TCXO, Temperature Compensated Crystal Oscillator
- an overview of the TCXO, used for providing a much higher levels of
temperature stability than are possible with a normal crystal oscillator
The temperature controlled xtal or crystal oscillator, TCXO,
is a form of crystal oscillator used where a high precision frequency source is
required within a small space and at reasonable cost. By compensating within the
oscillator for temperature changes, it is possible to considerably improve on
the basic performance of the oscillator.
Effect of temperature
Although crystal oscillators offer a highly stable form of
oscillator, they are nevertheless affected by temperature. The cut of the actual
crystal element from the overall grown crystal can help to minimize the effects
of temperature, but they are still affected to some degree. For a crystal cut
known as the AT cut, the drift with temperature can be minimized around normal
ambient temperature, but the rate of drift will rise above and below this.
The effects o temperature are, to a large degree, repeatable
and definable. Therefore it is possible to compensate for many of the effects
using a temperature compensated crystal oscillator, TCXO.
TCXO solution
A TCXO adjusts the frequency of the oscillator to compensate
for the changes that will occur as a result of temperature changes. To achieve
this, the main element within a TCXO is a Voltage Controlled Crystal Oscillator
(VCXO). This is connected to a circuit that senses the temperature and applies a
small correction voltage to the oscillator.
The temperature sensing and compensating network may take a
variety of forms these days. The traditional method was to use a thermistor
network. More recently digital techniques such as DSP have been used to enable
highly accurate compensation to be achieved.
The problem with applying the temperature compensation in a
TCXO is that the temperature coefficient of the crystal changes with
temperature, and it is not linear. Accordingly the design of the compensation
network is complicated.
Additionally TCXOs normally have an external adjustment to
enable the frequency to be reset periodically. This enables the effects of the
ageing of the crystal to be removed. The period between calibration adjustments
will depend upon the accuracy required, but may typically be six months or a
year. Shorter periods may be sued if very high levels of accuracy are required.
TCXO performance
The performance of a TCXO is generally considerably better
than that of a normal crystal oscillator. Figures within the range 1 to 5 ppm
are often achieved. However it should be mentioned that some of this figure
comes from a frequency / temperature hysteresis, dependent upon whether the
temperature was increasing or decreasing.
The power dissipation of a TCXO will be greater than an
ordinary oscillator in view of the additional circuitry required. Additionally
the cost is greater. It should also be remembered that it will take a short
while after start up for the oscillator to stabilize. This may be of the order
of 100 ms, or possibly longer, dependent upon the design.
TCXO packages
TCXOs can be supplied in a variety of packages dependent upon
the way they have been designed and the requirements of the end user. The most
common form of construction is to construct the circuit on a small printed
circuit board that can be house in a plat metal package. This is then suitable
for mounting onto the main circuit board of the overall equipment. As the
crystal itself is sealed, this means that sealing of the overall TCXO package is
not critical, or even required for most applications.
When TCXOs utilize ASIC technology this often enables much
smaller packages to be used. These may even be in surface mount packages that
are far more suitable for today's manufacturing techniques
Summary
TCXOs are widely used where accurate frequency sources are
needed. They are less expensive and smaller than oven controlled oscillators, as
such they offer an ideal solution for many portable units requiring a reasonably
accurate source.
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