by Norm Looper
Analog Scientific, Inc.
A Nuts & Volts Designer”s Guide to
reliable oscillators and timers.
“Deity resides in
— Ludwig Mies van der Rohe,
PART 3 — Crystals That Make The World Go ‘Round
Strike a crystal goblet with a spoon, and you
immediately have both the attention of your guests
and the sympathetic resonance of other goblets on
your sumptuous holiday table.
What is happening here? Well, you have created a crystal
oscillator that generates acoustic waves in the air. It’s an
impulse oscillator, wherein the peak movements of the glass
and of the ensuing waves of air pressure decay exponentially
over time. Your perceived “crispness” of this lovely chiming
sound correlates with the actual mechanical “Q” of the
singing glass: how sharply the resonant frequency peaks.
Now, common glass is an amorphous solid but
arrange the silicon dioxide atoms in a particular rigid
rectangular array and you produce a crystal capable of
acoustic oscillation in an electronic circuit. And don’t stop
there, obtain a diamond saw — by slicing the hexagonal
crystal at an appropriate angle to its long axis, you can
extract a thin slab whose natural acoustic resonance is
significantly independent of temperature changes.
Then, grind your slab to a suitable radius and
thickness, and you can accurately pre-determine its natural
mechanical resonance. Deposit a thin foil of gold on
either side of the resultant quartz disc (Figure 1), and
bring each foil out on conductive leads.
Voilà! You are now in possession of an oscillator
technology of formidable power for use in the everyday
world of audio, radio, precision filters, television, radar,
72 May 2008
microwave communications, missiles, space probes,
computers, power tools, calculators, cell phones, children’s
toys, alarm systems, and digital watches. It is not surprising
that this same everyday world demands several billion
quartz crystals every year!
Quartz is a natural mineral, consisting of silicon dioxide
in crystalline form. Today, quartz crystals are grown
artificially — a vital process in a world of finite resources.
Quartz is a piezoelectric material. This means that although
it does not conduct electricity, voltages across it can produce
internal mechanical strains in the crystal lattice, and these
strains can reverse the effect, producing voltages. By
applying an AC excitation voltage across the crystal at its
natural acoustic resonance, the strains can be optimized
and used in feedback to control the frequency of the
exciting voltage itself, forcing a sustained, accurate, and
We are used to thinking of the crystal oscillator as
solely an electronic device but, in reality, it contains an
acoustic control element, vibrating at frequencies usually
well above that of human hearing. Piezoelectric oscillators
are not just limited to quartz elements, since many
ceramics work well also, and these are referred to as
ceramic resonators. Physical packages range from those
the size of a cigarette lighter down to TO- 5 cans, . 3” DIP
packages, and recently, to surface-mount sizes and pin-outs.
Quartz crystals span an impressive frequency range