Resonators are frequency-selective electronic circuits that can produce a fixed (resonant) frequency when properly excited. The value of the resonant frequency depends on the circuit’s components. There are several basic types of resonators. Ceramic resonators are built with piezoelectric ceramic materials that have suitable mechanical resonance properties. They produce a vibrating frequency that is a function of the resonant frequency applied. Surface acoustic wave (SAW) technology generates elastic waves that propagate on the surfaces of specific materials. The energy density of the waves is confined to a depth of no more than one wavelength. Typically, SAW devices have a high frequency selectivity, low insertion loss, and can be manufactured in small size chips or dies. Resonators with quartz crystals or passive electronic components are also available. RC resonators include resistors (R) and capacitors (C). LC resonators include inductors (L) and capacitors (C).
Performance specifications for resonators include oscillation frequency, frequency tolerance, total frequency stability, 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). Operating temperature is the full-required range of ambient operating temperatures.
There are several packaging methods and form factors for resonators. 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.
Features and Standards
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 resonators are suitable for programmable devices or contain a build-in capacitor. Others are designed for military applications.