Crystal-Controlled Oscillators

In applications where extremely stable operating frequencies are required, the oscillators that we have studied so far come up short. They can experience variations in both frequency and amplitude for several reasons:

• If the transistor is replaced, it may have slightly different gain characteristics.
• If the inductor or capacitor is changed, the operating frequency may change.
• If circuit temperature changes, the resistive components will change, which can cause a change in both frequency and amplitude.

In any system where stability is paramount, crystal-controlled oscillators are used. Crystal-controlled oscillators use a quartz crystal to control the operating frequency.
The key to the operation of a crystal-controlled oscillator is the piezoelectric effect, which means that the crystal vibrates at a constant rate when it is exposed to an electric field. The physical dimensions of the crystal determine the frequency of vibration. Thus, by cutting the crystal to specific dimensions, we can produce crystals that have very exact frequency ratings. There are three commonly used crystals that exhibit piezoelectric properties. They are Rochelle salt, quartz, and tourmaline. Rochelle salt has the best piezoelectric properties but is very fragile. Tourmaline is very tough, but its vibration rate is not as stable. Quartz crystals fall between the two extremes and are the most commonly used.
Quartz crystals are made from silicon dioxide (). They develop as six-sided crystals as shown in Figure 18.20 of the text. When used in electronic components, a thin slice of crystal is placed between two conductive plates, like those of a capacitor. Remember that its physical dimensions determine the frequency at which the crystal vibrates.