The main disadvantage of an OCXO is power consumption, unit size, warm-up time and cost.  The amount of oven power required is determined mainly by the quality of insulation used and the temperature differential between the oven and the external environment.  Increasing amount of insulation to reduce heat loss requires an increase in size, resulting in a tradeoff between power and size.  Warm-up time is the time required for the oven to reach operating temperature and for the frequency to stabilize.  It is largely dependent on available power, the thermal mass of the oven, the quality of insulation, and ambient temperature.  Typical warm-up times are from 15 seconds to 5 minutes.

Setting Oven Temperature

      The oven operating temperature (crystal turnover temperature) must be several degrees higher than the highest ambient temperature in which the oscillator is to operate in order that the oven may maintain good control (considering the internal heat rise generated by the oscillator itself).

     However, there are disadvantages associated with high oven temperature operation. First, the crystal's frequency vs. temperature characteristic is sharper with higher turnover crystals resulting in more sensitivity to minute changes in oven temperature.  Second, and more important, crystal aging degrade with an increasing temperature.  Therefore, in designing an oven controlled crystal oscillator, one is faced with a compromise in determining the desired oven operating temperature; it should be low as practicable, but it must be high enough to provide good control at the maximum ambient operating temperature.

Warm-up on Crystal Resonator

     Changing the temperature surrounding a crystal unit produces thermal gradients when, for example, heat flows to or from the active area of the resonator plate through the mounting clips.  The static F vs. T characteristic is modified by the thermal-transient effect resulting from the thermal-gradient-induced stresses.  When an OCXO is turned on, there can be a significant thermal-transient effect. 

     For an OCXO utilizing the AT-cut resonator, the crystal resonator frequency rapidly decreases as the oven warms up.  This is simply due to the fact that the frequency of an AT cut crystal is considerably higher at room temperature than at its upper turnover temperature.  In a standard OCXO, the oven balances in 10 to 15 minutes, but the thermal gradients in the AT-cut crystal produce a large frequency undershoots (rubber band effect) that anneals out to its final frequency several minutes after the oven reaches equilibrium.  Typically, relatively high degree of stability is achieved within 30 minutes after turn-on and this time can be reduced to less than 5 minutes in special fast warm-up designs.  On the other hand, the SC-cut crystal, being "stress-compensated" and thereby insensitive to such thermal-transient-induced stresses, reaches the equilibrium frequency as soon as the oven stabilizes.

Oven Stability

       The oven stability depends on the temperature range outside the OCXO and the thermal gain of the oven.  The thermal gain is defined as the external to internal temperature excursion ratio.  For example, if during an external temperature excursion from -40^oC to +60^oC, the temperature inside the oven changes by 0.1^oC, the thermal gain is 10³.  In addition, the thermal transient effect makes small oven offsets more difficult and time consuming to achieve with AT-cut resonators than with SC-cut designs.

     When the required temperature stability is beyond that which can be achieved with a standard proportionally controlled oven, a double oven system can be employed in which the standard oven is housed within a second oven. The outer oven then buffers the ambient temperature changes to the inner oven, which contain the oscillator circuit.