■ FIGURE 3.Crystal oscillator in a surface-mount package.
Fox Electronics XO and VCXO SMD oscillators are available
in frequencies from 750 kHz to 1.35 GHz with stabilities to
20 ppm. Phase jitter is typically less than 1 ps. Typical
package sizes are 5 x 3. 2 mm and 7 x 5 mm.
sidebar). Most crystals are selected based on the needs of
the applications (the FCC states the accuracy for many
Next comes frequency stability. This is a measure of
how the frequency varies over time with temperature and
other variations. This is usually stated in ppm over a given
temperature range like - 40 to +85 degrees C. Part of the
stability spec is aging; that is how the
frequency may vary over a longer
period of time (like weeks or months).
Output voltage and interface are
one more specification. Most crystal
oscillators generate a rectangular wave
compatible with old style TTL circuits
or (in some cases) with the newer
lower voltage interfaces like HCMOS,
LVDS, PECL, or CML. Sine wave
output is also available if needed.
Phase noise is a critical spec
especially for crystals in above the 100
MHz range. These are used in
Phase noise is really just some short
term frequency variations around the
center frequency. It is sometimes
called jitter. In any case, it is really
small frequency or phase variations
that look like FM or PM.
The spec is given in dBc/Hz or
the decibel value of the sidebands
produced compared to the center or
carrier frequency a specific number of
Hz ( 10 kHz, 100 kHz, or 1 MHz) from
the carrier. A typical value may be
-120 dBc/10 kHz.
Finally, the package. An older
(but still used) package is the metal
can like that in Figure 2. They have
designations like HC- 45, HC- 49,
HC- 50, or HC- 51. The newer packages
are surface-mount devices (SMD) like
the one in Figure 3. This one is 5 x 7
mm — a common size. Even smaller
units are available now.
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While crystal oscillators are the
most common, you can get several
other very precise and stable
oscillators, depending on the