A Simple Crystal Oscillator

Author: J.B. Hoag

A properly cut quartz crystal is the equivalent of a high-Q tuned circuit. In the oscillator circuit of Fig. 14 F, such a crystal replaces the tuned-grid circuit of a tuned-plate tuned-grid circuit. The crystal is mounted between the plates of a small condenser, one of which is connected to the grid and the other to the cathode of the tube. When the grid voltage changes, the electrostatic field across the crystal changes. Due to an inherent property of the crystal, known as the piezo-electric effect, small charges of electricity then appear on the surface of the crystal, accompanied by real but very small changes in the dimensions of the crystal. The small potentials so created act back upon the grid of the tube.

Fig. 14 F. A quartz crystal oscillator circuit

When the frequency of oscillations of the circuit correspond to the natural mechanical frequency of the crystal, the voltages will be sufficiently augmented to sustain oscillation. As a matter of fact, the frequency of the tuned-plate circuit can differ a little from that of a crystal, yet the circuit will oscillate precisely at that of the crystal itself because of its inherent high Q (9,000 to 16,000). Inasmuch as the crystal frequency is largely determined by its thickness, this offers an unusually satisfactory method of stabilizing the frequency.