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8-27. OVEN HEATER AND CONTROLLER THEORY
In the following theory of operation the term OVEN MASS will be used to
describe the cast aluminum block in which the crystal and crystal
electronics are located. The exploded view of the oscillator shows the
oven mass.
8-28.
The purpose of the oven is to shield the oscillator crystal and electronics from normal ambient
temperature changes. The oven controller does this by maintaining a constant oven temperature
which is higher than the highest expected ambient temperature.
8-29.
Three main blocks make up the oven circuits. See
1. Thermistor
2. Amplifier (controller)
3. Heaters
8-30.
In the 10811A/B oven, a thermistor (RT1) is secured with epoxy into a hole in the oven mass. U3
is the amplifier and Q7 (not shown) and Q8 are the heaters. It is the thermistor that senses the
oven mass temperature. Since it is in one leg of the bridge circuit, when the mass temperature
changes slightly, a voltage change occurs across the bridge (RT1, R18, R19, R20, R21).
Amplifier U3 boosts this voltage change and then uses it to control the current through Q7 and Q8.
The current flowing through transistors Q7 and Q8 causes power dissipation in the form of heat,
and it is this heat that warms the oven mass. Therefore, when the mass temperature starts to
change, the heaters are told to adjust their power to cancel the impending temperature change.
8-31.
WARM-UP: GENERAL OPERATION. If the oscillator has been off for several hours, the mass
and thermistor will be at the ambient temperature. Assuming this is below the normal oven
operating temperature (80 to 84°C) the resistance at the thermistor RT1 is higher than that of R18
+ R20 and therefore V
current to Q8 through Q6. A separate circuit limits the collector current of Q8 and is described
later. As the oven mass warms up, the thermistor's resistance gets lower causing both V2 and V1
to lower (V2 lowers because Vo lowers due to RT1 getting smaller). V1 decreases at a faster rate
than V2 and eventually V
back" and begins to operate in a linear mode, adjusting the collector current in Q8 (and therefore
the power dissipated in Q7 and Q8) to keep the oven precisely at its set temperature.
46
Figure 8-9. Voltage References
NOTE
> V
. This causes the output of U3 to be ≈ (Vcc - 1.5V) and supply base
1
2
= V
when RT1 = R20 + R18. At this time, the oven controller "cuts
1
2
Figure 8-10.
Model 10811A/B
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