Keithley 630 Instruction Manual page 18

MODEL 630 POTENTIOMETRIC ELECTROMETER
CIRCUIT DESCRIPTION
through a cathode follower, half of tube V1003, and drives the meter.
1.
VClOl has a moveable plate driven by an electromagnet which varies the distance
between the two plates. Since the capacitance varies in inverse proportion to the dis-
tance between the plates, a time varying capacitance is produced.
2. If a charge is placed on the capacitor its value is
Q = CE
Equation 3
where Q is the charge;
C is the capacitance;
E is the wltage across the capacitor.
In terms of the voltage, the equation becomes E = Q/C. If Q is constant and C varies
periodically,,E must also vary. Therefore, an ac voltage which is proportional to the
magnitude of the input voltage will be produced.
e. Resistors R147 through Rl52 form the feedback network, which divides the output
voltage for all voltmeter and null ranges except the 500-volt and l-volt ranges. Total
output is fed back for these two ranges. The feedback voltage is returned to the low
end of the vibrating capacitor to provide stability.
f.
The Model 630 uses a "driven guard" at its input: the feedback voltage is also
applied to the second conductor of the input cable. The potential between the input high
and low is the difference between the applied and feedback voltages.
Therefore, the cable
capacitance charges only to the difference between the two voltages. This reduces the
effect of cable capacitance,
providing faster response speed.
g.
The zero-control network consists of a 1.35-volt mercury battery, BlOl, and resis-
tors R109 through R123. The rear panel COARSE ZERO Switch, S103, has a span of Cl00 milli.
volts in 20-millivolt steps.
The front panel METER ZERO potentiometer, R112, has a 220
millivolt span.
h. The recorder output is developed across the top end of meter MlOl. It is one volt
for a full-scale meter deflection for all ranges above 1 millivolt. However, using a re-
corder with a l-milliampere movement reduces the output to 0.9 volt on the l-millivolt
range since the output impedance is 100 ohms on this range.
3-5. OSCILLATOR.
a.
The oscillator drives the vibrating capacitor and bridge demodulator and supplies
the filament voltage for ,tubes VlOOl and V1002.
b. The output of transistor
Q203, connected as a regulator, is divided and compared
to zener diode D207. Any voltage difference is amplified by transistors Q235 and Q206,
operating as a differential voltage amplifier.
The signal is further amplified by trans-
istors 4204, Q202, and 4201 and applied to the regulator, Q2U3.
c. A dc-to-ac converter forms the voltage to drive the vibrating capacitor, VClOl, and
the bridge demodulator, diodes D106 through D109.
Transistors Q207 and Q208 form a switch-
ing network to supply an interrupted dc voltage to transformer T202.
An extra winding
from the transformer supplies feedback to the network to sustain switching action.
Re-
sistor R213 determines the feedback magnitude.
0166R
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