AC EQUIVALENT CIRCUIT OF CRYSTAL:-
R=FRICTION LOSSES
L=INERTIA
C=SHIFTNESS
C'=MAUNTING CAPACITANCE
Principle of crystal Oscillator:-
- A materrial such as a quartx which is having an unique properties called as a "piezo electric " property.
- it states that if we applied a force to this crystal then it generates a electric potential.
- also if we applied an electric signal to crystal then it shows the vibrations mechanically.
- The biggest advantage of a crystal oscillator is its frequency stability.
- frequency of the crystal oscillator is remains so stable with any changes in temp,voltage,humidity, or other parameters.
OPERATION:-
- Colpitt oscillator modifed to act as crystal oscillator.Only change is addition of crystal (y) f/b network.
- circuit shows parallel resonance circuit of crystal,for this impedence of crystal is maximum.
- Hence maximum energy transfer through feedback network.
- since here feedback is positive ,180 degree phase shift is produced bu crystal oscillator and further 180 degree produced by capacitor volage divider from input crystal.
- even smallest deviation from fp will cause oscillator to act as effective short, so it is example of extremly stable oscillator.
Resonant frequencies:-
- There are two resonant circuits in a eqiuvalent circuit of a crystal oscillator.
- RLC circuit forms an series resonant circuit and RLC with C combines the parallel resonant circuit.
- resonant frequency of RLC circuit is given by:$ fs = {1\over 2\pi\sqrt {LC} }$
Types of Crystal Oscillator:-
- Pierce Crystal Oscillator
- Miller Crystal Oscillator
Advantages:-
- High frequency stability with respective any changes in parameters like temp,voltage etc.
- Q factor of crystal Oscillator is too high .It is too high as compare to 100 LC tank.
Disadvantages:-
- Used at only lowe power circuits.
- Frequency of circuit can not be changed.
Applications:-
- Used as a frequency generator in microprocessor and microcontroller kits.
- The crystal oscillator is used in research and measurement for celestrial navigation, space tracking purpose, in the measuring instruments and medical devices, and so on.
- Used in a frequency synthesizers.
- In tv and radio transmitter.
- The use of crystal oscillator in military and aerospace, is to establish an efficient communication system, for the navigation purpose, electronic warfare, in the guidance systems, and so on.
- Crystal oscillator is used for engine controlling, stereo, clock and to trip computer, and in GPS system.