OCXOs - Oven-Controlled Crystal Oscillators

High performance crystal oscillators employ temperature control circuitry to
hold the crystal and critical circuitry at a precise, constant temperature. The
best controllers are proportional, providing a steady heating current which
changes with the ambient temperature to hold the oven at a precise set-point,
usually about 10 degrees above the highest expected ambient temperature.
Temperature induced frequency variations can be greatly reduced by an amount
approaching the thermal gain of the oven. The crystal for the oven is selected
to have a "turning-point" at or near the oven temperature further reducing the
sensitivity to temperature. The combination of the high oven gain with operation
near turning point yields temperature stabilities as good as 0.0001 PPM over a
temperature range that would cause the crystal to change by 10 PPM.

Highly polished, high-Q crystals which often have significant activity dips
may be designed with no activity dips near the operating temperature providing
better stability and phase noise than crystals designed for wide temperature
ranges. Ovens also allow the use of SC-cut crystals which offer superior
characteristics but which are impractical for ordinary TCXOs because of their
steep frequency drop at cooler temperatures.

Oven oscillators come in all levels of performance and complexity. Low cost
oven oscillators provide better temperature stability than more expensive TCXOs
but the power consumption is much higher. At the other end of the spectrum are
high-performance oven oscillators exhibiting superb short-term stability, very
low aging rates and high immunity to temperature and other enviornmental
effects. Oven oscillators insulated with Dewar flasks instead of foam are
available for superior temperature performance and lower power consumption.

Oven oscillators require a few minutes to warm and the power consumption is
typically one or two watts at room temperature. SC-cut crystals stabilize as
soon as they reach the operating temperature but AT-cut crystals exhibit a
significant thermal transient effect which can take many minutes to settle. A
typical AT-cut crystal will drop in frequency well below any point on the static
crystal curve due to the sudden application of oven heat. The frequency will
exponentially drift back up to the nominal frequency in most oscillators. In
some designs the oven controller overshoots significantly during initial warm-up
and then cools back down to the final oven temperature. This cooling transient
can kick the AT-cut in the other direction and may actually result in a shorter
warm-up time even though the controller takes longer to settle. Hand tweaked
designs can achieve fairly good warm-up times with carefully selected overshoot
but the advent of quick settling SC-cut crystals obsoleted this approach.

Typical Oven Oscillators

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Catalog