HP 5334B Service Manual page 156

8-267. RF OUTPUT IMPEDANCE MATCHING AND OUTPUT BUFFER. The signal for the output
amplifiers is taken from the same point as the AGC (across C10). The voltage is buffered by Q5, which is an im-
pedance matching stage. Resistors R14 and R15 set the dc bias level; R14 is bypassed by C14. The signal is then
applied to the output buffer stage of Q9. R40 provides a 50fk source impedance when transformed by TI. The
typical gain of Q9 (base-to-collector) is approximately 2.
8-268. VOLTAGE REFERENCES. Constant current diode CR2 feeds 1 mA to zener diode CR3 providing
6.4V dc for the EFC varactor reference. R12 and C15 form a filter to attenuate noise from the zener diode. R13
provides current limiting for Q4 if the 5.7V line is shorted.
8-269. Oven Heater and Controller Circuit Descriptions
NOTE
In the following theory of operation, the term OVEN MASS is used
to
describe the cast aluminum block in which the crystal and crystal
electronics are located.
8-270. The purpose of the oven is to shield the oscillator crystal and electronics from normal ambient tempera-
ture changes. The oven controller does this by maintaining a constant oven temperature which is higher than the
highest expected ambient temperature. The oven circuit is made up of three main blocks: thermistor, amplifier
(controller), and heaters.
8-271.
A
thermistor (RT1) is secured with epoxy into a hole in the oven mass. U3 is the amplifier, and Q7 and
Q8 are the heaters. It is the thermistor that senses the oven mass temperature. The thermistor is in one leg of a
bridge circuit consisting of RT1, R18, R19, R20, and R21. When the mass temperature changes slightly, a voltage
change occurs across the bridge. Amplifier U3 boosts this voltage change and then uses it to control the current
through Q7 and Q8. The current flowing through Q7 and Q8 causes a 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
biased to adjust their power to cancel the impending temperature change.
8-272. WARM-UP: GENERAL OPERATION. If the oscillator has been off for several hours, the mass ther-
mistor will be at the ambient temperature. Assuming this is below the normal oven operating temperature (80" to
84"C), the resistance of the thermistor RT1 is higher than that of R18
+
R20, and therefore the voltage at U3(3)
is more positive than at U3(2). This causes the output of U3 to be approximately (Vcc
- 1.5V), supplying base
current to Q8 through Q6. A separate circuit limits the collector current of Q8 and is described later.
8-273. As the oven mass warms up, the thermistor's resistance begins to drop, causing the voltage at both U3 in-
puts to drop (the other U3 input voltage drops because the voltage at the junction of R17 and R18, R19 drops
due to the lower RT1 resistance). The voltage at U3(3) decreases at a faster rete than at U3(2) and eventually
the U3 inputs are equal when RT1
=
R20
+
R18. At this time, the oven contrdner "cuts 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.
8-274. The purpose of R17 is mainly to reduce the power dissipated in the thermistor which causes it to self-
heat above the oven operating temperature.
8-275. R38 and R39, in parallel provide a means of sensing the heater current. During warm-up, the volthge
across the parallel resistors is used in the current limit circuit (described later). During normal linear operation,
the junction of R38 and R39 is essentially the feedback point for the oven controller loop.
HP 5334B
-
Service Manual
8-36