CRYSTAL OSCILLATOR

CRYSTAL OSCILLATOR

Devices like quartz watches use crystals to accurately keep time. They oscillate at extremely precise high frequencies and are used as clocking devices.

How it works:

Piezoelectric effect is a primary characteristic of a quartz or ceramic crystal. In this effect, when a voltage is applied to a crystal it distorts, producing a resonant frequency. When the voltage is removed, it returns to its original state.

When voltage is applied in precision pulses, the crystal resonates at a fixed frequency, producing waves of sound of very high frequency. The resonating frequency can vary as per changes in temperature, thermal shocks, humidity and other external disturbances.

Selecting a crystal oscillator:

Standard crystals are made of quartz or ceramic. They are mainly classified as: voltage-controlled, oven-controlled, frequency-controlled, programmable crystal oscillators, and monolithic crystal. Crystals are designated by the symbol ‘X’ while the oscillator is given the designation ‘XO’. Important parameters are:

Output frequency: The fundamental resonance of the crystal measured in MHz.
Jitter: Defined as the short-term stability for a fixed duration. Jitter is an undesired characteristic and should be low. It is measured in RMS.
Operating temperature: Temperature range in which the oscillator operates.
Supply voltage and current: Voltage and current that should be given to the oscillator measured in V and mA.
Phase noise: Signal power divided by the noise that is produced and measured as dBc/Hz. It should be low.
Pull range: It is the maximum amount the frequency of the crystal can be changed and is measured in ppm.
Ageing: As the oscillator undergoes prolonged use of, say, 10 years, the frequency can change. This is called ageing.
Transition time: It is the time in nanoseconds for the oscillator to change from logic 1 to 0 and vice versa. It is measured as rise transition time and fall transition time depending on the change of state of the oscillator.

Applications:

These are used in quartz watches and circuits for timing and clocking. They are also used in signal generators, counters, oscilloscopes, and quartz stabilized PLL.