Just as most electronic products today contain at least one embedded controller, most also have at least one crystal oscillator. In fact, some multiprotocol networking and telecom equipment can contain 10 or more different crystals.
A crystal oscillator usually sets the processor clock frequency and operational frequencies of networking speed or wireless channels. Crystals provide the accurate timing required by most modern products, in addition to the precision demanded by the FCC in setting operational wireless and networking frequencies.
When designing your products, you can opt to make your own crystal oscillator or design in one of the many available pre-packaged crystal oscillators. In some cases, all you do is connect the appropriate crystal (plus two capacitors) to the processor or other chip, which has the oscillator circuitry built in. Other cases require a separate oscillator.
In these instances, investing the development time and money in designing and building your own crystal oscillator no longer makes economic or time-to-market sense. Electronic design today is more about putting together components and chips to form a system rather than creating detailed circuits. Now, crystal oscillators have evolved into an off-the-shelf subsystem component.
THE MAGIC CRYSTAL
Crystal oscillators are virtually mandatory in more complex modern electronic products. Made of pure quartz, these thin slivers vibrate at a precise and very stable frequency. Their ability to be set to almost any desired frequency and maintain that frequency over a wide range of temperature and voltage variations makes them inordinately better than any RC or LC oscillator.
Quartz is a crystalline structure found in nature and the second most common material found in the earth’s surface next to feldspar. Its chemical composition is silicon dioxide (SiO2), but its piezoelectric characteristics make it special. Piezoelectricity is a material’s ability to generate a voltage when stressed mechanically or to vibrate at a precise frequency if excited by a voltage. This latter characteristic makes quartz the frequency-determining component of choice for most applications.
While quartz crystals are readily found in nature, they can be synthesized. Pure quartz crystal is formed by melting a mined material called lasca in an autoclave and using a seed crystal. Such crystals are then cut into slivers and ground to the desired thickness that sets the frequency of operation.
The geometry and angle of the slice cut from the crystal determines its stability and other characteristics. Different cuts are referred to by designations such as AT, SC, and X cuts. Two plates of silver are deposited on opposite faces of the crystal, and mounting leads are attached to them. The completed assembly is mounted in an enclosure, usually metal.
The crystal itself looks like a series resonant circuit with equivalent inductive, capacitive, and resistive components (Fig. 1a). Placing the crystal in a holder produces a parallel capacitance, with the crystal serving as the dielectric between the two holding plates. This combination produces a unique circuit with both series and parallel resonances (Fig. 1b).
A crystal may be operated in either its series or parallel or anti-resonant modes, depending on the oscillator circuit used. The parallel mode is usually avoided because it’s less stable. However, the frequency range between the series and parallel resonant points is commonly used. This area is known as the parallel mode range.
When operating in the parallel mode, the external capacitance across the crystal will determine the operating frequency. Called the load capacitance, this reactance is any stray or distributed capacitance on the printed-circuit board (PCB) and in the oscillator circuit. Usually in the 3- to 20-pF range, it must be specified when ordering a crystal to be used in a parallelmode circuit.
You can also add a series or parallel capacitor to a crystal to “pull” its resonant frequency over a narrow range. This feature permits minor adjustments to the frequency, as well as the ability to produce a variable-frequency crystal oscillator for use in phase-locked loops (PLLs).
Most crystals also oscillate at higher overtone frequencies. The third and fifth overtones are the most common. An overtone is an approximate third, fifth, or other odd multiple of the primary resonant frequency. A harmonic of a fundamental frequency is an exact multiple, while the overtone is a close but not exact multiple.
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