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a.
Adjust the C ZERO COARSE control for a null indication on the meter.
b.
Adjust the C ZERO FINE control for a null
indication.
c.
Adjust the R/G ZERO control for a null indication.
d.
Repeat steps b and c until a complete null is obtained.
NOTE
Whenever the DIVIDE G/MULT1PLY R switch is in the I
position, adjust the CONDUCTANCE control for a null
indication before perform! ng steps a through d above.
3.5.1
TEST LEVEL control: This control adjusts the oscillator voltage applied to the
bridge and, of course, to the specimen being measured; this voltage can be \*jried over
the range of 1 mV to approximately 300 mV. When measuring semiconductor devices with
zero bias, a low test level is necessary so the excursion on the V-l characteristic is con¬
fined to small enough a section to be considered nearly linear.
That is, an increase in the
test level should not cause a significant change in the measured capacitance.
The signal voltage applied to the specimen can easily be measured at the front
panel with an RF Millivoltmeter such as the Boonton Model 92 Series.
The probe of the
voltmeter is plugged into the LO TEST coaxial jack and the TEST LEVEL control varied
to obtain the correct voltage.
The millivoltmeter can then be removed and the spe¬
cimen connected.
NOTE: When the TEST LEVEL control has been adjusted, the bridge
must be rebalanced before making any measurements.
If it is desired to monitor the applied voltage continuously, the Millivoltmeter
can be permanently connected by using a Tee adapter.
Presence of the probe does not
affect bridge operation.
3.5.2 GAIN control: This adjusts the gain of the first amplifier stage, VI01, over a
range of approximately 26 dB. When this control is set at a maximum, as called for in
Table 4, Initial Switcn and Control Settings, the instrument is operating at its maximum
resolution capability and the balancing process can be rather tedious.
On the high-capaci¬
ty ranges, at least, the resolution afforded at this setting is really more than is needed.
In these circumstances, it will be found that a balance is©asier to reach if the
GAIN control i s turned down a bit, as the resolution varies almost directly as the gain.
The optimum setting of the GAIN control is best determined experimentally by the operator
to suit the nature of the measurements being made.
3.5.3
BIAS arrangements: Many semiconductor devices, such as transistors and voltage-
variable capacitors, must be measured with normal dc bias applied.
The 75D includes an
adjustable internal bias supply with a range of -6 volts to +150 volts, controlled by the
panel BIAS ADJUST control.
These bias voltages appear accross the HI and Ground posts
of the front panel.
The polarity can be switched by the DC BIAS switch.
The bias voltage and current applied to a specimen can be measured at the front
panel.
A dc voltmeter can be connected across the E posts, and a dc milliameter can be
plugged into the I jack on the BIAS panel.
A current-limiting resistor is part of the
bias circuits.
75D
b-370
23
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