Oscillators and Crystals

An oscillator is a mechanical or electronic device that produces a periodic oscillating output, often a sine wave or a square wave. A crystal is a passive component that also produces a periodic oscillating output but has a narrow frequency passband. Oscillators and crystals are often used together in digital integrated circuits to provide an accurate clock source. Although crystal frequencies can be in the high kilohertz to low megahertz, a phase-locked loop (PLL) can accurately multiply the input frequency into the high MHz and low GHz ranges which are useful in clocking high-speed circuitry.
There are two primary types of oscillators: linear and non-linear. Linear oscillators tend to produce sinusoidal output voltages and are comprised of operational amplifiers (op-amps) or a single transistor arranged in a feedback loop through either an inductor or capacitor. Non-linear oscillators usually employ digital components and a capacitor. One example is a ring oscillator, which is a cascade of odd number of inverters connected in a series.
A crystal is a piezoelectric device that uses mechanical resonance to achieve a very precise frequency output. The most common type of crystal material is quartz, but different piezoelectric materials may be used. The electrical behavior of a crystal may be modeled with ideal resistors, inductors , and capacitors through characterization.