Power Requirements and Warm-up Time

The power consumed by an ovenized oscillator is primarily determined by the
quality of the insulation, the size of the internal oven and the ambient
temperature. Oven oscillator power consumption is frequently specified during
warm-up and after stabilization at room temperature. The steady-state power
requirements may be projected from knowledge of the oscillator's thermal
conductance. If an oscillator has a thermal conductance of 10 mw per degree, the
power consumption will increase by 50 mw for an ambient temperature drop of 5
degrees. The temperature at which the oven will "lose control" may be projected
from the room temperature power consumption since the oven controller cannot
drop the current below the amount needed to operate the circuitry. Typically,
foam insulated oscillators require 20mw per degree centigrade and will consume
between 1 and 2 watts at room temperature. Dewar insulated oscillators can
require less than 10mw per degree and consume below 1 watt at room temperature.
The room temperature consumption depends on the operating temperature of the
oven as the watts per degree concept would suggest. Increasing the oven setpoint
from 80 to 90 degrees causes a typical foam insulated oscillator to consume
about 200 milliwatts more power. The oven setpoint must be above the highest
ambient temperature specified plus the temperature rise due to the fixed power
consumption (oven oscillators do not have air-conditioning).

Warm-up time depends on the warm-up power, starting temperature and oven
mass. It takes more joules to heat a thick copper oven structure than a thin
aluminum structure and the required joules will enter the oven faster if the
turn-on power is higher. The warm-up current should be significantly higher than
the current required to keep the oven warm at the coolest ambient temperature or
the warm-up time may be unacceptably long at the cold temperature extreme. After
the oven reaches the desired operating temperature the oscillator will continue
to drift as the crystal and circuitry reach thermal equilibrium. SC-cut crystals
are superior to AT-cut crystals during warm-up because thermal gradients across
the crystal have far less effect on the frequency. Smaller, faster oven
oscillators are appealing but the more massive ovens will exhibit better
short-term stability (all other factors being equal) so avoid specifying short
warm-up times with little warm-up power whenever practical. Certain new
oscillator designs employ superior insulation techniques including evacuated
housings and Dewar flasks. Dewar flasks are a superior choice since the
evacuated space is free from out-gassing components and the removable flask
makes manufacture and repair vastly easier. Smaller ovens will consume less
power and precision ovens are shrinking to a size barely larger than the crystal
thanks to surface-mount components and hybrid techniques. SC-cut crystals are
more immune to thermal transients and are the favorite in the smaller oven
designs. Wenzel associates has developed a 1.3 inch cube Dewar insulated
oscillator with specifications rivaling much larger designs. The WAMO (Wenzel
Associates Miniature Oscillator) is available in commercial and fully space
qualified versions and was designed as a lower cost alternative to more exotic
designs.

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