single integrated circuit. And they are cheap —
less than $2 in single quantities. The crystal is
already wired into the on-chip oscillator circuitry.
All you need to do is apply DC power (usually
3. 3 or 5 volts) and out comes either a sine wave
or rectangular wave signal ready to use. The
output voltage range is several volts.
The frequency range for crystals is very wide
so you can get almost anything. The lowest
frequencies are 32.768 kHz and 100 kHz. The
highest is about 300 MHz. Most popular crystal
frequencies are in the 10 MHz to 200 MHz
TYPES OF CRYSTAL
Crystal oscillators come in a variety of types,
shapes, and sizes. The simplest device is the plain
vanilla crystal oscillator (XO). It comes in a fixed
frequency value that is commonly with an
accuracy of ( 10) several hundred parts per million
(ppm) or in the 10-4 to 10-5. These are for the
least critical applications such as embedded
controller clocks. You can get one at a common
frequency of 10 MHz or 20 MHz for less than a dollar.
A variation of the basic XO is the TCXO or
temperature compensated crystal oscillator. Since all
crystal oscillators are subject to frequency changes with
temperature — small as they are — for more critical
applications special circuitry is used to correct for
frequency variations. A feedback loop is created with a
thermistor temperature sensor in a circuit that drives a
varactor or voltage variable capacitor. This varactor
connects to the crystal to provide frequency adjustments.
The result is an amazingly precise and stable oscillator. A
frequency accuracy and stability in the 10-5 to 10-7 is
typical. End products requiring that kind of precision and
stability are cell phones and other two-way radios.
Another type of oscillator is the VCXO or voltage
controlled crystal oscillator. A varactor is connected in
series or parallel with the crystal to “pull” its frequency
over a narrow range. These units are used primarily in
phase-locked loops (PLLs) for frequency multiplication or
The other type of oscillator is the oven-controlled
crystal oscillator or OCXO (see Figure 2). As the name
implies, the oscillator comes in a thermally enclosed
package with a small electric heating element that is
cycled off and on in a closed-loop controller to maintain a
super constant temperature. With this arrangement,
■ FIGURE 1. The equivalent electrical circuit of a quartz crystal and its
schematic symbols. The curve shows the reactance variations over
frequency. Courtesy McGraw Hill, Principles of
Electronic Communication, 3rd edition, 2008.
accuracy and stability is in the 10-7 to 10-10 range. This is
good enough for real critical applications like cellular
basestations and other telecommunications networks.
Obviously, the main specification is the frequency of
operation. Frequencies up to about 200 MHz are
available. Higher frequencies can also be had, but in some
cases a PLL frequency multiplier is used. Most crystal
companies stock common frequencies, but you can
always order a special one (at extra cost).
Frequency accuracy is the next most important spec.
It is just a measure of how close to the desired frequency
the crystal actually is. It is usually given in ppm or as a
percentage, such as 10-4 or 0.0001 or 0.01% (see the
■ FIGURE 2. A common crystal oscillator in a metal can—
the Bliley Technologies NV47AG OCXO. It comes in
frequencies from 5 to 13 MHz. It is designed for use in
wireless basestations, GPS infrastructures, and test
equipment. The basic stability is ± 4. 6 ppm.
August 2010 63