OCXO, Oven Controlled Crystal Oscillator
- an overview of the basics of the oven controlled crystal oscillator, OCXO,
which is used for generating stable frequency reference signals.
Oven Controlled crystal (Xtal) Oscillators,
OCXOs, are used in applications where a very high degree of frequency stability
is required. While crystal oscillators show a high degree of stability even when
the outside temperature is varied over a significant range, for some
applications even higher levels of temperature stability are required. In these
applications oven controlled crystal oscillators (OCXO) are often used.
Stability of a crystal
Like any physical item, quartz crystals are subject to slight
changes as a result of temperature variations. These changes reflect back into
the resonant frequency of the crystal causing slight variations. The degree of
variation is highly dependent upon the way the crystal is cut during
manufacture. The angles of the plane of the blank with reference to the axes of
the original crystal determine many of its properties. These include the mode of
vibration, the degree of the piezo-electric effect - i.e. its activity, and of
course the temperature stability.
The type of crystal cut most used for general RF applications
is known as the AT cut. This provides a crystal with very good all round
properties as required for RF applications. For the temperature stability it is
found that the change of frequency measured Δf/f in ppm
(parts per million) reaches a minimum at around 25�C rising at temperatures
above this and typically falling at temperatures below this figure. Some
variation is found dependent upon the exact angle the crystal blank is cut with
respect to the crystal axes.
OCXO
Despite this it is still sometimes necessary to ensure a
better degree of stability. This can be achieved by placing the crystal in a
thermally insulated container with a thermostatically controlled heater. By
heating the crystal to a temperature above that which would normally be
encountered within the electronic equipment the temperature of the crystal can
be maintained at a constant temperature. This results in a far greater degree of
temperature stability. Additionally the crystal in the OCXO will be cut to
ensure that its temperature stability is optimised for the internal operating
temperature of the OCXO.
The typical specification for an OCXO might be �5 x 10-8
per degree Celsius (0.05 ppm), whereas a non-oven controlled oscillator may be
between 10 and 100 times poorer. As the oscillator assembly will also contain
buffering circuitry as well as supply voltage regulation the other
characteristics of the oscillator should also be good. Typically it might be
expected that frequency stability would be around �5 x 10-9 (0.005
ppm) per day and �5 x 10-7 (0.5 ppm) per year and 1 x 10-7
for a 5% change in supply voltage. All of these are far better than would be
expected from a simple crystal oscillator.
In order to ensure that the optimum overall accuracy is
maintained, combating elements such as ageing of the crystal itself, a periodic
calibration of the OCXO may be required. Typical calibration periods for OCXOs
may be of the order of six months to a year, but the actual period will depend
upon the OCXO itself and the requirements of the application in which it is
being used.
OCXO physical considerations
OCXOs are physically much larger than a simple crystal
oscillator. Not only do they need to incorporate the crystal oscillator itself,
but also the heater, control circuitry and the thermal insulation around the
crystal oscillator.
Typically the heater will be run from a different supply to
the oscillator. It does not need the same level of regulation, and indeed the
oscillator is most likely to have its own regulator to remove any stray noise
and RF that may appear on the supply line and thereby degrade the performance of
the OCXO.
The supply for the heater in the OCXO may be quite current
hungry. Some OCXO heaters may require an Amp or so on warm up. This figure will
reduce as the temperature inside the OCXO rises and less heat is needed. As will
be imagined the temperature of the OCXO is thermostatically controlled.
Summary
These OCXO units are naturally more expensive than crystals
on their own, but the performance of an OCXO is considerably enhanced on that of
a simple crystal in an unregulated electrical and physical environment.
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