MM0474
j. With the 431C RANGE switch at NULL, a stable reading greater than zero
indicates an audio-bias oscillation state.
While changing the substituted
resistances, the operator can determine when oscillations cease by noting a
change of meter readings to zero. The operating resistance of the detection
thermistor element is measured by reading the resistance deviation in percent
directly from the switch settings that cause the power meter to change from an
oscillation to a non-oscillation state.
6.
PRINCIPLES OF OPERATION.
a. Overall Description.
(1) The block diagram of the 8402B is shown in Figure 4.
The 8402B
precision DC generator circuit consists of a differential amplifier and two
output transistors connected in a Darlington-combination.
Eleven switchable
power ranges are selected by precision resistances in the emitter circuit of
the output transistors. The differential amplifier may be switched to either
of two types of internal voltage references.
Both voltage references are
furnished -30 volts by a regulated power supply.
One voltage reference is
fixed and the other is continuously variable. The fixed voltage reference is
used in the CALIBRATION mode of operation, the variable voltage reference is
used in the SUBSTITUTE mode. The variable voltage reference is used to provide
an adjustable DC power output, over the power range selected, for DC
substitution power measurements made with a 431C Power Meter.
output provides a DC voltage proportional to the output current.
(2) The thermistor resistance function circuit of the 8402B is passive.
Precision resistances are substituted for the RF detection thermistor in the
431C Power Meter RF detection bridge. The power meter is used to indicate the
bridge balance condition identical to chat during an actual power measurement.
This indication provides a means to measure the RF detection thermistor
operating resistance.
b. Precision Current Generator.
(1) The schematic diagram of the 8402B is shown in Figure 5.
The
precision DC generator consists of transistors A1Q8A thru A1Q7 and associated
circuits. Transistors A1Q8A and 8B form a differential amplifier. The circuit
loop from the collector of A1Q8B thru the base and emitter of A1Q6, and the
base and emitter of A1Q7, returning to the base of A1Q8B is a negative feedback
loop of moderate loop gain.
The function of the loop is to maintain the
voltage at the base of A1Q8B equal to the voltage at the base of A1Q8A. The
voltage at the base of A1Q8A is a reference voltage that is derived from one of
two sources, depending on the mode of operation.
In the CALIBRATE mode, a
fixed reference voltage is precisely developed across the temperature-
compensated breakdown diode pair, A1VR2. In the SUBSTITUTE mode, the reference
is continuously variable by the CURRENT CONTROL AND VERNIER controls.
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