Selasa, 28 Juni 2011


Oscillators are devices that are used to generate repetitive signals. They produce output signals without an input signal. There are two major types of electronic oscillators: harmonic oscillators and relaxation oscillators. Harmonic oscillators produce sine wave outputs. Relaxation oscillators produce square wave, rectangular wave, and sawtooth outputs. For both types of devices, an active component (normally, in the feedback loop) determines the frequency of oscillation. This active component can be either an electronic circuit or a crystal.

Voltage-controlled oscillators (VCO) and voltage-controlled crystal oscillators (VCXO) change the frequency in direct proportion to a control voltage. Basic crystal oscillators (XO) do not. Temperature-compensated crystal oscillators (TCXO) use a thermistor network to generate a correction voltage to reduce frequency deviations over temperature. Oven-controlled crystal oscillators (OCXO) use temperature control circuitry to hold the crystal and its circuitry at a precise, constant temperature. Other oscillator types include voltage-controlled, temperature-compensated crystal oscillators (VCTCXO); oven-controlled, voltage-controlled crystal oscillators (OCVCXO); and digital, temperature-compensated, crystal oscillators (DTCXO). Microcomputer-compensated crystal oscillators (MCXO), rubidium crystal oscillators (RbXO), voltage-controlled SAW oscillators (VCSO), dielectric resonator oscillators (DRO), and simple packaged crystal oscillators (SPXO) are also available.

There are several packaging methods and form factors for oscillators. 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. Oscillators that are modules or boards are also available. Connectorized devices attach with coaxial or radio frequency (RF) 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.

Performance specifications for oscillators include oscillation frequency, frequency tolerance, total frequency stability, resistance, operating temperature and supply voltage. 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. Typical supply voltages include 1.8 V, 2.5 V, 3.3 V, 5.0 V, 12.0 V, and 15 V.

Oscillators vary in terms of output type. Circuits with transistor-transistor logic (TTL) use a 5 V power supply. TTL signals are "low" when between 0 V and 0.8 V with respect to the ground terminal and "high" when between 2 V and 5 V. A complementary metal-oxide semiconductor (CMOS) is a low-power technology that can achieve high switching speeds. High-speed CMOS (HCMOS) provides faster switching speeds than conventional CMOS. Emitter coupled logic (ECL) is an extremely high-speed logic that consumes a great amount of power. Variants include positive emitter coupled logic (ECL) and low voltage, positive emitter coupled logic (LVPECL). Low voltage differential signaling (LVDS) is also available.

There are several important features and standards for oscillators. 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 oscillators are suitable for programmable devices. Others are designed for military applications.

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