High Frequency Crystal Oscillator

 

• A Crystal oscillator is basically a tuned circuit oscillator using a piezoelectric crystal as a resonant tank circuit.
• Crystal oscillators are used when accuracy and stability of oscillation frequency are important.
• When a mechanical stress is applied across the faces of the crystal, a potential difference develops across opposite faces of the crystal. This property of a crystal is called the piezoelectric effect.

·         Similarly, when a.c. voltage is applied across one set of faces of crystal, they vibrate at a frequency of the applied voltage. The main substances that produce the piezoelectric effect are quartz, Rochelle salt and tourmaline.

The following fig. shows the electrical equivalent circuit of a crystal along with some of the practical oscillators available in the market.

■       In the above circuit we will have two resonant frequencies. One of the resonating condition occurs when the reactance at series RLC leg are equal (and opposite). 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 equal the reactance of capacitor, CM. This is a parallel resonance of antiresonance condition of the crystal. At this frequency, the crystal offers very high impedance to the external circuit.

■       The graph of crystal impedance versus frequency is shown in following fig.

Fig. Crystal impedance versus frequency

In order 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 anti-resonant operating mode is selected.

Crystal oscillators operate on the principle of inverse piezoelectric effect in which an alternating voltage applied across the crystal surfaces causes it to vibrate at its natural frequency. It is these vibrations which eventually get converted into oscillations.

To excite a crystal for operation in the series resonant mode, it may be connected as a series element in a feedback path. At the series resonant frequency of the crystal, its impedance is smallest and amount of (positive) feedback is largest. An op-amp can be used in a crystal oscillator as shown in following fig.

Fig. Crystal oscillator using op-amp

The crystal is connected in the series resonant path and operates at the crystal series-resonant frequency. The circuit sown in fig. has a high gain so that an output is square wave signal as shown. A pair of Zener diodes is connected to provide output amplitude at exactly the Zener voltage (Vz).