Page 251 - Complete Wireless Design

Oscillator Design

250 Chapter Four

SAW-based oscillators are becoming more popular in the VHF and above
regions, and are similar in design and concept to crystal oscillators. However,
surface acoustic wave (SAW) devices are limited in design usefulness; unless
the center frequency is a common one, no SAW resonators are available with-
out an expensive custom production run. And the initial frequency stability,
temperature stability, Q, and aging characteristics are many times worse than
those of an average crystal resonator. Nonetheless, replacing crystals with
SAW resonators makes it possible to operate at very high frequencies of up to
2 GHz, and with powers of up to 22 dBm at 500 MHz.
Depending on the application, a crystal oscillator may require higher fre-
quency accuracy over temperature than a normal noncompensated crystal oscil-
lator (XO) can supply. This will demand that some type of compensated device
be used, such as a temperature-controlled crystal oscillator (TCXO) or an oven-
controlled crystal oscillator (OCXO). However, increased size, cost, current con-
sumption, and complexity are the tradeoffs if such a compensated oscillator is to
be adopted. And, depending on the angle of the cut for AT crystals, frequency
stability over a desired temperature range can be optimized for an uncompen-
sated crystal oscillator, sometimes making compensated oscillators unneces-
sary: Frequency stabilities of ±5 ppm from 25 to 70°C are possible with the
appropriate AT cut angle in an XO. Wider temperature variations than this will
quickly degrade an AT cut’s frequency stability dramatically (down to ±20 ppm
from 40 to 80°C), necessitating the use of a TCXO, or even an OCXO.
Most of the components making up any oscillator are temperature sensitive,
especially important being the crystal and the ceramic capacitors of its res-
onator network. Even the finest crystal oscillator, if built with poor or inap-
propriate ceramic capacitors, may have unacceptable frequency drift. In a
well-designed oscillator, the majority of the long and short-term frequency
drift should originate only from within the crystal—and any circuit that adds
more than double the drift of a lone crystal is improperly designed. The use of
incorrect temperature-compensating ceramic capacitors, or capacitors with
poor temperature tolerance versus capacitance, can destroy frequency stabili-
ty of an otherwise good oscillator. However, if high frequency stability of bet-
ter than a few ppm is required, then both the crystal and the entire oscillator
circuit itself must be ovenized within an OCXO. The OCXO ovenizes not only
the crystal, but all of the temperature-sensitive components. It has the high-
est stability commonly available in compensated crystal oscillators, with bet-
ter than 0.001 ppm being common with SC-cut or AT-cut crystals over a wide
temperature range. The oscillator itself is kept in a temperature-controlled
oven that maintains the crystal and circuits at a temperature that is 10°C
above the highest specified ambient temperature. The OCXO can even be
tuned very slightly (by a few ppm) by a small screw located within the case.
However, OCXOs are high in cost, consume much more current than a stan-
dard oscillator, have a certain warm-up period to reach full frequency accura-
cy, and may have poor aging characteristics because of the high heat that the
crystal is constantly subjected to.

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