OPENCOMMUNICATION
THE LATEST IN NETWORKING AND WIRELESS TECHNOLOGIES
■ BY LOUIS E. FRENZEL W5LEF
CRYSTAL CLOCK OSCILLATORS
ARE THE HEART OF ALL
COMMUNICATIONS PRODUCTS
If you look inside any electronic product today, there are a few circuits and
components that are common to all of them. For example, every product
contains at least one microcontroller that runs it. Another circuit that you
will see — especially in communications products — is a crystal oscillator
or clock. This circuit generates precise timing signals that control everything
else — including that microcontroller. In communications gear like radios,
that crystal is the source of the exact operating frequencies needed for
wireless operations. Here is a look at these critical circuits.
WHY CRYSTALS?
62 August 2010
The need for precise frequency control in electronics
is critical. In microcontroller applications, the need is for
accurate timing of all processor functions. In
communications, precise frequencies are needed for all
wireless operations because they are restricted to specific
frequencies and bands by the Federal Communications
Commission (FCC). Penalties for frequency violations are
expensive and severe. Crystals ensure not only accurate
timing in processor operations but also in setting the
operating frequencies of any radio.
You can make oscillators out of resonant inductor-capacitor (LC) circuits or RC charge/discharge circuits.
They work well, but their limitations are severe. First, it is
difficult to get just the right frequency. Component
tolerances are just not good enough, even if the
components are made variable. Second, even if you can
tune an LC or RC oscillator to the desired frequency, it just
won’t be stable enough to maintain that frequency.
Oscillators are not so stable. Their frequency changes with
temperature, vibration, and other physical conditions.
These oscillators drift off frequency over time and quickly
lose their timing accuracy or frequency settings.
The solution is to use a crystal oscillator. A crystal is a
piece of natural quartz that has been cut into a thin
element that will vibrate at a specific frequency. The shape
and thickness of the quartz sets the frequency. That
frequency can be very precisely determined to within a
fraction of a percent. Furthermore, the quartz crystal has
amazing stability. It can maintain that precise frequency
over a long period despite temperature variations and
power supply changes.
HOW CRYSTALS WORK
The equivalent circuit of a crystal is a simple series-parallel RLC circuit as shown in Figure 1. The components
R, L, and Cs form a serial resonant circuit. At resonance,
the reactance of the inductor and capacitor cancel
out leaving only the equivalent resistance R in parallel
with Cp.
At a slightly higher frequency, the inductance and the
parallel capacitance Cp form a parallel resonant circuit
with its near infinite impedance. Figure 1 shows the
reactance variations. Note also the schematic symbols for
a crystal are sometimes abbreviated XTAL. The lower
symbol is the more widely used. The crystal has an
incredibly high Q so the resonant frequencies are very
sharply defined.
The idea is to connect this crystal into a simple
oscillator circuit. You can do this with a single transistor.
While you can still do that, it is more common today to
just buy a crystal oscillator. They come packaged as a
