Boonton Electronics 71A, 71AR - Extending the Range of the; DC Bias for Capacitance Tests

for connection. Samples connected for differential measure-
ment may both be biased, since any voltage applied to the dc
bias input appears across both the "TEST" and "DIFF" terminals.
Remote differential measurements may be made by connecting the
specimens to the "TEST" and "DIFF" terminals with coaxial cables.
The "COMP ADJ" adjustment procedures for remote differential
measurements are the same as those for absolute measurements
(see paragraph 3.4.2) except that the required connections and
disconnections are made simultaneously at both sets of terminals.
3.7 Extending the Range of the "C ZERO" Control
The front panel "c ZERO" control has sufficient range to zero
out capacitance up to approximately 7 pF between the "HI" and
"LO" test terminals. If it is found that greater range is re-
quired to zero out the capacitance of an external fixture, the
range may be extended up to approximately 30 pF by connecting
a high quality mica capacitor between the "HI" and "LO" "DIFF"
posts. The amount of "c ZERO" range extension will equal the
value of the capacitor at the "DIFF" terminals.
3.8 DC Bias for Capacitance Tests
3.8.1 Application at the "BIAS" Terminals
The capacitance specimen may be biased by applying a dc
voltage up to ±200 volts at 250 rnA to the dc bias binding
posts at the rear of the instrument. since any voltage
applied to these binding posts appears across both the "TEST"
and "DIFF" terminals, specimens for differential measure-
ment are subjected to identical bias conditions.
NOTE
In instruments having serial number 140 or above, a
fuse has been inserted in the dc bias circuit. This
prevents internal damage in case the bias voltage is
short-circuited at the "TEST" or "DIFF" terminals either
by accidental contact or by a shorted test specimen.
If no bias appears at the "TEST" and "DIFF" terminals,
regardless of the indication of the monitor of the bias
supply, the condition of this fuse (located at the rear
panel) should be checked. If frequent blowing of this
fuse is experienced, an external current limiting re-
sistor of approximately 100 K ohms may be connected in
series with the bias terminal.
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