• A Crystal Oscillator is
basically a tuned circuit Oscillator using a piezoelectric
crystal as a resonant circuit.
• The crystal ( usually quartz)
has a greater stability in holding constant at
whatever frequency the crystal is
originally cut to operate.
• Crystal Oscillators are used
whenever great stability is required, such as
communication transmitters and
receivers.
Characteristics
of a Quartz Crystal
• A quartz crystal exhibits the
property that when mechanical stress is applied
across one set of its faces, a
difference of potential develops across the opposite
faces.
• This property of a Crystal is
called ‘ Piezoelectric effect’.
• Similarly, a voltage applied
across one set of faces of the Crystal causes
mechanical distortion in the
Crystal shape.
• When alternating voltage is
applied to a crystal, mechanical vibrations are set up
– these vibrations having a
natural resonant frequency dependent on the Crystal.
• Although the Crystal has
electromechanical resonance, we can represent the
Crystal action by equivalent electrical circuit as
shown.
The inductor L and the capacitor
C represent electrical equivalents of Crystal mass
and compliance respectively,
whereas resistance R is an electrical equivalent of the
crystal structures internal
friction. The shunt capacitance CM represents the
capacitance due to mechanical
mounting of the crystal. Because the crystal losses,
represented by R, are small, the
equivalent crystal Q factor is high – typically 20,000.
Values of Q up to almost 106 can
be achieved by using Crystals. The Crystal can
have two resonant frequencies.
One resonant condition occurs when the reactances of
the series RLC leg are equal. For
this condition, the series – resonant impedance is
very low ( equal to R). The other
resonant condition occurs at a higher frequency
when the reactance of the series
resonant leg equals the reactance of the capacitor CM.
This is parallel resonance or
antiresonance condition of the Crystal,
At this frequency, the crystal offers very high
impedance to the external circuit.
To use the crystal properly, it
must be connected in a circuit so that its low
impedance in the series resonant
operating mode or high impedance in the
antiresonant operating mode is selected.
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