TABLE OF CONTENTS Short-term frequency stability is a function of the resonator Q, and long-term stability is a function of the drift of the resonant frequency. Piezoelectric quartz crystals have resonances that are much more stable than the LC circuits previously discussed, and also have very high Q. Consequently for stable oscillators at a fixed or only slightly variable frequency, quartz crystal resonators are generally used. Other piezoelectric crystal materials have been used in filters and to control oscillators. However, their temperature stability is considerably poorer than that of quartz; so for oscillator use, quartz is generally preferred. A piezoelectric material is one which develops a voltage when it is under a mechanical strain or is placed under strain by an applied voltage. A physical piece of such material, depending upon its shape, can have a number of mechanical resonances. By appropriate shaping and location of the electrodes, one or other resonant mode of vibration can be favored, so that the resonance can be excited by the external voltage. When the material is crystalline, such as quartz, the properties of the resonator are not only affected by the shape and placement of the electrodes, but also by the relationship of the resonator cut to the principal crystal axes. Table 7.9 lists designations of various quartz crystal cuts.
The temperature performance of a quartz crystal resonator varies with the angles at which it is cut. Figure 7.115 shows how different cuts react to temperature variations. Most crystals...
