OCXO Datasheet, PDF (1/7 Pages) MICORO CRYSTAL SWITZERLAND – Oven Controlled Crystal Oscillator

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Note
OCXO: Oven Controlled Crystal Oscillator
Application Note
Introduction
If stability requirements are too stringent to be met by a basic crystal oscillator or TCXO, the crystal and
critical circuits may be temperature controlled by an oven. The block diagram for a Vectron oven controlled
crystal oscillator is similar to that for a Vectron TCXO except that the varactor diode and associated
thermistor compensation network are deleted and the oscillator is instead temperature controlled by a
proportionally controlled oven.
Proportional Oven Controlled
A proportional control is an electronic servo system which continuously supplies power to the oven; it varies
the amount of oven power, continuously compensating for the ambient temperature changes. In many
Vectron oven controlled oscillators, a thermistor is heat sunk to the oven’s metal shell to sense temperature.
The thermistor is one leg of a resistance bridge, as shown in the following diagram.
B+
Differential
Power
T
Amplifer
Amplifier
Figure 1.
The bridge operates such that if the temperature at the oven decreases due to an ambient temperature
change, the change in thermistor resistance causes the bridge to unbalance, developing an increase in
bridge output voltage. This voltage is amplified in a high-gain differential amplifier. The output of the
differential amplifier is further amplified in a power amplifier which drives directly into the oven winding.
Thus, the small voltage increase resulting from bridge unbalance generates a large voltage increase
across the oven winding. This increase in power to the oven generates more heat, compensating for the
temperature decrease which was initially sensed by the thermistor. Similarly, an increase in temperature at
the oven causes a reduction in bridge output voltage, which results in reduced power into the oven and a
compensating temperature decrease.
An alternative to this design, used in some Vectron OCXOs, has the power amplifier(s) heat sunk to the oven
shell as the heat transfer mechanism, in lieu of having a heater winding. The concept is the same, the only
difference being the vehicle by which the heat is applied to the oven.
Vectron International • 267 Lowell Road, Hudson, NH 03051 • Tel: 1-88-VECTRON-1 • http://www.vectron.com
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