The term parts per million (ppm) is an expression of how
far a crystal frequency varies from its main value. This is
also expressed as a percentage of deviation from the
base frequency. For example, a 200 ppm reading on a
10 MHz crystal means that the deviation is:
(200/1,000,000) x 10,000,000 = 2,000 Hz or 2 kHz.
That translates into a maximum deviation of
2000/10,000,000 or 0.0002 = 2 x 10-4or 0.02%.
application. A really good but cheap alternative is the
ceramic resonator. It is like a crystal, just not as precise or
stable. It is still good enough for some
microcontroller clocks and the like.
Another variation is the device
that uses a PLL frequency multiplier.
The crystal and the oscillator with a
VCO are all packaged into a single PLL
and the frequency of a basic crystal in
the 10 to 50 MHz range is multiplied
up into the hundreds of MHz range.
These are not as stable as a standard
oscillator, but are still good. Phase
noise is also not good enough for
Another interesting choice is a
surface acoustic wave (SAW) oscillator.
SAW devices use an interdigital pattern
or a quartz or other base that has a
peak response at a fixed frequency.
SAW devices are mainly used as VHF
and UHF filters but when put into a
feedback loop, they oscillate at a
specific frequency. You can get
frequencies from about 300 MHz to
over 1 GHz. The precision and
stability are not as good as a crystal
but for some uses, they are more than
MEMS oscillators are also
available. These micro
electromechanical systems devices are
tiny vibrating arms that oscillate at a
precise frequency. They are made
entirely of silicon on one chip. They
are more than accurate enough for
some uses but again, not as good as a
One really interesting option is the
programmable crystal oscillator. These
feature a fixed frequency crystal
oscillator in a PLL with a
programmable frequency divider in the
feedback loop. The divider is
programmed by an external digital
input. A digital PLL is used so precision
and stability are just as good as most
pure XOs. A good example is Silicon Laboratories Si530,
Si550, or Si570 XO/VCXO oscillators that can be ordered
in any frequency from 10 MHz to 1,417 MHz.
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