OCXO Datasheet, PDF (5/7 Pages) MICORO CRYSTAL SWITZERLAND – Oven Controlled Crystal Oscillator

Application Note
C. Restabilization And Retrace - When a crystal oscillator is turned off for a period of time and then turned
on again (as occurs when the unit is shipped), the crystal requires a restabilization period. The characteristic
is similar to the initial factory aging characteristic, but high stability is achieved significantly more quickly
because the crystal has been factory pre-aged.
In most applications, oven-controlled crystal oscillators are continuously energized. This being the case, ag-
ing is the critical characteristic with turn-off/turn-on characteristic being of little or no significance. However,
certain applications require that oven controlled crystal oscillators be frequently deenergized and re-ener-
gized (a practice which should be avoided whenever possible). When applications require frequent turn-off,
an additional series of characteristics should be considered.
Figure 6.
f1
f1
f2
f3
T1
T2
T3
T4
T5
Time
In Figure 6, assume that an oscillator is energized until time T2 when it is turned off for a period of
time and then turned on again at time T3. Three characteristics may then be of significance:
1.How close does the oscillator return to the output frequency at turn-off, a specified time after
turnon. This is called the retrace characteristic. Retrace error at T4 = f1 - f3.
2.How much will the frequency change over moderate periods of time (hours) after the oven has
stabilized. This is called the restabilization, or warmup, characteristic. Restabilization rate from T4
to T5 = (f3 - f2) / (T5 - T4)
3.How long does it take the oscillator to achieve its specified aging rate following a specified off
period (This is called “reaging”).
Many factors affect retrace, restabilization and reaging characteristics. Proper circuit design and com-
ponent selection minimize their effects, leaving (1) the crystal and, (2) the length of off-period prior
to oscillator turn-on as the prime factors. There is significant variation in these characteristics from
crystal to crystal and they should only be specified when absolutely required and then only to the
degree needed, as “tight” specifications in this area can have a major impact upon oscillator cost due
to low yield. These characteristics are of little consequence in oscillators which are energized continu-
ously.
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