MM0474
(3) To achieve -30 volts at the start of the regenerative process, A1CR5
is reverse biased by the negative voltage placed on its anode by the conduction
of A1CR6. Diode A1CR5 is reverse biased at this time so that all the current
from A1R2 and A1R3 is routed through the base of A1Q2 portion of the
differential amplifier instead of through A1R6. After the power supply reaches
-30 volts, the biases on A1CR5 and A1CR6 are reversed. Diode A1CR5 is forward
biased by the greater negative potential from the output of the series
is reverse biased by a relative negative potential on its anode by -9 volts
from A1VR1 with respect to the negative voltage at the junction of A1R2 and
A1R3.
With these final circuit conditions, the current through A1VR1 is
derived from the output of the series regulator Q1.
The action of the
differential amplifier A1Q2-A1Q3 stabilizes the output voltage of the series
regulator at the level necessary to supply -9 volts to the base of A1Q3. The
output of the power supply is set at -30 volts by adjusting A1R14 to provide
exactly -9 volts to the base of A1Q3 portion of the differential amplifier.
d. Thermistor Resistance Function.
(1) Thermistor Resistance Function is shown in the schematic in Figure
7. The purpose of this function is to determine the exact operating resistance
of a RF detection thermistor element when used with a 431C Power Meter for
microwave power measurement.
During an actual power measurement, the RF
detection thermistor element resistance is determined by the application of 10
kHz audio bias power from the poker meter.
The audio bias magnitude is
dependent on the amount of detection bridge unbalance.
At balance, the
thermistor resistance is made nearly equal to the balancing resistance arm of
the bridge. However, this operating resistance may not be exactly the nominal
operating resistance imprinted on the thermistor mount label.
The actual
operating resistance of the detection thermistor element may deviate as much as
0.5% from the nominal value. The thermistor resistance function of the 8402B
measures this deviation by measuring the balancing resistance arm of the 431C
detection bridge network under simulated operating conditions.
(2) The thermistor resistance function circuit is entirely passive.
Resistive circuits connected to the RESISTANCE STANDARD connector, J4, appear
as a thermistor mount to the RF detection bridge network. However, instead of
heat sensitive thermistors as the resistive elements, precision 0.01% resistors
are used. The substituted resistance values are determined by the THERMISTOR
RESISTANCE (PERCENT) switch. The resistors are designated A5R1 through A5R10,
plus an infinite resistance for the 0.5% position of the switch. Resistance
values placed in the 431C RF detection bridge network determine the feedback
loop gain of the bridge 10 kHz amplifier combination in the power meter. If
the resistance substituted for the thermistor is less than the balancing
resistance arm of the bridge, the loop gain is less than unity since feedback
is negative.
If the substituted resistance is greater than the balancing
resistance, the loop gain is greater than unity since feedback is positive.
These two conditions result in a no-oscillation/oscillation
93
Previous Page
