Crystal Oscillator

  
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In crystal oscillators, the usual electrical resonant circuit is replaced by a mechanically vi ­brating crystal. The crystal (usually quartz) has a high degree of stability in holding con ­stant at whatever frequency the crystal is originally cut to operate. The crystal oscillators are, therefore, used whenever great stability is needed, for examp
Crystal Oscillator - schematic

le, in communication trans ­mitters, and receivers, digital clocks etc. A quartz crystal exhibits a very important property known as piezo-electric effect. Whena mechanical pressure is applied across the faces of the crystal, a voltage proportional tothe applied mechanical pressure appears across thecrystal. Conversely, when a voltage is applied across the crystal surfaces, the crystal is distorted by an amount proportional to the applied voltage. An alter ­nating voltage applied to a crystal causes it to vibrate at its natural frequency. Besides quartz, the other substances that exhibit the piezo-electric effect are Rochelle salt and tourmaline. Rochelle salt exhibits the greatest piezoelectric effect, but its applications are limited to manufacture of microphones, headsets and loudspeakers. It is because the Rochelle salt is mechanically the weakest and strongly affected by moisture and heat. Tourmaline is most rugged but shows the least piezo-electric effect. Quartz is a compromise between the piezoelectric effect of Rochelle salt and the mechani ­cal strength of tourmaline. It is inexpensive and readily available in nature. It is mainly the quartz crystal that is used in radio-frequency (RF) oscillators. For use in electronic oscillators, the crystal is suitably cut and then mounted between two metal plates, as shown in fig (a). Although the crystal has electro-mechanical resonance but the crystal action can be...



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