crystals and crystal oscillators
Crystals are naturally occurring materials that can be induced to resonate or vibrate at an exact frequency. Quartz, a piezoelectric crystal that provides excellent mechanical and electrical stability, acquires a charge when compressed, twisted, or distorted. Typically, quartz crystals are used as active elements in oscillators. Applying mechanical stress to quartz produces a voltage that is proportional to the mechanical frequency of the vibration. Conversely, applying an AC voltage induces vibrations at the frequency of the applied voltage. These mechanical vibrations provide a standard of time that is superior to that of mechanical clocks, but inferior to that of atomic vibrations. In some circuits, crystals are made from thin sheets of quartz and plated like integrated circuits (IC). Amplifying noise at the crystal’s frequency induces continuous oscillation, the periodic movement between two points. The oscillator output is then converted to pulses that are suitable for digital circuits. In turn, these pulses divide down the crystal’s frequency and translate it into a format for display.
Performance specifications for crystals include oscillation frequency, frequency tolerance, total frequency stability, load capacitance, drive level, equivalent series resistance, and operating temperature. Oscillation frequency is a nominal frequency value. Frequency tolerance or frequency error is the allowed, stated deviation from the nominal oscillation frequency. Typically, frequency tolerance is expressed as a percentage. Total frequency stability is the maximum frequency deviation from the nominal value for all conditions, including supply voltage. This amount is usually expressed in parts per million (ppm). Load capacitance is the total capacitance across crystals. Drive level is the total power that crystals dissipate internally. Expressed as a maximum amount, equivalent series resistance (ESR) is the ohmic resistance when crystal-based devices are in operation. Operating temperature is the full-required range of ambient operating temperatures.
There are several packaging methods and form factors for crystals. Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board. By contrast, through hole technology (THT) mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Connectorized devices attach with coaxial or other types of connectors. Waveguide assemblies consist of a hollow metallic conductor with a rectangular, elliptical, or circular cross-section. Some conductors contain solid or gaseous dielectric materials. Most are used in microwave waveguide systems.
There are several important features and standards for crystals. Restriction of Hazardous Substances (RoHS) is a European Union (EU) directive that requires all manufacturers of electronic and electrical equipment sold in Europe to demonstrate that their products contain only minimal levels of the following hazardous substances: lead, mercury, cadmium, hexavalent chromium, polybrominated biphenyl and polybrominated diphenyl ether. RoHS will become effective on July 1, 2006. By definition, lead-free devices contain less than 1000 ppm lead by weight. Some crystals are suitable for programmable devices. Others are designed for military applications.