Crystal Oscillator
A
crystal oscillator is an electronic circuit that produces electrical
oscillations at a particular designed frequency determined by the
physical characteristics of one or more crystals, generally of
quartz, positioned in the circuit feedback loop.
Crystal
Oscillators is the base of many electronic and communications systems
that depend on accurate time measurement and coordination. They are
used to provide stability to time-frequency generators, which in turn
provide carrier and pilot signals for electronic communication and
navigation systems. Crystal Oscillators also provide the clock
signals used by data processing equipment, as well as the reference
signals for other special-purpose systems. The required accuracy and
stability of oscillator output frequency depend on the application,
ranging from about ±1000 PPM in the case of simple
microprocessor clocks, to less than ±5 PPM for applications
requiring very precise frequency control.
A
Crystal Oscillator consist of an amplifier and feedback network that
selects a portion of the amplifier output and returns it to the
amplifier input. The oscillator circuit is dependent on two key
conditions: First, the gain loop needs to be greater than losses
around the oscillator loop, or equal to unity. Second, the loop phase
shift must be equal to 0 or 360 degrees. Loop phase angle shifts
determine the frequency at which the oscillator will work. A little
bit of change in net loop phase angle results in a change in output
frequency of the oscillator circuit. In order to minimize the net
phase shift, a quartz crystal is placed in the feedback loop. Because
of the changes in impedance of the quartz crystal due to changes in
applied frequency
, all other components of the circuit should be
considered constant reactance resulting in an adjustment of the
quartz crystal frequency to a reactance that satisfies the loop phase
characteristics.
Crystal
oscillator circuits using crystal have a number of advantages in
actual application since crystals show high frequency stability and
stable temperature characteristic as well as excellent processing
ability. Also crystal oscillator can achieve a high degree of
accuracy and frequency stability.
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