By David Herres
Some materials acquire an electrical charge when they are compressed or otherwise dimensionally stressed. They are said to exhibit the piezoelectric effect. If the property is to be manifest on any but an atomic level, the material must be a crystal, with the atoms organized in a lattice.
The piezoelectric effect was first described in the early 1880s. Researchers found that when a weight was placed on a quartz crystal, electrical charges could be measured at its surface. It was further observed that when voltage was applied, the crystal changed shape. This property is known as electrostriction or inverse piezoelectricity. Removing the field lets the quartz generate an electric field as it returns to its previous shape. Consequently, a quartz crystal behaves like a circuit composed of an inductor, capacitor and resistor, with a precise resonant frequency. These properties make it possible for a quartz crystal to facilitate stability in an oscillator circuit.
Quartz is well-suited for this application because it is inexpensive, being abundant in nature and suitable for milling. A crystal such as quartz that exhibits the piezoelectric effect can be used to good effect in an oscillator.
An important attribute of an oscillator circuit is its stability, by which is meant that its frequency remains constant regardless of changes in ambient temperature, electrical load and dc power supply voltage. (Specific behavior over temperature will depend on the mode of vibration and the angle at which the quartz is cut relative to its crystallographic axes.)
The quartz crystal is a small wafer accurately milled to size and shape. Two parallel surfaces have metal plates bonded to or deposited onto them for connection to the electrical circuit. The geometry of this crystal determines the fundamental or characteristic frequency.
Mechanical and electrical characteristics of a quartz crystal in an oscillator circuit are a two-way street. The mechanical vibrations result in electrical output and voltage drop across the crystal makes it vibrate. The surrounding circuit sustains oscillations by taking the voltage from the quartz crystal, amplifying it, and feeding it back to the crystal. When the energy of the generated output frequencies matches the losses in the circuit, the oscillation continues.
During startup, the circuit puts the quartz crystal into an unstable equilibrium. The positive feedback in the system amplifies any electrical noise in the circuit that effectively ramps up the oscillation.
The quartz crystal can also be viewed as a highly frequency-selective filter in that it will only pass a narrow range of frequencies centered around the resonant frequency and attenuates everything else. As the oscillator circuitry amplifies signals from the crystal, frequencies near the crystal’s resonant frequency will become stronger and will eventually dominate the oscillator output.
The output signal of a quartz crystal oscillator can be either at the resonant frequency or at a multiple of that resonance, called a harmonic frequency. Harmonics are an exact integer multiple of the resonant frequency. Crystals exhibit vibrational energy at several frequencies that are usually at odd integer multiples of the resonant frequency. These are called overtone modes and circuits can be designed to excite them.
A quartz crystal has a much higher Q factor than a conventional electronic tuned circuit. (Higher Q indicates slower energy loss relative to the stored energy. Oscillations die out more slowly in a high Q oscillator.) A typical Q for a quartz oscillator ranges from 104 to 106, compared to perhaps 102 for an LC oscillator. The maximum Q for a high-stability quartz oscillator can be estimated as Q = 1.6 × 107/f, where f is the resonance frequency in megahertz. The fundamental frequency, as seen in an oscilloscope’s frequency domain display, contains far greater power than the harmonics on either side of it.
Microprocessors and central processing units (CPUs) in computers and related equipment make use of quartz crystal oscillators to set the frequency, generating continuous square-wave pulse constituting a clock waveform of greater accuracy and stability than could be provided by an RC or LC oscillator.