostriction oscillator.
  • When the current through coil L2, changes it causes a corresponding change in the magnetisation of the rod, due to magnetostriction effect a small change in the kength of rod will be noticed.
  • This change in length will give rise to change of flux linked with the coil L1 and it will induce an voltage.
  • This induce voltage will change grid voltage. The changed grid voltage will be amplified and come out at plate circuit, and cycle will continue.
  • Oscillator circuit will develop the frequency:f=12π1L2C
  • Electric field of frequency f is transfered to plates A and B through coil L2 and L.
  • Between plates A and B, a crystal of natural frequencyη is fixed.
  • For resonance we take, f =η, under this condition crystal will generate oscillations with highest amplitude.

Advantages:

  • It has higher frequency range.
  • It is small in size and economical.
  • It has better waveform.
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