TABLE OF CONTENTS The quartz crystal has been widely used as a frequency-determining component for many years. It is small, robust, accurate and stable. Also, like other components, it has its vices. This section will look briefly at crystal theory before reviewing some application pitfalls, and also cover its cheaper but more popular cousin the ceramic resonator.
Quartz (silica, SiO 2) exhibits a piezoelectric effect whereby mechanical stress generates a directionally related electric field, and conversely an applied electric field causes a directionally related force across the crystal. An alternating voltage applied to the crystal will cause it to vibrate, and if its frequency is close to the mechanical resonance the generated electric field will be amplified and can be used to stabilise the applied frequency.
The crystals used in electronic circuits are in the form of plates or elements cut from a synthetic crystal. The resonant properties vary depending on the angle of cut referred to the base crystal s major axis. X- and Y-cut units, where the direction of cut is perpendicular to the major axis, show subsidiary responses which can reduce the Q-factor of the element and impose a fairly low upper limit on the achievable frequency range. Also, the temperature coefficient of these cuts is large.
Happily, a particular angle of cut of 35 21 from the major axis known as the AT-cut, shows very small coupling between the principal and other modes of vibration, therefore lacks subsidiary resonances and is capable...
