OCXO Datasheet, PDF (4/7 Pages) MICORO CRYSTAL SWITZERLAND – Oven Controlled Crystal Oscillator

Application Note
Second, and more important, crystal aging (discussed below) degrades with an increasing temperature.
Therefore, in designing an oven controlled crystal oscillator, one is faced with a compromise in determin-
ing 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.
Stability
A. Aging - Aging refers to the continuous change in crystal oscillator frequency with time, all other param-
eters held constant. Prior to delivery, each Vectron oven controlled oscillator is pre-aged until it achieve its
specified aging rate. Aging rate is often used synonymously with the word stability; thus, an oscillator with
an aging rate of one part in 10^-8 per day (1x10^-8/day) is sometimes referred to as one part in 108 oscillator.
This is incorrect terminology, as aging rate (long term stability) must be referred to time, and represents only
one facet of oscillator stability.
B. Temperature Stability - As previously noted, because no oven control system is perfect, a change in am-
bient temperature causes a small change output frequency. The frequency shift is an offset from the oscil-
lator’s aging curve. This deviation from the normal aging characteristic is not related to time, but is a fixed
offset. Thus, the frequency offset vs. temperature (temperature stability), for a given temperature change is,
for example, 5x10^-9, not 5x10-⁹/day. This characteristic is shown below.
Figure 5.
Frequency Offset
Change in
ambient temp.
Return to initial
ambient temp.
Aging characteristic
at constant ambient
Frequency curve with
temp. change
Time
Ambient temperature changes do not produce hysteresis effects; that is, if there is a change in ambient tem-
perature followed by a return to the original temperature, the final frequency will be essentially that which
would have resulted had there been no ambient temperature change.
When the required temperature stability is beyond that which can be achieved with a standard proportion-
ally 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 con-
tain the oscillator circuit.
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