Crystal-controlled oscillators

Quartz crystals can be used in place of tuned LC circuits in RF oscillators, if it is not essential to change the frequency. Crystal oscillators provide excellent frequency stability far superior to that of the LC-tuned VFOs.

There are many ways that crystals can be connected in the bipolar or FET circuits to obtain oscillation. One common circuit is Pierce oscillator. An N-channel JFET and quartz crystal are connected in the Pierce configuration as shown in the diagram below.The crystal frequency is varied somewhat (by about 0.1 %) by means of an inductor or capacitor in parallel with crystal. But the frequency can be determined mainly by thickness of crystal, and by angle at which it is cut from the quartz rock.

The crystals change in frequency as temperature changes. But they are more stable than the LC circuits. Some crystal oscillators are kept in temperature-controlled chambers known as ovens. They sustain their frequency so well that

                                                        Figure-- A JFET Pierce oscillator.

they are used as frequency standards,  against which other oscillators are calibrated. The accuracy is within a few Hertz at working frequencies of mega- hertz.