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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