is the frequency error after power is applied, relative to the previous value and aging rate before power was removed. The normal period that the OCXO is powered off is 24 hours. The normal period powered up is to sufficient time to allow complete thermal equilibrium. Good retrace is obtained by proper design of the oscillator, oven mechanics and resonator. All of these require careful processing and assembly to achieve consistent results. This is of the order of ±20 to ±50 ppb. In addition to the crystal related effects described above, thermal stresses from heating and cooling the oven structure can contribute to the retrace, and changes in aging rate.